Linux中的system函数的实现和解释

#include	<sys/wait.h>
#include	<errno.h>
#include	<signal.h>
#include	<unistd.h>

int
system(const char *cmdstring)	/* with appropriate signal handling */
{
pid_t				pid;
int					status;
struct sigaction	ignore, saveintr, savequit;
sigset_t			chldmask, savemask;

if (cmdstring == NULL)
return(1);		/* always a command processor with UNIX */

ignore.sa_handler = SIG_IGN;	/* ignore SIGINT and SIGQUIT */
sigemptyset(&ignore.sa_mask);
ignore.sa_flags = 0;
if (sigaction(SIGINT, &ignore, &saveintr) < 0)
return(-1);
if (sigaction(SIGQUIT, &ignore, &savequit) < 0)
return(-1);
sigemptyset(&chldmask);			/* now block SIGCHLD */
sigaddset(&chldmask, SIGCHLD);
if (sigprocmask(SIG_BLOCK, &chldmask, &savemask) < 0)
return(-1);

if ((pid = fork()) < 0) {
status = -1;	/* probably out of processes */
} else if (pid == 0) {			/* child */
/* restore previous signal actions & reset signal mask */
sigaction(SIGINT, &saveintr, NULL);
sigaction(SIGQUIT, &savequit, NULL);
sigprocmask(SIG_SETMASK, &savemask, NULL);

execl("/bin/sh", "sh", "-c", cmdstring, (char *)0);
_exit(127);		/* exec error */
} else {						/* parent */
while (waitpid(pid, &status, 0) < 0)
if (errno != EINTR) {
status = -1; /* error other than EINTR from waitpid() */
break;
}
}

/* restore previous signal actions & reset signal mask */
if (sigaction(SIGINT, &saveintr, NULL) < 0)
return(-1);
if (sigaction(SIGQUIT, &savequit, NULL) < 0)
return(-1);
if (sigprocmask(SIG_SETMASK, &savemask, NULL) < 0)
return(-1);

return(status);
}