C++标准库中的std::endl究竟做了什么？

时间：2019-11-13 16:39:25 阅读：138 评论：0 收藏：0 [点我收藏+]

先抓出std::endl的源代码：

/**

* @file ostream

* @brief Write a newline and flush the stream.

* This manipulator is often mistakenly used when a simple newline is

* desired, leading to poor buffering performance. See

* http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt11ch25s02.html

* for more on this subject.

template<typename _CharT, typename _Traits>

inline basic_ostream<_CharT, _Traits>&

endl(basic_ostream<_CharT, _Traits>& __os)

{

return flush(__os.put(__os.widen(‘\n‘)));

}

可以看到endl实际是名字空间std中的一个全局内联函数（记住std::endl是一个函数），它做了两件事：

1) 输出一个换行符（为何要输出一个换行符，如果不输出会怎么样？）；

2) 调用flush。

继续看flush这个函数（也是std名字空间内的全局内联函数）：

/**

* @brief Flushes the output stream.

* This manipulator simply calls the stream‘s @c flush() member function.

template<typename _CharT, typename _Traits>

inline basic_ostream<_CharT, _Traits>&

flush(basic_ostream<_CharT, _Traits>& __os)

{

return __os.flush(); // 注意这里的os不是操作系统的意思，而是类basic_ostream的缩写

}

进一步查看std::basic_ostream::flush的代码：

/**

* @file ostream.tcc

template<typename _CharT, typename _Traits>

basic_ostream<_CharT, _Traits>&

basic_ostream<_CharT, _Traits>::flush()

{

// _GLIBCXX_RESOLVE_LIB_DEFECTS

// DR 60. What is a formatted input function?

// basic_ostream::flush() is *not* an unformatted output function.

ios_base::iostate __err = ios_base::goodbit;

__try

{

// 刷新发生在pubsync，底层调用的是LIBC库函数fflush

if (this->rdbuf() && this->rdbuf()->pubsync() == -1)

__err |= ios_base::badbit;

}

__catch(__cxxabiv1::__forced_unwind&)

{

this->_M_setstate(ios_base::badbit);

__throw_exception_again;

}

__catch(...)

{

this->_M_setstate(ios_base::badbit);

}

if (__err)

this->setstate(__err);

return *this;

}

下段小代码，可以看到两个帮助释疑的调用栈，：

$ cat xxx.cpp

#include <iostream>

int main() {

std::cout << std::endl;

return 0;

}

写换符符：

#0 0x00007ffff72e0840 in write () from /lib64/libc.so.6

#1 0x00007ffff726cfb3 in _IO_new_file_write () from /lib64/libc.so.6

#2 0x00007ffff726e41c in __GI__IO_do_write () from /lib64/libc.so.6

#3 0x00007ffff726e7f3 in __GI__IO_file_overflow () from /lib64/libc.so.6

#4 0x00007ffff726ac49 in putc () from /lib64/libc.so.6

#5 0x00007ffff7b694c6 in std::ostream::put(char) () from /lib64/libstdc++.so.6

#6 0x00007ffff7b69712 in std::basic_ostream<char, std::char_traits<char> >& std::endl<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&) ()

from /lib64/libstdc++.so.6

#7 0x0000000000400803 in main () at xxx.cpp:3

刷新流：

#0 0x00007ffff7262030 in fflush () from /lib64/libc.so.6

#1 0x00007ffff7b68d5e in std::ostream::flush() () from /lib64/libstdc++.so.6

#2 0x0000000000400803 in main () at xxx.cpp:3

实际上，还可以看到更多，如全局对象std::cout、std::cerr和std::clog等的析构：

#0 0x00007ffff7262030 in fflush () from /lib64/libc.so.6

#1 0x00007ffff7b68d5e in std::ostream::flush() () from /lib64/libstdc++.so.6

#2 0x00007ffff7b41bc8 in std::ios_base::Init::~Init() () from /lib64/libstdc++.so.6

#3 0x00007ffff722fa69 in __run_exit_handlers () from /lib64/libc.so.6

#4 0x00007ffff722fab5 in exit () from /lib64/libc.so.6

#5 0x00007ffff7218c0c in __libc_start_main () from /lib64/libc.so.6

#6 0x0000000000400729 in _start ()

Init析构的同时析构cout等：

/**

* @file ios_init.cc

ios_base::Init::~Init()

{

// Be race-detector-friendly. For more info see bits/c++config.

_GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_S_refcount);

if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, -1) == 2)

{

_GLIBCXX_SYNCHRONIZATION_HAPPENS_AFTER(&_S_refcount);

// Catch any exceptions thrown by basic_ostream::flush()

__try

{

// Flush standard output streams as required by 27.4.2.1.6

cout.flush();

cerr.flush();

clog.flush();

#ifdef _GLIBCXX_USE_WCHAR_T

wcout.flush();

wcerr.flush();

wclog.flush();

#endif

}

__catch(...)

{

}

文件ios_init.cc其它相关函数源码：

1) 类std::ios_base::Init的构造函数

ios_base::Init::Init()

{

if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0) // 防止重复初始化

{

// Standard streams default to synced with "C" operations.

