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A signal is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.
The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default.
If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.
Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.
This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.
A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or generate, or raise) a signal:
kill or raise by
the same process.kill from
another process. Signals are a limited but useful form of interprocess
communication.Each
of these kinds of events (excepting explicit calls to kill and raise)
generates its own particular kind of signal. The various kinds of signals are
listed and described in detail in section Standard
Signals.
In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.
An
error means that a program has done something invalid and cannot continue
execution. But not all kinds of errors generate signals--in fact, most do not.
For example, opening a nonexistent file is an error, but it does not raise a
signal; instead, open returns -1. In general, errors
that are necessarily associated with certain library functions are reported by
returning a value that indicates an error. The errors which raise signals are
those which can happen anywhere in the program, not just in library calls. These
include division by zero and invalid memory addresses.
An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process.
An
explicit request means the use of a library function such as kill whose purpose is specifically to
generate a signal.
Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later.
Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process.
A given type of signal is either typically synchrous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.
When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely--until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See section Blocking Signals.
When the signal is delivered, whether right away or after a
long delay, the specified
action for that signal
is taken. For certain signals, such as SIGKILL andSIGSTOP,
the action is fixed, but for most signals, the program has a choice: ignore the
signal, specify a handler function, or
accept the default
action for that kind of
signal. The program specifies its choice using functions such as signal or sigaction (see
section Specifying
Signal Actions). We sometimes say that a
handlercatches the signal. While the handler is
running, that particular signal is normally blocked.
If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.
If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see section Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing.
When
a signal terminates a process, its parent process can determine the cause of
termination by examining the termination status code reported by the wait or waitpidfunctions. (This
is discussed in more detail in section Process
Completion.) The information it can get includes the fact that termination
was due to a signal, and the kind of signal involved. If a program you run from
a shell is terminated by a signal, the shell typically prints some kind of error
message.
The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.
If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.
This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.
The signal names are defined in the header file `signal.h‘.
NSIG is
also one greater than the largest defined signal number.The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there‘s usually no way to continue the computation which encountered the error.
Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
Termination
is the sensible ultimate outcome from a program error in most programs. However,
programming systems such as Lisp that can load compiled user programs might need
to keep executing even if a user program incurs an error. These programs have
handlers which use longjmp to
return control to the command level.
The
default action for all of these signals is to cause the process to terminate. If
you block or ignore these signals or establish handlers for them that return
normally, your program will probably break horribly when such signals happen,
unless they are generated by raise or kill instead
of a real error.
When one of these program error signals terminates a process,
it also writes a core
dump file which records
the state of the process at the time of termination. The core dump file is
named `core‘ and is written in whichever directory
is current in the process at the time. (On the GNU system, you can specify the
file name for core dumps with the environment variable COREFILE.)
The purpose of core dump files is so that you can examine them with a debugger
to investigate what caused the error.
SIGFPE signal
reports a fatal arithmetic error. Although the name is derived from
"floating-point exception", this signal actually covers all arithmetic errors,
including division by zero and overflow. If a program stores integer data in a
location which is then used in a floating-point operation, this often causes
an "invalid operation" exception, because the processor cannot recognize the
data as a floating-point number.
Actual floating-point exceptions are a complicated subject because there
are many types of exceptions with subtly different meanings, and the SIGFPE signal
doesn‘t distinguish between them. The IEEE Standard for Binary
Floating-Point Arithmetic (ANSI/IEEE Std 754-1985) defines
various floating-point exceptions and requires conforming computer systems to
report their occurrences. However, this standard does not specify how the
exceptions are reported, or what kinds of handling and control the operating
system can offer to the programmer.
BSD
systems provide the SIGFPE handler
with an extra argument that distinguishes various causes of the exception. In
order to access this argument, you must define the handler to accept two
arguments, which means you must cast it to a one-argument function type in order
to establish the handler. The GNU library does provide this extra argument, but
the value is meaningful only on operating systems that provide the information
(BSD systems and GNU systems).
FPE_INTOVF_TRAPFPE_INTDIV_TRAPFPE_SUBRNG_TRAPFPE_FLTOVF_TRAPFPE_FLTDIV_TRAPFPE_FLTUND_TRAPFPE_DECOVF_TRAPSIGILL typically
indicates that the executable file is corrupted, or that you are trying to
execute data. Some common ways of getting into the latter situation are by
passing an invalid object where a pointer to a function was expected, or by
writing past the end of an automatic array (or similar problems with pointers
to automatic variables) and corrupting other data on the stack such as the
return address of a stack frame.
SIGILL can
also be generated when the stack overflows, or when the system has trouble
running the handler for a signal.
Common ways of getting a SIGSEGV condition
include dereferencing a null or uninitialized pointer, or when you use a
pointer to step through an array, but fail to check for the end of the array.
It varies among systems whether dereferencing a null pointer generates SIGSEGV or SIGBUS.
SIGSEGV, this signal
is typically the result of dereferencing an uninitialized pointer. The
difference between the two is that SIGSEGV indicates
an invalid access to valid memory, while SIGBUS indicates
an access to an invalid address. In particular,SIGBUS signals
often result from dereferencing a misaligned pointer, such as referring to a
four-word integer at an address not divisible by four. (Each kind of computer
has its own requirements for address alignment.)
The name of this signal is an abbreviation for "bus error".
abort.
See section Aborting
a Program.SIGABRT.SIGTRAP if
it is somehow executing bad instructions.These signals are all used to tell a process to terminate, in one way or another. They have different names because they‘re used for slightly different purposes, and programs might want to handle them differently.
The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
The (obvious) default action for all of these signals is to cause the process to terminate.
SIGTERM signal is a generic signal used to
cause program termination. Unlike SIGKILL, this signal
can be blocked, handled, and ignored. It is the normal way to politely ask a
program to terminate.
SIGINT ("program interrupt") signal is
sent when the user types the INTR character (normally C-c).
See section Special
Characters, for information about terminal driver support for C-c.SIGQUIT signal
is similar to SIGINT, except that
it‘s controlled by a different key--the QUIT character, usually C-\---and
produces a core dump when it terminates the process, just like a program error
signal. You can think of this as a program error condition "detected" by the
user.
See section Program Error Signals, for information about core dumps. See section Special Characters, for information about terminal driver support.
Certain kinds of cleanups are best omitted in handling SIGQUIT.
For example, if the program creates temporary files, it should handle the
other termination requests by deleting the temporary files. But it is better
for SIGQUIT not to delete them, so that the
user can examine them in conjunction with the core dump.
SIGKILL signal
is used to cause immediate program termination. It cannot be handled or
ignored, and is therefore always fatal. It is also not possible to block this
signal.
This signal is usually generated only by explicit request. Since it cannot
be handled, you should generate it only as a last resort, after first trying a
less drastic method such as C-c or SIGTERM. If a process
does not respond to any other termination signals, sending it a SIGKILL signal will almost always cause it
to go away.
In fact, if SIGKILL fails
to terminate a process, that by itself constitutes an operating system bug
which you should report.
The system will generate SIGKILL for
a process itself under some unusual conditions where the program cannot
possible continue to run (even to run a signal handler).
