让父进程设置管道更为常见。父进程保持写端打开（pipefd[1]），关闭读端（pipefd[0]）。子进程关闭写端（pipefd[1]），并设置读取结束（pipefd[1]）非阻塞。
这样子进程就可以使用read(pipefd[0], buffer, 1)检查父进程是否仍然存在。如果父进程仍在运行，它将返回-1 with errno == EAGAIN (or errno == EINTR).
现在，在Linux中，子进程还可以设置读取结束异步，在这种情况下，它将发送一个信号（SIGIO默认情况下）当父进程退出时：
fcntl(pipefd[0], F_SETSIG, desired_signal);
fcntl(pipefd[0], F_SETOWN, getpid());
fcntl(pipefd[0], F_SETFL, O_NONBLOCK | O_ASYNC);
使用 siginfo 处理程序desired_signal. If info->si_code == POLL_IN && info->si_fd == pipefd[0]，父进程要么退出，要么向管道写入内容。因为read()异步信号安全，并且管道是非阻塞的，您可以使用read(pipefd[0], &buffer, sizeof buffer)在信号处理程序中，父级是否写入了某些内容，或者父级是否退出（关闭管道）。在后一种情况下，read()将返回0.
据我所知，这种方法没有竞争条件（如果您使用实时信号，则信号不会因为用户发送的信号已经挂起而丢失），尽管它非常特定于 Linux。设置信号处理程序后，在子进程生命周期的任何时刻，子进程始终可以显式检查父进程是否仍然存在，而不会影响信号生成。
因此，用伪代码回顾一下：
Construct pipe
Fork child process
Child process:
Close write end of pipe
Install pipe signal handler (say, SIGRTMIN+0)
Set read end of pipe to generate pipe signal (F_SETSIG)
Set own PID as read end owner (F_SETOWN)
Set read end of pipe nonblocking and async (F_SETFL, O_NONBLOCK | O_ASYNC)
If read(pipefd[0], buffer, sizeof buffer) == 0,
the parent process has already exited.
Continue with normal work.
Child process pipe signal handler:
If siginfo->si_code == POLL_IN and siginfo->si_fd == pipefd[0],
parent process has exited.
To immediately die, use e.g. raise(SIGKILL).
Parent process:
Close read end of pipe
Continue with normal work.
我不指望你相信我的话。
下面是一个粗略的示例程序，您可以使用它自己检查此行为。它很长，但只是因为我希望很容易看到运行时发生的情况。要在普通程序中实现这一点，您只需要几十行代码。示例.c:
#define _GNU_SOURCE
#define _POSIX_C_SOURCE 200809L
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>
#include <signal.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
static volatile sig_atomic_t done = 0;
static void handle_done(int signum)
{
if (!done)
done = signum;
}
static int install_done(const int signum)
{
struct sigaction act;
memset(&act, 0, sizeof act);
sigemptyset(&act.sa_mask);
act.sa_handler = handle_done;
act.sa_flags = 0;
if (sigaction(signum, &act, NULL) == -1)
return errno;
return 0;
}
static int deathfd = -1;
static void death(int signum, siginfo_t *info, void *context)
{
if (info->si_code == POLL_IN && info->si_fd == deathfd)
raise(SIGTERM);
}
static int install_death(const int signum)
{
struct sigaction act;
memset(&act, 0, sizeof act);
sigemptyset(&act.sa_mask);
act.sa_sigaction = death;
act.sa_flags = SA_SIGINFO;
if (sigaction(signum, &act, NULL) == -1)
return errno;
return 0;
}
int main(void)
{
pid_t child, p;
int pipefd[2], status;
char buffer[8];
if (install_done(SIGINT)) {
fprintf(stderr, "Cannot set SIGINT handler: %s.\n", strerror(errno));
return EXIT_FAILURE;
}
if (pipe(pipefd) == -1) {
fprintf(stderr, "Cannot create control pipe: %s.\n", strerror(errno));
return EXIT_FAILURE;
}
child = fork();
if (child == (pid_t)-1) {
fprintf(stderr, "Cannot fork child process: %s.\n", strerror(errno));
return EXIT_FAILURE;
}
if (!child) {
/*
* Child process.
