在init进程的main函数中有调用sigchld_handler_init() 设置SIGCHLD signal。
在子进程比如收到signal时，init进程会收到通知，在SIGCHLD_handler()中write；然后在handle_signal()中read；然后会调用ReapOneProcess()；然后调用这个service对象的reap()
void sigchld_handler_init() {
// Create a signalling mechanism for SIGCHLD.
int s[2];
if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0, s) == -1) {
PLOG(FATAL) << "socketpair failed in sigchld_handler_init";
}
signal_write_fd = s[0];
signal_read_fd = s[1];
// Write to signal_write_fd if we catch SIGCHLD.
struct sigaction act;
memset(&act, 0, sizeof(act));
act.sa_handler = SIGCHLD_handler;
act.sa_flags = SA_NOCLDSTOP;
sigaction(SIGCHLD, &act, 0);
ReapAnyOutstandingChildren();
register_epoll_handler(signal_read_fd, handle_signal);
}
在Reap()中，以media service为例，它应该是没有SVC_ONESHOT flag的，所以会调Service::KillProcessGroup()，这个函数会发signal SIGKILL(9)给media进程以杀掉它。
然后调用onrestart_.ExecuteAllCommands()
void Service::Reap(const siginfo_t& siginfo) {
if (!(flags_ & SVC_ONESHOT) || (flags_ & SVC_RESTART)) {
KillProcessGroup(SIGKILL);
}
// Remove any descriptor resources we may have created.
std::for_each(descriptors_.begin(), descriptors_.end(),
std::bind(&DescriptorInfo::Clean, std::placeholders::_1));
for (const auto& f : reap_callbacks_) {
f(siginfo);
}
if (flags_ & SVC_EXEC) UnSetExec();
if (flags_ & SVC_TEMPORARY) return;
pid_ = 0;
flags_ &= (~SVC_RUNNING);
start_order_ = 0;
// Oneshot processes go into the disabled state on exit,
// except when manually restarted.
if ((flags_ & SVC_ONESHOT) && !(flags_ & SVC_RESTART)) {
flags_ |= SVC_DISABLED;
}
// Disabled and reset processes do not get restarted automatically.
if (flags_ & (SVC_DISABLED | SVC_RESET)) {
NotifyStateChange("stopped");
return;
}
// If we crash > 4 times in 4 minutes, reboot into recovery.
boot_clock::time_point now = boot_clock::now();
if ((flags_ & SVC_CRITICAL) && !(flags_ & SVC_RESTART)) {
if (now < time_crashed_ + 4min) {
if (++crash_count_ > 4) {
LOG(FATAL) << "critical process ‘" << name_ << "‘ exited 4 times in 4 minutes";
}
} else {
time_crashed_ = now;
crash_count_ = 1;
}
}
flags_ &= (~SVC_RESTART);
flags_ |= SVC_RESTARTING;
// Execute all onrestart commands for this service.
onrestart_.ExecuteAllCommands();
NotifyStateChange("restarting");
return;
}
这个onrestart_.ExecuteAllCommands()对于media进程来说，它没有添加command（如下面的mediaserver.rc），所以不会做什么事情。
frameworks/av/media/mediaserver/mediaserver.rc
service media /system/bin/mediaserver
class main
user media
group audio camera inet net_bt net_bt_admin net_bw_acct drmrpc mediadrm
ioprio rt 4
writepid /dev/cpuset/foreground/tasks /dev/stune/foreground/tasks
原文：https://www.cnblogs.com/aspirs/p/11407611.html