popen.c

该代码为unix环境高级编程的源代码

#include	<sys/wait.h>
#include	<errno.h>
#include	<fcntl.h>
#include	"ourhdr.h"

static pid_t	*childpid = NULL;
						/* ptr to array allocated at run-time */
static int		maxfd;	/* from our open_max(), {Prog openmax} */

#define	SHELL	"/bin/sh"

FILE *
popen(const char *cmdstring, const char *type)
{
	int		i, pfd[2];
	pid_t	pid;
	FILE	*fp;

			/* only allow "r" or "w" */
	if ((type[0] != 'r' && type[0] != 'w') || type[1] != 0) {
		errno = EINVAL;		/* required by POSIX.2 */
		return(NULL);
	}

	if (childpid == NULL) {		/* first time through */
				/* allocate zeroed out array for child pids */
		maxfd = open_max();
		if ( (childpid = calloc(maxfd, sizeof(pid_t))) == NULL)
			return(NULL);
	}

	if (pipe(pfd) < 0)
		return(NULL);	/* errno set by pipe() */

	if ( (pid = fork()) < 0)
		return(NULL);	/* errno set by fork() */
	else if (pid == 0) {							/* child */
		if (*type == 'r') {
			close(pfd[0]);
			if (pfd[1] != STDOUT_FILENO) {
				dup2(pfd[1], STDOUT_FILENO);
				close(pfd[1]);
			}
		} else {
			close(pfd[1]);
			if (pfd[0] != STDIN_FILENO) {
				dup2(pfd[0], STDIN_FILENO);
				close(pfd[0]);
			}
		}
			/* close all descriptors in childpid[] */
		for (i = 0; i < maxfd; i++)
			if (childpid[i] > 0)
				close(i);

		execl(SHELL, "sh", "-c", cmdstring, (char *) 0);
		_exit(127);
	}
								/* parent */
	if (*type == 'r') {
		close(pfd[1]);
		if ( (fp = fdopen(pfd[0], type)) == NULL)
			return(NULL);
	} else {
		close(pfd[0]);
		if ( (fp = fdopen(pfd[1], type)) == NULL)
			return(NULL);
	}
	childpid[fileno(fp)] = pid;	/* remember child pid for this fd */
	return(fp);
}

int
pclose(FILE *fp)
{
	int		fd, stat;
	pid_t	pid;

	if (childpid == NULL)
		return(-1);		/* popen() has never been called */

	fd = fileno(fp);
	if ( (pid = childpid[fd]) == 0)
		return(-1);		/* fp wasn't opened by popen() */

	childpid[fd] = 0;
	if (fclose(fp) == EOF)
		return(-1);

	while (waitpid(pid, &stat, 0) < 0)
		if (errno != EINTR)
			return(-1);	/* error other than EINTR from waitpid() */

	return(stat);	/* return child's termination status */
}