#include "apue.h"#include <errno.h>#include <fcntl.h>#include <sys/wait.h>/* * Pointer to array allocated at run-time. */static pid_t	*childpid = NULL;/* * From our open_max(), {Prog openmax}. */static int		maxfd;FILE *popen(const char *cmdstring, const char *type){	int		i;	int		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 */		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("/bin/sh", "sh", "-c", cmdstring, (char *)0);		_exit(127);	}	/* parent continues... */	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);}intpclose(FILE *fp){	int		fd, stat;	pid_t	pid;	if (childpid == NULL) {		errno = EINVAL;		return(-1);		/* popen() has never been called */	}	fd = fileno(fp);	if ((pid = childpid[fd]) == 0) {		errno = EINVAL;		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 */}