*/
/* Close write end of pipe. */
deathfd = pipefd[0];
close(pipefd[1]);
/* Set a SIGHUP signal handler. */
if (install_death(SIGHUP)) {
fprintf(stderr, "Child process: cannot set SIGHUP handler: %s.\n", strerror(errno));
return EXIT_FAILURE;
}
/* Set SIGTERM signal handler. */
if (install_done(SIGTERM)) {
fprintf(stderr, "Child process: cannot set SIGTERM handler: %s.\n", strerror(errno));
return EXIT_FAILURE;
}
/* We want a SIGHUP instead of SIGIO. */
fcntl(deathfd, F_SETSIG, SIGHUP);
/* We want the SIGHUP delivered when deathfd closes. */
fcntl(deathfd, F_SETOWN, getpid());
/* Make the deathfd (read end of pipe) nonblocking and async. */
fcntl(deathfd, F_SETFL, O_NONBLOCK | O_ASYNC);
/* Check if the parent process is dead. */
if (read(deathfd, buffer, sizeof buffer) == 0) {
printf("Child process (%ld): Parent process is already dead.\n", (long)getpid());
return EXIT_FAILURE;
}
while (1) {
status = __atomic_fetch_and(&done, 0, __ATOMIC_SEQ_CST);
if (status == SIGINT)
printf("Child process (%ld): SIGINT caught and ignored.\n", (long)getpid());
else
if (status)
break;
printf("Child process (%ld): Tick.\n", (long)getpid());
fflush(stdout);
sleep(1);
status = __atomic_fetch_and(&done, 0, __ATOMIC_SEQ_CST);
if (status == SIGINT)
printf("Child process (%ld): SIGINT caught and ignored.\n", (long)getpid());
else
if (status)
break;
printf("Child process (%ld): Tock.\n", (long)getpid());
fflush(stdout);
sleep(1);
}
printf("Child process (%ld): Exited due to %s.\n", (long)getpid(),
(status == SIGINT) ? "SIGINT" :
(status == SIGHUP) ? "SIGHUP" :
(status == SIGTERM) ? "SIGTERM" : "Unknown signal.\n");
fflush(stdout);
return EXIT_SUCCESS;
}
/*
* Parent process.
*/
/* Close read end of pipe. */
close(pipefd[0]);
while (!done) {
fprintf(stderr, "Parent process (%ld): Tick.\n", (long)getpid());
fflush(stderr);
sleep(1);
fprintf(stderr, "Parent process (%ld): Tock.\n", (long)getpid());
fflush(stderr);
sleep(1);
/* Try reaping the child process. */
p = waitpid(child, &status, WNOHANG);
if (p == child || (p == (pid_t)-1 && errno == ECHILD)) {
if (p == child && WIFSIGNALED(status))
fprintf(stderr, "Child process died from %s. Parent will now exit, too.\n",
(WTERMSIG(status) == SIGINT) ? "SIGINT" :
(WTERMSIG(status) == SIGHUP) ? "SIGHUP" :
(WTERMSIG(status) == SIGTERM) ? "SIGTERM" : "an unknown signal");
else
fprintf(stderr, "Child process has exited, so the parent will too.\n");
fflush(stderr);
break;
}
}
if (done) {
fprintf(stderr, "Parent process (%ld): Exited due to %s.\n", (long)getpid(),
(done == SIGINT) ? "SIGINT" :
(done == SIGHUP) ? "SIGHUP" : "Unknown signal.\n");
fflush(stderr);
}
/* Never reached! */
return EXIT_SUCCESS;
}
使用例如编译并运行上面的内容
gcc -Wall -O2 example.c -o example
./example
The parent process will print to standard output, and the child process to standard error. The parent process will exit if you press Ctrl+C; the child process will ignore that signal. The child process uses SIGHUP instead of SIGIO (although a realtime signal, say SIGRTMIN+0, would be safer); if generated by the parent process exiting, the SIGHUP signal handler will raise SIGTERM in the child.
为了使终止原因易于看到，孩子抓住SIGTERM，并退出下一次迭代（一秒钟后）。如果需要，处理程序可以使用例如raise(SIGKILL)立即终止自身。
父进程和子进程都会显示其进程 ID，因此您可以轻松地发送SIGINT/SIGHUP/SIGTERM来自另一个终端窗口的信号。 （子进程忽略SIGINT and SIGHUP从进程外部发送。）