SIGHUP ("hang-up") signal is used to
report that the user‘s terminal is disconnected, perhaps because a network or
telephone connection was broken. For more information about this, see
section Control
Modes.
This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see section Termination Internals.
These signals are used to indicate the expiration of timers. See section Setting an Alarm, for information about functions that cause these signals to be sent.
The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.
alarm function, for example.The
signals listed in this section are used in conjunction with asynchronous I/O
facilities. You have to take explicit action by calling fcntl to enable a particular file
descriptior to generate these signals (see section Interrupt-Driven
Input). The default action for these signals is to ignore them.
On most operating systems, terminals and sockets are the only kinds of
files that can generate SIGIO; other kinds,
including ordinary files, never generate SIGIOeven if you ask
them to.
In the GNU system SIGIO will
always be generated properly if you successfully set asynchronous mode
with fcntl.
SIGIO. It is
defined only for compatibility.These signals are used to support job control. If your system doesn‘t support job control, then these macros are defined but the signals themselves can‘t be raised or handled.
You should generally leave these signals alone unless you really understand how job control works. See section Job Control.
The default action for this signal is to ignore it. If you establish a
handler for this signal while there are child processes that have terminated
but not reported their status via wait or waitpid (see
section Process
Completion), whether your new handler applies to those processes or not
depends on the particular operating system.
SIGCONT signal to a process to make it
continue. This signal is special--it always makes the process continue if it
is stopped, before the signal is delivered. The default behavior is to do
nothing else. You cannot block this signal. You can set a handler, but SIGCONT always
makes the process continue regardless.
Most programs have no reason to handle SIGCONT; they simply
resume execution without realizing they were ever stopped. You can use a
handler for SIGCONT to
make a program do something special when it is stopped and continued--for
example, to reprint a prompt when it is suspended while waiting for
input.
SIGTSTP signal
is an interactive stop signal. Unlike SIGSTOP, this signal
can be handled and ignored.
Your program should handle this signal if you have a special need to leave
files or system tables in a secure state when a process is stopped. For
example, programs that turn off echoing should handle SIGTSTP so they can turn echoing back on
before stopping.
This signal is generated when the user types the SUSP character (normally C-z). For more information about terminal driver support, see section Special Characters.
SIGTTIN signal. The default action for this
signal is to stop the process. For more information about how this interacts
with the terminal driver, see section Access
to the Controlling Terminal.SIGTTIN,
but is generated when a process in a background job attempts to write to the
terminal or set its modes. Again, the default action is to stop the
process. SIGTTOU is
only generated for an attempt to write to the terminal if the TOSTOP output mode is set; see
section Output
Modes.While
a process is stopped, no more signals can be delivered to it until it is
continued, except SIGKILL signals
and (obviously) SIGCONT signals.
The signals are marked as pending, but not delivered until the process is
continued. The SIGKILL signal
always causes termination of the process and can‘t be blocked, handled or
ignored. You can ignore SIGCONT, but it always
causes the process to be continued anyway if it is stopped. Sending a SIGCONT signal
to a process causes any pending stop signals for that process to be discarded.
Likewise, any pending SIGCONT signals
for a process are discarded when it receives a stop signal.
When
a process in an orphaned process group (see section Orphaned
Process Groups) receives a SIGTSTP, SIGTTIN,
or SIGTTOU signal and does not handle it, the
process does not stop. Stopping the process would probably not be very useful,
since there is no shell program that will notice it stop and allow the user to
continue it. What happens instead depends on the operating system you are using.
Some systems may do nothing; others may deliver another signal instead, such
as SIGKILL or SIGHUP. In the GNU
system, the process dies with SIGKILL; this avoids the
problem of many stopped, orphaned processes lying around the system.
These signals are used to report various errors generated by an operation done by the program. They do not necessarily indicate a programming error in the program, but an error that prevents an operating system call from completing. The default action for all of them is to cause the process to terminate.
SIGPIPE signal.
If SIGPIPE is blocked, handled or ignored, the
offending call fails with EPIPE instead.
Pipes and FIFO special files are discussed in more detail in section Pipes and FIFOs.
Another cause of SIGPIPE is
when you try to output to a socket that isn‘t connected. See section Sending
Data.
In the GNU system, SIGLOST is
generated when any server program dies unexpectedly. It is usually fine to
ignore the signal; whatever call was made to the server that died just returns
an error.
These signals are used for various other purposes. In general, they will not affect your program unless it explicitly uses them for something.
SIGUSR1 and SIGUSR2 signals are set aside for you to
use any way you want. They‘re useful for simple interprocess communication, if
you write a signal handler for them in the program that receives the signal.
There is an example showing the use of SIGUSR1 and SIGUSR2 in
section Signaling
Another Process.
The default action is to terminate the process.
If a program does full-screen display, it should handle SIGWINCH.
When the signal arrives, it should fetch the new screen size and reformat its
display accordingly.
If the process is the leader of the process group, the default action is to print some status information about the system and what the process is doing. Otherwise the default is to do nothing.
We
mentioned above that the shell prints a message describing the signal that
terminated a child process. The clean way to print a message describing a signal
is to use the functions strsignal and psignal. These functions
use a signal number to specify which kind of signal to describe. The signal
number may come from the termination status of a child process (see section Process
Completion) or it may come from a signal handler in the same process.
This function is a GNU extension, declared in the header file `string.h‘.
stderr;
see section Standard
Streams.
If you call psignal with
a message that is either a null pointer or an
empty string, psignal just
prints the message corresponding to signum, adding a
trailing newline.
If you supply a non-null message argument,
then psignal prefixes its output with this
string. It adds a colon and a space character to separate the message from the string corresponding
to signum.
This function is a BSD feature, declared in the header file `signal.h‘.
There is also an array sys_siglist which
contains the messages for the various signal codes. This array exists on BSD
systems, unlike strsignal.
The
simplest way to change the action for a signal is to use the signal function. You can specify a built-in
action (such as to ignore the signal), or you canestablish a
handler.
The
GNU library also implements the more versatile sigaction facility. This section describes both
facilities and gives suggestions on which to use when.
The signal function
provides a simple interface for establishing an action for a particular signal.
The function and associated macros are declared in the header
file`signal.h‘.
void.
So, you should define handler functions like this:
void handler (int signum) { ... }
The name sighandler_t for this data type is a GNU
extension.
signal function
establishes action as
the action for the signal signum.
The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names described in section Standard Signals---don‘t use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.
The second argument, action, specifies the action to use for the signal signum. This can be one of the following:
SIG_DFLSIG_DFL specifies the default action for
the particular signal. The default actions for various kinds of signals are
stated in section Standard
Signals.SIG_IGNSIG_IGN specifies that the signal should
be ignored. Your program generally should not ignore signals that represent
serious events or that are normally used to request termination. You cannot
ignore the SIGKILL or SIGSTOP signals
at all. You can ignore program error signals like SIGSEGV,
but ignoring the error won‘t enable the program to continue executing
meaningfully. Ignoring user requests such as SIGINT, SIGQUIT, and SIGTSTP is
unfriendly. When you do not wish signals to be delivered during a certain
part of the program, the thing to do is to block them, not ignore them. See
section Blocking
Signals.handlerIf you set the action for a signal to SIG_IGN, or if you set
it to SIG_DFL and
the default action is to ignore that signal, then any pending signals of that
type are discarded (even if they are blocked). Discarding the pending signals
means that they will never be delivered, not even if you subsequently specify
another action and unblock this kind of signal.
