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);

}