基于此http://man7.org/tlpi/code/online/dist/procexec/fork_sig_sync.c.html
/* fork_sig_sync.c
Demonstrate how signals can be used to synchronize the actions
of a parent and child process.
*/
#include <signal.h>
#include "curr_time.h" /* Declaration of currTime() */
#include "tlpi_hdr.h"
#define SYNC_SIG SIGUSR1 /* Synchronization signal */
static void /* Signal handler - does nothing but return */
handler(int sig)
{
}
int
main(int argc, char *argv[])
{
pid_t childPid;
sigset_t blockMask, origMask, emptyMask;
struct sigaction sa;
setbuf(stdout, NULL); /* Disable buffering of stdout */
sigemptyset(&blockMask);
sigaddset(&blockMask, SYNC_SIG); /* Block signal */
if (sigprocmask(SIG_BLOCK, &blockMask, &origMask) == -1)
errExit("sigprocmask");
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
sa.sa_handler = handler;
if (sigaction(SYNC_SIG, &sa, NULL) == -1)
errExit("sigaction");
switch (childPid = fork()) {
case -1:
errExit("fork");
case 0: /* Child */
/* Child does some required action here... */
printf("[%s %ld] Child started - doing some work\n",
currTime("%T"), (long) getpid());
sleep(2); /* Simulate time spent doing some work */
/* And then signals parent that it's done */
printf("[%s %ld] Child about to signal parent\n",
currTime("%T"), (long) getpid());
if (kill(getppid(), SYNC_SIG) == -1)
errExit("kill");
/* Now child can do other things... */
_exit(EXIT_SUCCESS);
default: /* Parent */
/* Parent may do some work here, and then waits for child to
complete the required action */
printf("[%s %ld] Parent about to wait for signal\n",
currTime("%T"), (long) getpid());
sigemptyset(&emptyMask);
if (sigsuspend(&emptyMask) == -1 && errno != EINTR) // <<<<< Question
errExit("sigsuspend");
printf("[%s %ld] Parent got signal\n", currTime("%T"), (long) getpid());
/* If required, return signal mask to its original state */
if (sigprocmask(SIG_SETMASK, &origMask, NULL) == -1)
errExit("sigprocmask");
/* Parent carries on to do other things... */
exit(EXIT_SUCCESS);
}
}
问题
当父进程调用 sigsuspend 时,为什么它不验证发送的信号是SYNC_SIG?
http://pubs.opengroup.org/onlinepubs/7908799/xsh/sigsuspend.html
答案 0 :(得分:0)
代码假定程序运行时不会发生其他信号,所以它不会检查它是否收到正确的信号。这意味着如果发生任何其他信号,程序可能会失败。
添加强>
有很多方法可以检查是否发送了正确的信号。您可以让处理程序设置一些sig_atomic_t
全局变量,并在sigsuspend之后检查其设置是否返回(如果不是则循环重复sigsuspend调用)。或者您可以将传递给sigsuspend的掩码设置为阻止除SYNC_SIG
之外的所有内容。如果其他进程向您的进程发送虚假的SIGUSR1,这些都不会起作用,但只有在有人故意试图破坏您的程序时才会发生这种情况。