The signal function
returns the action that was previously in effect for the specified signum.
You can save this value and restore it later by calling signalagain.
If signal can‘t honor the request, it
returns SIG_ERR instead.
The following errno error
conditions are defined for this function:
EINVALSIGKILL or SIGSTOP.Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
if (signal (SIGINT, termination_handler) == SIG_IGN)
signal (SIGINT, SIG_IGN);
if (signal (SIGHUP, termination_handler) == SIG_IGN)
signal (SIGHUP, SIG_IGN);
if (signal (SIGTERM, termination_handler) == SIG_IGN)
signal (SIGTERM, SIG_IGN);
...
}
Note how if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.
We
do not handle SIGQUIT or
the program error signals in this example because these are designed to provide
information for debugging (a core dump), and the temporary files may give useful
information.
ssignal function
does the same thing as signal; it is provided
only for compatibility with SVID.signal to indicate an error.The sigaction function
has the same basic effect as signal: to specify how a
signal should be handled by the process. However, sigaction offers more control, at the expense
of more complexity. In particular, sigaction allows
you to specify additional flags to control when the signal is generated and how
the handler is invoked.
The sigaction function
is declared in `signal.h‘.
struct
sigaction are used in
the sigaction function
to specify all the information about how to handle a particular signal. This
structure contains at least the following members:
sighandler_t sa_handlersignal function.
The value can be SIG_DFL, SIG_IGN,
or a function pointer. See section Basic
Signal Handling.sigset_t sa_masksa_mask. If you want
that signal not to be blocked within its handler, you must write code in the
handler to unblock it.int sa_flagssigaction.signal function‘s
return value--you can check to see what the old action in effect for the
signal was, and restore it later if you want.)
Either action or old-action can be a null pointer. If old-action is a null pointer, this simply suppresses the return of information about the old action. Ifaction is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.
The return value from sigaction is
zero if it succeeds, and -1 on
failure. The following errno error
conditions are defined for this function:
EINVALSIGKILL or SIGSTOP.signal and sigactionIt‘s
possible to use both the signal and sigaction functions
within a single program, but you have to be careful because they can interact in
slightly strange ways.
The sigaction function
specifies more information than the signal function,
so the return value from signal cannot
express the full range of sigaction possibilities.
Therefore, if you use signal to
save and later reestablish an action, it may not be able to reestablish properly
a handler that was established with sigaction.
To
avoid having problems as a result, always use sigaction to
save and restore a handler if your program uses sigaction at all. Since sigaction is more general, it can properly save
and reestablish any action, regardless of whether it was established originally
with signal or sigaction.
On
some systems if you establish an action with signal and
then examine it with sigaction, the handler
address that you get may not be the same as what you specified with signal.
It may not even be suitable for use as an action argument with signal.
But you can rely on using it as an argument to sigaction.
This problem never happens on the GNU system.
So, you‘re better off using one or the other of the mechanisms consistently within a single program.
Portability
Note: The basic signal function
is a feature of ANSI C, while sigaction is
part of the POSIX.1 standard. If you are concerned about portability to
non-POSIX systems, then you should use the signal function
instead.
sigaction Function ExampleIn
section Basic
Signal Handling, we gave an example of establishing a simple handler for
termination signals using signal. Here is an
equivalent example usingsigaction:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
struct sigaction new_action, old_action;
/* Set up the structure to specify the new action. */
new_action.sa_handler = termination_handler;
sigemptyset (&new_action.sa_mask);
new_action.sa_flags = 0;
sigaction (SIGINT, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGINT, &new_action, NULL);
sigaction (SIGHUP, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGHUP, &new_action, NULL);
sigaction (SIGTERM, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGTERM, &new_action, NULL);
...
}
The
program just loads the new_action structure
with the desired parameters and passes it in the sigaction call. The usage of sigemptyset is described later; see sectionBlocking
Signals.
As
in the example using signal, we avoid
handling signals previously set to be ignored. Here we can avoid altering the
signal handler even momentarily, by using the feature of sigaction that lets us examine the current
action without specifying a new one.
Here
is another example. It retrieves information about the current action for SIGINT without
changing that action.
struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /*sigactionreturns -1 in case of error. */ else if (query_action.sa_handler == SIG_DFL) /*SIGINTis handled in the default, fatal manner. */ else if (query_action.sa_handler == SIG_IGN) /*SIGINTis ignored. */ else /* A programmer-defined signal handler is in effect. */
sigactionThe sa_flags member
of the sigaction structure
is a catch-all for special features. Most of the time, SA_RESTART is a good value to use for this
field.
The
value of sa_flags is
interpreted as a bit mask. Thus, you should choose the flags you want to set, OR
those flags together, and store the result in the sa_flagsmember
of your sigaction structure.
Each
signal number has its own set of flags. Each call to sigaction affects one particular signal number,
and the flags that you specify apply only to that particular signal.
In
the GNU C library, establishing a handler with signal sets all the flags to zero except
for SA_RESTART,
whose value depends on the settings you have made withsiginterrupt.
See section Primitives
Interrupted by Signals, to see what this is about.
These macros are defined in the header file `signal.h‘.
SIGCHLD signal. When the flag is set, the
system delivers the signal for a terminated child process but not for one that
is stopped. By default, SIGCHLD is
delivered for both terminated children and stopped children.
Setting this flag for a signal other than SIGCHLD has no effect.
SIGILL.open, read or write), and the signal
handler returns normally. There are two alternatives: the library function can
resume, or it can return failure with error code EINTR.
The choice is controlled by the SA_RESTART flag
for the particular kind of signal that was delivered. If the flag is set,
returning from a handler resumes the library function. If the flag is clear,
returning from a handler makes the function fail. See section Primitives
Interrupted by Signals.
When
a new process is created (see section Creating
a Process), it inherits handling of signals from its parent process.
However, when you load a new process image using the exec function (see section Executing
a File), any signals that you‘ve defined your own handlers for revert to
their SIG_DFL handling. (If you think about it a
little, this makes sense; the handler functions from the old program are
specific to that program, and aren‘t even present in the address space of the
new program image.) Of course, the new program can establish its own
handlers.
When
a program is run by a shell, the shell normally sets the initial actions for the
child process to SIG_DFL or SIG_IGN, as appropriate.
It‘s a good idea to check to make sure that the shell has not set up an initial
action of SIG_IGN before
you establish your own signal handlers.
Here
is an example of how to establish a handler for SIGHUP,
but not if SIGHUP is
currently ignored:
...
struct sigaction temp;
sigaction (SIGHUP, NULL, &temp);
if (temp.sa_handler != SIG_IGN)
{
temp.sa_handler = handle_sighup;
sigemptyset (&temp.sa_mask);
sigaction (SIGHUP, &temp, NULL);
}
This
section describes how to write a signal handler function that can be established
with the signal or sigaction functions.
