/*  
 *  popen.c     Written by W. Richard Stevens
 *  write 20180623 email: aixiao@aixiao.me
 */

#include    <stdio.h>
#include    <unistd.h>
#include    <stdlib.h>
#include    <sys/wait.h>
#include    <errno.h>
#include    <fcntl.h>
#include    <sys/resource.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 */
#if 0
        maxfd = open_max();
#else
        /* OPEN_MAX is canceled after linux-2.6.24, and become a part of RLIMIT_NOFILE.
           Execute shell 'ulimit -a' to get more information */
        struct rlimit limit;
        if (getrlimit(RLIMIT_NOFILE, &limit) == -1) {
            perror("getrlimit fail");
            return (NULL);
        }
        maxfd = limit.rlim_cur;
        //print("rlimit = %d.\n", maxfd);  
#endif
        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 */
}