[英]How does parent process valid the sent signal by child process
基於此http://man7.org/tlpi/code/online/dist/procexec/fork_sig_sync.c.ZFC35FDC70D5FC69D269883A822C7A53
/* 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);
}
}
問題
當父進程調用sigsuspend時,為什么它不驗證發送的信號是 SYNC_SIG?
http://pubs.opengroup.org/onlinepubs/7908799/xsh/sigsuspend.html
該代碼假定程序運行時不會出現其他信號,因此它不會費心檢查它是否收到了正確的信號。 這意味着如果出現任何其他信號,程序可能會失敗。
添加
有多種方法可以檢查是否發送了正確的信號。 您可以讓處理程序設置一些sig_atomic_t
全局變量,並在 sigsuspend 返回后檢查其設置(如果不是,則循環以重復 sigsuspend 調用)。 或者您可以設置傳遞給 sigsuspend 的掩碼以阻止除SYNC_SIG
的所有內容。 如果其他一些進程向您的進程發送虛假的 SIGUSR1,則這些都不起作用,但只有當有人故意試圖破壞您的程序時才會發生這種情況。
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