A
signal handler is just a function that you compile together with the rest of the
program. Instead of directly invoking the function, you use signal or sigaction to
tell the operating system to call it when a signal arrives. This is known
as establishing the handler. See section Specifying
Signal Actions.
There are two basic strategies you can use in signal handler functions:
You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.
Handlers
which return normally are usually used for signals such as SIGALRM and the I/O and interprocess
communication signals. But a handler for SIGINT might
also return normally after setting a flag that tells the program to exit at a
convenient time.
It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See section Program Error Signals.
Handlers
that return normally must modify some global variable in order to have any
effect. Typically, the variable is one that is examined periodically by the
program during normal operation. Its data type should be sig_atomic_t for reasons described in section Atomic
Data Access and Signal Handling.
Here
is a simple example of such a program. It executes the body of the loop until it
has noticed that a SIGALRM signal
has arrived. This technique is useful because it allows the iteration in
progress when the signal arrives to complete before the loop exits.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;
/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
keep_going = 0;
signal (sig, catch_alarm);
}
void
do_stuff (void)
{
puts ("Doing stuff while waiting for alarm....");
}
int
main (void)
{
/* Establish a handler for SIGALRM signals. */
signal (SIGALRM, catch_alarm);
/* Set an alarm to go off in a little while. */
alarm (2);
/* Check the flag once in a while to see when to quit. */
while (keep_going)
do_stuff ();
return EXIT_SUCCESS;
}
Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.
The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this:
volatile sig_atomic_t fatal_error_in_progress = 0;
void
fatal_error_signal (int sig)
{
/* Since this handler is established for more than one kind of signal,
it might still get invoked recursively by delivery of some other kind
of signal. Use a static variable to keep track of that. */
if (fatal_error_in_progress)
raise (sig);
fatal_error_in_progress = 1;
/* Now do the clean up actions:
- reset terminal modes
- kill child processes
- remove lock files */
...
/* Now reraise the signal. Since the signal is blocked,
it will receive its default handling, which is
to terminate the process. We could just call
exit or abort, but reraising the signal
sets the return status from the process correctly. */
raise (sig);
}
You
can do a nonlocal transfer of control out of a signal handler using the setjmp and longjmp facilities
(see section Non-Local
Exits).
When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.
There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See section Blocking Signals.
The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.
Here is a rather schematic example showing the reinitialization of one global variable.
#include <signal.h>
#include <setjmp.h>
jmp_buf return_to_top_level;
volatile sig_atomic_t waiting_for_input;
void
handle_sigint (int signum)
{
/* We may have been waiting for input when the signal arrived,
but we are no longer waiting once we transfer control. */
waiting_for_input = 0;
longjmp (return_to_top_level, 1);
}
int
main (void)
{
...
signal (SIGINT, sigint_handler);
...
while (1) {
prepare_for_command ();
if (setjmp (return_to_top_level) == 0)
read_and_execute_command ();
}
}
/* Imagine this is a subroutine used by various commands. */
char *
read_data ()
{
if (input_from_terminal) {
waiting_for_input = 1;
...
waiting_for_input = 0;
} else {
...
}
}
What happens if another signal arrives while your signal handler function is running?
When
the handler for a particular signal is invoked, that signal is automatically
blocked until the handler returns. That means that if two signals of the same
kind arrive close together, the second one will be held until the first has been
handled. (The handler can explicitly unblock the signal using sigprocmask,
if you want to allow more signals of this type to arrive; see section Process
Signal Mask.)
However,
your handler can still be interrupted by delivery of another kind of signal. To
avoid this, you can use the sa_mask member
of the action structure passed tosigaction to
explicitly specify which signals should be blocked while the signal handler
runs. These signals are in addition to the signal for which the handler was
invoked, and any other signals that are normally blocked by the process. See
section Blocking
Signals for a Handler.
When
the handler returns, the set of blocked signals is restored to the value it had
before the handler ran. So using sigprocmask inside
the handler only affects what signals can arrive during the execution of the
handler itself, not what signals can arrive once the handler returns.
Portability
Note: Always use sigaction to
establish a handler for a signal that you expect to receive asynchronously, if
you want your program to work properly on System V Unix. On this system, the
handling of a signal whose handler was established with signal automatically sets the signal‘s
action back to SIG_DFL, and the handler
must re-establish itself each time it runs. This practice, while inconvenient,
does work when signals cannot arrive in succession. However, if another signal
can arrive right away, it may arrive before the handler can re-establish itself.
Then the second signal would receive the default handling, which could terminate
the process.
If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.
Here
is an example of a handler for SIGCHLD that
compensates for the fact that the number of signals recieved may not equal the
number of child processes generate them. It assumes that the program keeps track
of all the child processes with a chain of structures as follows:
struct process
{
struct process *next;
/* The process ID of this child. */
int pid;
/* The descriptor of the pipe or pseudo terminal
on which output comes from this child. */
int input_descriptor;
/* Nonzero if this process has stopped or terminated. */
sig_atomic_t have_status;
/* The status of this child; 0 if running,
otherwise a status value from waitpid. */
int status;
};
struct process *process_list;
This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero.
/* Nonzero means some child‘s status has changed
so look at process_list for the details. */
int process_status_change;
Here is the handler itself:
void
sigchld_handler (int signo)
{
int old_errno = errno;
while (1) {
register int pid;
int w;
struct process *p;
/* Keep asking for a status until we get a definitive result. */
do
{
errno = 0;
pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
}
while (pid <= 0 && errno == EINTR);
if (pid <= 0) {
/* A real failure means there are no more
stopped or terminated child processes, so return. */
errno = old_errno;
return;
}
/* Find the process that signaled us, and record its status. */
for (p = process_list; p; p = p->next)
if (p->pid == pid) {
p->status = w;
/* Indicate that the status field
has data to look at. We do this only after storing it. */
p->have_status = 1;
/* If process has terminated, stop waiting for its output. */
if (WIFSIGNALED (w) || WIFEXITED (w))
if (p->input_descriptor)
FD_CLR (p->input_descriptor, &input_wait_mask);
/* The program should check this flag from time to time
to see if there is any news in process_list. */
++process_status_change;
}
/* Loop around to handle all the processes
that have something to tell us. */
}
}
Here
is the proper way to check the flag process_status_change:
if (process_status_change) {
struct process *p;
process_status_change = 0;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.
The
loop which checks process status avoids examining p->status until it sees that status has been
validly stored. This is to make sure that the status cannot change in the middle
of accessing it. Once p->have_status is set, it means that the child
process is stopped or terminated, and in either case, it cannot stop or
terminate again until the program has taken notice. See section Atomic
Usage Patterns, for more information about coping with interruptions during
accessings of a variable.
Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked.
sig_atomic_t process_status_change;
sig_atomic_t last_process_status_change;
...