_S_synced_with_stdio = true;

new (&buf_cout_sync) stdio_sync_filebuf<char>(stdout);

new (&buf_cin_sync) stdio_sync_filebuf<char>(stdin);

new (&buf_cerr_sync) stdio_sync_filebuf<char>(stderr);

// The standard streams are constructed once only and never destroyed.

new (&cout) ostream(&buf_cout_sync);

new (&cin) istream(&buf_cin_sync);

new (&cerr) ostream(&buf_cerr_sync);

new (&clog) ostream(&buf_cerr_sync);

cin.tie(&cout);

cerr.setf(ios_base::unitbuf);

// _GLIBCXX_RESOLVE_LIB_DEFECTS

// 455. cerr::tie() and wcerr::tie() are overspecified.

cerr.tie(&cout);

#ifdef _GLIBCXX_USE_WCHAR_T

new (&buf_wcout_sync) stdio_sync_filebuf<wchar_t>(stdout);

new (&buf_wcin_sync) stdio_sync_filebuf<wchar_t>(stdin);

new (&buf_wcerr_sync) stdio_sync_filebuf<wchar_t>(stderr);

new (&wcout) wostream(&buf_wcout_sync);

new (&wcin) wistream(&buf_wcin_sync);

new (&wcerr) wostream(&buf_wcerr_sync);

new (&wclog) wostream(&buf_wcerr_sync);

wcin.tie(&wcout);

wcerr.setf(ios_base::unitbuf);

wcerr.tie(&wcout);

#endif

// NB: Have to set refcount above one, so that standard

// streams are not re-initialized with uses of ios_base::Init

// besides <iostream> static object, ie just using <ios> with

// ios_base::Init objects.

__gnu_cxx::__atomic_add_dispatch(&_S_refcount, 1);

}

2) 全局函数ios_base::sync_with_stdio

bool

ios_base::sync_with_stdio(bool __sync)

{

// _GLIBCXX_RESOLVE_LIB_DEFECTS

// 49. Underspecification of ios_base::sync_with_stdio

bool __ret = ios_base::Init::_S_synced_with_stdio;

// Turn off sync with C FILE* for cin, cout, cerr, clog iff

// currently synchronized.

if (!__sync && __ret)

{

// Make sure the standard streams are constructed.

ios_base::Init __init;

ios_base::Init::_S_synced_with_stdio = __sync;

// Explicitly call dtors to free any memory that is

// dynamically allocated by filebuf ctor or member functions,

// but don‘t deallocate all memory by calling operator delete.

buf_cout_sync.~stdio_sync_filebuf<char>();

buf_cin_sync.~stdio_sync_filebuf<char>();

buf_cerr_sync.~stdio_sync_filebuf<char>();

#ifdef _GLIBCXX_USE_WCHAR_T

buf_wcout_sync.~stdio_sync_filebuf<wchar_t>();

buf_wcin_sync.~stdio_sync_filebuf<wchar_t>();

buf_wcerr_sync.~stdio_sync_filebuf<wchar_t>();

#endif

// Create stream buffers for the standard streams and use

// those buffers without destroying and recreating the

// streams.

new (&buf_cout) stdio_filebuf<char>(stdout, ios_base::out);

new (&buf_cin) stdio_filebuf<char>(stdin, ios_base::in);

new (&buf_cerr) stdio_filebuf<char>(stderr, ios_base::out);

cout.rdbuf(&buf_cout);

cin.rdbuf(&buf_cin);

cerr.rdbuf(&buf_cerr);

clog.rdbuf(&buf_cerr);

#ifdef _GLIBCXX_USE_WCHAR_T

new (&buf_wcout) stdio_filebuf<wchar_t>(stdout, ios_base::out);

new (&buf_wcin) stdio_filebuf<wchar_t>(stdin, ios_base::in);

new (&buf_wcerr) stdio_filebuf<wchar_t>(stderr, ios_base::out);

wcout.rdbuf(&buf_wcout);

wcin.rdbuf(&buf_wcin);

wcerr.rdbuf(&buf_wcerr);

wclog.rdbuf(&buf_wcerr);

#endif

}

return __ret;

}

前面提到的endl为何要输出一个换行符，这是因为fflush只是将数据刷到标准输出，标准输出本身也是有缓存的，而这个换行符的作用是让数据sync，因为标准输出是字符型设备，对应的是行缓存（_IOLBF line buffered）。

是否可将标准输出设置为全缓存（_IOFBF fully buffered）了？答案是不可以，因为标准输出是字符设备（Character Device，只能顺序读取），并不是块设备（Block Device，支持随机存取）。因此下列代码即使已将标准输出设置为全缓存，但仍会不缓存立即输出，因为输出的字符串最后跟了换行符：

$ cat eee.cpp

#include <stdio.h>

int main(void) {

char buf[BUFSIZ];

setvbuf(stdin, buf, _IOFBF, BUFSIZ);

printf("Hello, world!\n");