{
sig_atomic_t prev = last_process_status_change;
last_process_status_change = process_status_change;
if (last_process_status_change != prev) {
struct process *p;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
}
Handler
functions usually don‘t do very much. The best practice is to write a handler
that does nothing but set an external variable that the program checks
regularly, and leave all serious work to the program. This is best because the
handler can be called at asynchronously, at unpredictable times--perhaps in the
middle of a primitive function, or even between the beginning and the end of a C
operator that requires multiple instructions. The data structures being
manipulated might therefore be in an inconsistent state when the handler
function is invoked. Even copying one int variable
into another can take two instructions on most machines.
This means you have to be very careful about what you do in a signal handler.
volatile. This tells
the compiler that the value of the variable might change asynchronously, and
inhibits certain optimizations that would be invalidated by such
modifications.A function can be non-reentrant if it uses memory that is not on the stack.
gethostbyname. This
function returns its value in a static object, reusing the same object each
time. If the signal happens to arrive during a call to gethostbyname,
or even after one (while the program is still using the value), it will
clobber the value that the program asked for. However, if the program does not
use gethostbyname or any other function that returns
information in the same object, or if it always blocks signals around each
use, then you are safe. There are a large number of library functions that
return values in a fixed object, always reusing the same object in this
fashion, and all of them cause the same problem. The description of a function
in this manual always mentions this behavior.fprintf. Suppose that
the program was in the middle of an fprintf call
using the same stream when the signal was delivered. Both the signal handler‘s
message and the program‘s data could be corrupted, because both calls operate
on the same data structure--the stream itself. However, if you know that the
stream that the handler uses cannot possibly be used by the program at a time
when signals can arrive, then you are safe. It is no problem if the program
uses some other stream.malloc and free are
not reentrant, because they use a static data structure which records what
memory blocks are free. As a result, no library functions that allocate or
free memory are reentrant. This includes functions that allocate space to
store a result. The best way to avoid the need to allocate memory in a handler
is to allocate in advance space for signal handlers to use. The best way to
avoid freeing memory in a handler is to flag or record the objects to be
freed, and have the program check from time to time whether anything is
waiting to be freed. But this must be done with care, because placing an
object on a chain is not atomic, and if it is interrupted by another signal
handler that does the same thing, you could "lose" one of the objects. The
relocating allocation functions (see section Relocating
Allocator) are certainly not safe to use in a signal handler.errno is
non-reentrant, but you can correct for this: in the handler, save the original
value of errno and
restore it before returning normally. This prevents errors that occur within
the signal handler from being confused with errors from system calls at the
point the program is interrupted to run the handler. This technique is
generally applicable; if you want to call in a handler a function that
modifies a particular object in memory, you can make this safe by saving and
restoring that object.Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily atomic. This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler might in the middle of reading or writing the object.
There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see section Blocking Signals).
Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)
#include <signal.h>
#include <stdio.h>
struct two_words { int a, b; } memory;
void
handler(int signum)
{
printf ("%d,%d\n", memory.a, memory.b);
alarm (1);
}
int
main (void)
{
static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
signal (SIGALRM, handler);
memory = zeros;
alarm (1);
while (1)
{
memory = zeros;
memory = ones;
}
}
This
program fills memory with
zeros, ones, zeros, ones, alternating forever; meanwhile, once per second, the
alarm signal handler prints the current contents. (Calling printf in the handler is safe in this
program because it is certainly not being called outside the handler when the
signal happens.)
Clearly,
this program can print a pair of zeros or a pair of ones. But that‘s not all it
can do! On most machines, it takes several instructions to store a new value
in memory, and the
value is stored one word at a time. If the signal is delivered in between these
instructions, the handler might find that memory.a is
zero andmemory.b is
one (or vice versa).
On
some machines it may be possible to store a new value in memory with just one instruction that cannot
be interrupted. On these machines, the handler will always print two zeros or
two ones.
To
avoid uncertainty about interrupting access to a variable, you can use a
particular data type for which access is always atomic: sig_atomic_t.
Reading and writing this data type is guaranteed to happen in a single
instruction, so there‘s no way for a handler to run "in the middle" of an
access.
The
type sig_atomic_t is
always an integer data type, but which one it is, and how many bits it contains,
may vary from machine to machine.
In
practice, you can assume that int and
other integer types no longer than int are
atomic. You can also assume that pointer types are atomic; that is very
convenient. Both of these are true on all of the machines that the GNU C library
supports, and on all POSIX systems we know of.
Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.
An interrupt in the middle of testing the flag is safe because either it‘s recognized to be nonzero, in which case the precise value doesn‘t matter, or it will be seen to be nonzero the next time it‘s tested.
An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)
Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. See sectionSignals Close Together Merge into One, for an example.
A
signal can arrive and be handled while an I/O primitive such as open or read is
waiting for an I/O device. If the signal handler returns, the system faces the
question: what should happen next?
POSIX
specifies one approach: make the primitive fail right away. The error code for
this kind of failure is EINTR. This is flexible,
but usually inconvenient. Typically, POSIX applications that use signal handlers
must check for EINTR after
each library function that can return it, in order to try the call again. Often
programmers forget to check, which is a common source of error.
The
GNU library provides a convenient way to retry a call after a temporary failure,
with the macro TEMP_FAILURE_RETRY:
EINTR, TEMP_FAILURE_RETRY evaluates it again, and over and
over until the result is not a temporary failure.
The value returned by TEMP_FAILURE_RETRY is whatever value expression produced.
BSD
avoids EINTR entirely and provides a more
convenient approach: to restart the interrupted primitive, instead of making it
fail. If you choose this approach, you need not be concerned with EINTR.
You
can choose either approach with the GNU library. If you use sigaction to establish a signal handler, you
can specify how that handler should behave. If you specify the SA_RESTART flag, return from that handler will
resume a primitive; otherwise, return from that handler will cause EINTR.
See section Flags
for sigaction.
Another
way to specify the choice is with the siginterrupt function.
See section BSD
Function to Establish a Handler.
When
you don‘t specify with sigaction or siginterrupt what
a particular handler should do, it uses a default choice. The default choice in
the GNU library depends on the feature test macros you have defined. If you
define _BSD_SOURCE or _GNU_SOURCE before
calling signal, the
default is to resume primitives; otherwise, the default is to make them fail
with EINTR. (The
library contains alternate versions of the signal function,
and the feature test macros determine which one you really call.) See
section Feature
Test Macros.
The
description of each primitive affected by this issue lists EINTR among the error codes it can
return.
There
is one situation where resumption never happens no matter which choice you make:
when a data-transfer function such as read or write is
interrupted by a signal after transferring part of the data. In this case, the
function returns the number of bytes already transferred, indicating partial
success.
This
might at first appear to cause unreliable behavior on record-oriented devices
(including datagram sockets; see section Datagram
Socket Operations), where splitting one read or write into
two would read or write two records. Actually, there is no problem, because
interruption after a partial transfer cannot happen on such devices; they always
transfer an entire record in one burst, with no waiting once data transfer has
started.
Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.
A
process can send itself a signal with the raise function.
This function is declared in `signal.h‘.
raise function
sends the signal signum to
the calling process. It returns zero if successful and a nonzero value if it
fails. About the only reason for failure would be if the value of signum is invalid.gsignal function
does the same thing as raise; it is provided
only for compatibility with SVID.One
convenient use for raise is
to reproduce the default behavior of a signal that you have trapped. For
instance, suppose a user of your program types the SUSP character (usually C-z; see section Special
Characters) to send it an interactive stop stop signal
(SIGTSTP), and you want to clean up some internal data buffers
before stopping. You might set this up like this:
#include <signal.h>
/* When a stop signal arrives, set the action back to the default
and then resend the signal after doing cleanup actions. */
void
tstp_handler (int sig)
{
signal (SIGTSTP, SIG_DFL);
/* Do cleanup actions here. */
...
raise (SIGTSTP);
}
/* When the process is continued again, restore the signal handler. */
void
cont_handler (int sig)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
}
/* Enable both handlers during program initialization. */
int
main (void)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
...
}
Portability
note: raise was
invented by the ANSI C committee. Older systems may not support it, so
using kill may be more portable. See
section Signaling
Another Process.
The kill function
can be used to send a signal to another process. In spite of its name, it can be
used for a lot of things other than causing a process to terminate. Some
examples of situations where you might want to send signals between processes
are:
This section assumes that you know a little bit about how processes work. For more information on this subject, see section Processes.
The kill function
is declared in `signal.h‘.
kill function
sends the signal signum to
the process or process group specified by pid. Besides the signals
listed in section Standard
Signals, signum can
also have a value of zero to check the validity of the pid.
The pid specifies the process or process group to receive the signal:
pid > 0pid ==
0pid < -1pid ==
-1A process can send a signal signum to
itself with a call like kill (getpid(), signum).
If kill is used by a process to send a
signal to itself, and the signal is not blocked, then kill delivers at least one signal (which
might be some other pending unblocked signal instead of the signal signum)
to that process before it returns.
The return value from kill is
zero if the signal can be sent successfully. Otherwise, no signal is sent, and
a value of -1 is
returned. If pid specifies
sending a signal to several processes, kill succeeds
if it can send the signal to at least one of them. There‘s no way you can tell
which of the processes got the signal or whether all of them did.
The following errno error
conditions are defined for this function:
EINVALEPERMESCRHkill,
but sends signal signum to
the process group pgid. This function is
provided for compatibility with BSD; using kill to
do this is more portable.As
a simple example of kill, the call kill (getpid (), sig) has the same effect as raise
(sig).
killThere
are restrictions that prevent you from using kill to
send signals to any random process. These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another user. In
typical use, kill is
used to pass signals between parent, child, and sibling processes, and in these
situations you normally do have permission to send signals. The only common
execption is when you run a setuid program in a child process; if the program
changes its real UID as well as its effective UID, you may not have permission
to send a signal. The su program
does this.
Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in section The Persona of a Process.
Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root‘), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID‘s don‘t match, and other implementations might enforce other restrictions.
The SIGCONT signal
is a special case. It can be sent if the sender is part of the same session as
the receiver, regardless of user IDs.
kill for
CommunicationHere is a longer example showing how signals can be used for
interprocess communication. This is what the SIGUSR1 and SIGUSR2 signals
are provided for. Since these signals are fatal by default, the process that is
supposed to receive them must trap them through signal or sigaction.
In
this example, a parent process forks a child process and then waits for the
child to complete its initialization. The child process tells the parent when it
is ready by sending it a SIGUSR1 signal,
using the kill function.
#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
/* When a SIGUSR1 signal arrives, set this variable. */
volatile sig_atomic_t usr_interrupt = 0;
void
synch_signal (int sig)
{
usr_interrupt = 1;
}
/* The child process executes this function. */
void
child_function (void)
{
/* Perform initialization. */
printf ("I‘m here!!! My pid is %d.\n", (int) getpid ());
/* Let parent know you‘re done. */
kill (getppid (), SIGUSR1);
/* Continue with execution. */
puts ("Bye, now....");
exit (0);
}
int
main (void)
{
struct sigaction usr_action;
sigset_t block_mask;
pid_t child_id;
/* Establish the signal handler. */
sigfillset (&block_mask);
usr_action.sa_handler = synch_signal;
usr_action.sa_mask = block_mask;
usr_action.sa_flags = 0;
sigaction (SIGUSR1, &usr_action, NULL);
/* Create the child process. */
child_id = fork ();
if (child_id == 0)
child_function (); /* Does not return. */
/* Busy wait for the child to send a signal. */
while (!usr_interrupt)
;
/* Now continue execution. */
puts ("That‘s all, folks!");
return 0;
}
This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in section Waiting for a Signal.
Blocking
a signal means telling the operating system to hold it and deliver it later.
Generally, a program does not block signals indefinitely--it might as well
ignore them by setting their actions to SIG_IGN. But it is
useful to block signals briefly, to prevent them from interrupting sensitive
operations. For instance:
sigprocmask function to block signals while you
modify global variables that are also modified by the handlers for these
signals.sa_mask in
your sigaction call
to block certain signals while a particular signal handler runs. This way, the
signal handler can run without being interrupted itself by signals.Temporary
blocking of signals with sigprocmask gives
you a way to prevent interrupts during critical parts of your code. If signals
arrive in that part of the program, they are delivered later, after you unblock
them.
One
example where this is useful is for sharing data between a signal handler and
the rest of the program. If the type of the data is not sig_atomic_t (see sectionAtomic
Data Access and Signal Handling), then the signal handler could run when the
rest of the program has only half finished reading or writing the data. This
would lead to confusing consequences.
To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data.
Blocking
signals is also necessary when you want to perform a certain action only if a
signal has not arrived. Suppose that the handler for the signal sets a flag of
type sig_atomic_t;
you would like to test the flag and perform the action if the flag is not set.
This is unreliable. Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will perform the
action even though the signal has arrived.
The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.
All of the signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function.
These facilities are declared in the header file `signal.h‘.
sigset_t data
type is used to represent a signal set. Internally, it may be implemented as
either an integer or structure type.
For portability, use only the functions described in this section to
initialize, change, and retrieve information from sigset_t objects--don‘t try to manipulate
them directly.
There
are two ways to initialize a signal set. You can initially specify it to be
empty with sigemptyset and
then add specified signals individually. Or you can specify it to be full
with sigfillset and
then delete specified signals individually.
You
must always initialize the signal set with one of these two functions before
using it in any other way. Don‘t try to set all the signals explicitly because
thesigset_t object
might include some other information (like a version field) that needs to be
initialized as well. (In addition, it‘s not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)
0.0.sigaddset does
is modify set; it
does not block or unblock any signals.
The return value is 0 on
success and -1 on
failure. The following errno error
condition is defined for this function:
EINVALsigdelset does
is modify set; it
does not block or unblock any signals. The return value and error conditions
are the same as for sigaddset.Finally, there is a function to test what signals are in a signal set:
sigismember function tests whether the
signal signum is a member of the signal set set. It returns 1 if
the signal is in the set, 0 if
not, and -1 if there is an error.
The following errno error
condition is defined for this function:
EINVALThe collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see sectionCreating a Process), it inherits its parent‘s mask. You can block or unblock signals with total flexibility by modifying the signal mask.
The
prototype for the sigprocmask function
is in `signal.h‘.
sigprocmask function is used to examine or
change the calling process‘s signal mask. The how argument determines how the signal
mask is changed, and must be one of the following values:
SIG_BLOCKset---add
them to the existing mask. In other words, the new mask is the union of the
existing mask and set.SIG_UNBLOCKSIG_SETMASKThe last argument, oldset, is used to
return information about the old process signal mask. If you just want to
change the mask without looking at it, pass a null pointer as the oldset argument. Similarly, if you want to
know what‘s in the mask without changing it, pass a null pointer for set (in
this case the howargument is not
significant). The oldset argument
is often used to remember the previous signal mask in order to restore it
later. (Since the signal mask is inherited over fork and exec calls, you can‘t predict what its
contents are when your program starts running.)
If invoking sigprocmask causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before sigprocmask returns. The order in which pending
signals are delivered is not specified, but you can control the order
explicitly by making multiple sigprocmask calls to unblock various signals
one at a time.
The sigprocmask function returns 0 if successful, and -1 to indicate an error. The
following errno error
conditions are defined for this function:
EINVALYou can‘t block the SIGKILL and SIGSTOP signals,
but if the signal set includes these, sigprocmask just ignores them instead of
returning an error status.
Remember, too, that blocking program error signals such as SIGFPE leads to undesirable results for
signals generated by an actual program error (as opposed to signals sent
with raise or kill).
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again. See section Program
Error Signals.
Now
for a simple example. Suppose you establish a handler for SIGALRM signals that sets a flag whenever a
signal arrives, and your main program checks this flag from time to time and
then resets it. You can prevent additional SIGALRM signals
from arriving in the meantime by wrapping the critical part of the code with
calls tosigprocmask, like this:
/* This variable is set by the SIGALRM signal handler. */
volatile sig_atomic_t flag = 0;
int
main (void)
{
sigset_t block_alarm;
...
/* Initialize the signal mask. */
sigemptyset (&block_alarm);
sigaddset (&block_alarm, SIGALRM);
while (1)
{
/* Check if a signal has arrived; if so, reset the flag. */
sigprocmask (SIG_BLOCK, &block_alarm, NULL);
if (flag)
{
actions-if-not-arrived
flag = 0;
}
sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
...
}
}
When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.
When
a handler function is invoked on a signal, that signal is automatically blocked
(in addition to any other signals that are already in the process‘s signal mask)
during the time the handler is running. If you set up a handler for SIGTSTP,
for instance, then the arrival of that signal forces further SIGTSTP signals to wait during the execution
of the handler.
However, by default, other kinds of signals are not blocked; they can arrive during handler execution.
The
reliable way to block other kinds of signals during the execution of the handler
is to use the sa_mask member
of the sigaction structure.
Here is an example:
#include <signal.h>
#include <stddef.h>
void catch_stop ();
void
install_handler (void)
{
struct sigaction setup_action;
sigset_t block_mask;
sigemptyset (&block_mask);
/* Block other terminal-generated signals while handler runs. */
sigaddset (&block_mask, SIGINT);
sigaddset (&block_mask, SIGQUIT);
setup_action.sa_handler = catch_stop;
setup_action.sa_mask = block_mask;
setup_action.sa_flags = 0;
sigaction (SIGTSTP, &setup_action, NULL);
}
This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicity in the handler, you can‘t avoid at least a short interval at the beginning of the handler where they are not yet blocked.
You
cannot remove signals from the process‘s current mask using this mechanism.
However, you can make calls to sigprocmask within
your handler to block or unblock signals as you wish.
In any case, when the handler returns, the system restores the mask that was in place before the handler was entered. If any signals that become unblocked by this restoration are pending, the process will receive those signals immediately, before returning to the code that was interrupted.
You
can find out which signals are pending at any time by calling sigpending.
This function is declared in `signal.h‘.
sigpending function
stores information about pending signals in set. If there is a
pending signal that is blocked from delivery, then that signal is a member of
the returned set. (You can test whether a particular signal is a member of
this set using sigismember; see
section Signal
Sets.)
The return value is 0 if
successful, and -1 on
failure.
Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.
Here is an example.
#include <signal.h>
#include <stddef.h>
sigset_t base_mask, waiting_mask;
sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);
/* Block user interrupts while doing other processing. */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
...
/* After a while, check to see whether any signals are pending. */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) {
/* User has tried to kill the process. */
}
else if (sigismember (&waiting_mask, SIGTSTP)) {
/* User has tried to stop the process. */
}
Remember
that if there is a particular signal pending for your process, additional
signals of that same type that arrive in the meantime might be discarded. For
example, if a SIGINT signal
is pending when another SIGINT signal
arrives, your program will probably only see one of them when you unblock this
signal.
Portability
Note: The sigpending function is new in POSIX.1. Older
systems have no equivalent facility.
Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock". Here is an example:
/* If this flag is nonzero, don‘t handle the signal right away. */
volatile sig_atomic_t signal_pending;
/* This is nonzero if a signal arrived and was not handled. */
volatile sig_atomic_t defer_signal;
void
handler (int signum)
{
if (defer_signal)
signal_pending = signum;
else
... /* "Really" handle the signal. */
}
...
void
update_mumble (int frob)
{
/* Prevent signals from having immediate effect. */
defer_signal++;
/* Now update mumble, without worrying about interruption. */
mumble.a = 1;
mumble.b = hack ();
mumble.c = frob;
/* We have updated mumble. Handle any signal that came in. */
defer_signal--;
if (defer_signal == 0 && signal_pending != 0)
raise (signal_pending);
}
Note
how the particular signal that arrives is stored in signal_pending.
That way, we can handle several types of inconvenient signals with the same
mechanism.
We
increment and decrement defer_signal so
that nested critical sections will work properly; thus, if update_mumble were called with signal_pending already nonzero, signals would be
deferred not only within update_mumble, but also
within the caller. This is also why we do not check signal_pending if defer_signal is
still nonzero.
The
incrementing and decrementing of defer_signal require
more than one instruction; it is possible for a signal to happen in the middle.
But that does not cause any problem. If the signal happens early enough to see
the value from before the increment or decrement, that is equivalent to a signal
which came before the beginning of the increment or decrement, which is a case
that works properly.
It
is absolutely vital to decrement defer_signal before
testing signal_pending, because
this avoids a subtle bug. If we did these things in the other order, like
this,
if (defer_signal == 1 && signal_pending != 0)
raise (signal_pending);
defer_signal--;
then
a signal arriving in between the if statement
and the decrement would be effetively "lost" for an indefinite amount of time.
The handler would merely setdefer_signal, but the program having
already tested this variable, it would not test the variable again.
Bugs like these are called timing errors. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can‘t expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them.
(You
would not be tempted to write the code in this order, given the use of defer_signal as
a counter which must be tested along with signal_pending. After
all, testing for zero is cleaner than testing for one. But if you did not
use defer_signal as
a counter, and gave it values of zero and one only, then either order might seem
equally simple. This is a further advantage of using a counter for defer_signal:
it will reduce the chance you will write the code in the wrong order and create
a subtle bug.)
If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.
pauseThe
simple way to wait until a signal arrives is to call pause.
Please read about its disadvantages, in the following section, before you use
it.
pause function
suspends program execution until a signal arrives whose action is either to
execute a handler function, or to terminate the process.
If the signal causes a handler function to be executed, then pause returns. This is considered an
unsuccessful return (since "successful" behavior would be to suspend the
program forever), so the return value is -1. Even if you
specify that other primitives should resume when a system handler returns (see
sectionPrimitives
Interrupted by Signals), this has no effect on pause;
it always fails when a signal is handled.
The following errno error
conditions are defined for this function:
EINTRIf the signal causes program termination, pause doesn‘t return (obviously).
The pause function is declared in `unistd.h‘.
pauseThe
simplicity of pause can
conceal serious timing errors that can make a program hang mysteriously.
It
is safe to use pause if
the real work of your program is done by the signal handlers themselves, and the
"main program" does nothing but call pause. Each time a
signal is delivered, the handler will do the next batch of work that is to be
done, and then return, so that the main loop of the program can call pause again.
You
can‘t safely use pause to
wait until one more signal arrives, and then resume real work. Even if you
arrange for the signal handler to cooperate by setting a flag, you still can‘t
use pause reliably. Here is an example of this
problem:
/* usr_interrupt is set by the signal handler. */
if (!usr_interrupt)
pause ();
/* Do work once the signal arrives. */
...
This
has a bug: the signal could arrive after the variable usr_interrupt is checked, but before the call
to pause. If no
further signals arrive, the process would never wake up again.
You
can put an upper limit on the excess waiting by using sleep in a loop, instead of using pause. (See section Sleeping,
for more about sleep.) Here is what
this looks like:
/* usr_interrupt is set by the signal handler.
while (!usr_interrupt)
sleep (1);
/* Do work once the signal arrives. */
...
For
some purposes, that is good enough. But with a little more complexity, you can
wait reliably until a particular signal handler is run, using sigsuspend.
sigsuspendThe
clean and reliable way to wait for a signal to arrive is to block it and then
use sigsuspend. By
using sigsuspend in
a loop, you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.
If the process is woken up by deliver of a signal that invokes a handler
function, and the handler function returns, then sigsuspend also returns.
The mask remains set only
as long as sigsuspend is
waiting. The function sigsuspend always
restores the previous signal mask when it returns.
The return value and error conditions are the same as for pause.
With sigsuspend, you can
replace the pause or sleep loop
in the previous section with something completely reliable:
sigset_t mask, oldmask; ... /* Set up the mask of signals to temporarily block. */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); ... /* Wait for a signal to arrive. */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL);
This
last piece of code is a little tricky. The key point to remember here is that
when sigsuspend returns,
it resets the process‘s signal mask to the original value, the value from before
the call to sigsuspend---in this
case, the SIGUSR1 signal
is once again blocked. The second call to sigprocmask is
necessary to explicitly unblock this signal.
One
other point: you may be wondering why the while loop
is necessary at all, since the program is apparently only waiting for one SIGUSR1 signal.
The answer is that the mask passed to sigsuspend permits
the process to be woken up by the delivery of other kinds of signals, as
well--for example, job control signals. If the process is woken up by a signal
that doesn‘t set usr_interrupt, it just
suspends itself again until the "right" kind of signal eventually arrives.
This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines.
A
signal stack is a special area of memory to be used as the execution stack
during signal handlers. It should be fairly large, to avoid any danger that it
will overflow in turn; the macro SIGSTKSZ is
defined to a canonical size for signal stacks. You can use malloc to allocate the space for the stack.
Then call sigaltstack orsigstack to tell the system to use that space
for the signal stack.
You don‘t need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. (Some non-GNU debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.)
There
are two interfaces for telling the system to use a separate signal stack. sigstack is
the older interface, which comes from 4.2 BSD. sigaltstack is the newer interface, and comes
from 4.4 BSD. The sigaltstack interface
has the advantage that it does not require your program to know which direction
the stack grows, which depends on the specific machine and operating system.
void *ss_spsize_t ss_sizeSIGSTKSZMINSIGSTKSZSIGSTKSZ for ss_size is sufficient. But if you know
how much stack space your program‘s signal handlers will need, you may
want to use a different size. In this case, you should allocate MINSIGSTKSZ additional bytes for the signal
stack and increase ss_size accordinly.int ss_flagssigaltstack function specifies an alternate
stack for use during signal handling. When a signal is received by the process
and its action indicates that the signal stack is used, the system arranges a
switch to the currently installed signal stack while the handler for that
signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on
success and -1 on
failure. If sigaltstack fails, it sets errno to one of these values:
EINVALENOMEMMINSIGSTKSZ.Here
is the older sigstack interface.
You should use sigaltstack instead
on systems that have it.
void *ss_spint ss_onstacksigstack function
specifies an alternate stack for use during signal handling. When a signal is
received by the process and its action indicates that the signal stack is
used, the system arranges a switch to the currently installed signal stack
while the handler for that signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on
success and -1 on
failure.
This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.
There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are:
int bit
mask, rather than as a sigset_t object.The BSD facilities are declared in `signal.h‘.
struct
sigaction (see
section Advanced
Signal Handling); it is used to specify signal actions to the sigvec function. It contains the following
members:
sighandler_t sv_handlerint sv_maskint sv_flagssv_onstack.These
symbolic constants can be used to provide values for the sv_flags field of a sigvec structure. This field is a bit mask
value, so you bitwise-OR the flags of interest to you together.
sv_flags field of a sigvec structure, it means to use the
signal stack when delivering the signal.sv_flags field of a sigvec structure, it means that system
calls interrupted by this kind of signal should not be restarted if the
handler returns; instead, the system calls should return with a EINTR error status. See section Primitives
Interrupted by Signals.sv_flags field of a sigvec structure, it means to reset the
action for the signal back to SIG_DFL when
the signal is received.sigaction (see section Advanced
Signal Handling); it installs the action action for the signal signum,
returning information about the previous action in effect for that signal
in old-action.EINTR. See
section Primitives
Interrupted by Signals.sigmask together to specify more than one
signal. For example,
(sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU))
specifies a mask that includes all the job-control stop signals.
sigprocmask (see section Process
Signal Mask) with a how argument
of SIG_BLOCK: it
adds the signals specified by mask to
the calling process‘s set of blocked signals. The return value is the previous
set of blocked signals.sigprocmask (see section Process
Signal Mask) with a how argument
of SIG_SETMASK:
it sets the calling process‘s signal mask to mask.
The return value is the previous set of blocked signals.sigsuspend (see section Waiting
for a Signal): it sets the calling process‘s signal mask to mask,
and waits for a signal to arrive. On return the previous set of blocked
signals is restored.原文：http://www.cnblogs.com/davidwang456/p/3519986.html