source: azure_iot_hub/trunk/musl-1.1.18/src/stdio/popen.c@ 389

#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <spawn.h>
#include "stdio_impl.h"
#include "syscall.h"

extern char **__environ;

FILE *popen(const char *cmd, const char *mode)
{
        int p[2], op, e;
        pid_t pid;
        FILE *f;
        posix_spawn_file_actions_t fa;

        if (*mode == 'r') {
                op = 0;
        } else if (*mode == 'w') {
                op = 1;
        } else {
                errno = EINVAL;
                return 0;
        }
        
        if (pipe2(p, O_CLOEXEC)) return NULL;
        f = fdopen(p[op], mode);
        if (!f) {
                __syscall(SYS_close, p[0]);
                __syscall(SYS_close, p[1]);
                return NULL;
        }
        FLOCK(f);

        /* If the child's end of the pipe happens to already be on the final
         * fd number to which it will be assigned (either 0 or 1), it must
         * be moved to a different fd. Otherwise, there is no safe way to
         * remove the close-on-exec flag in the child without also creating
         * a file descriptor leak race condition in the parent. */
        if (p[1-op] == 1-op) {
                int tmp = fcntl(1-op, F_DUPFD_CLOEXEC, 0);
                if (tmp < 0) {
                        e = errno;
                        goto fail;
                }
                __syscall(SYS_close, p[1-op]);
                p[1-op] = tmp;
        }

        e = ENOMEM;
        if (!posix_spawn_file_actions_init(&fa)) {
                if (!posix_spawn_file_actions_adddup2(&fa, p[1-op], 1-op)) {
                        if (!(e = posix_spawn(&pid, "/bin/sh", &fa, 0,
                            (char *[]){ "sh", "-c", (char *)cmd, 0 }, __environ))) {
                                posix_spawn_file_actions_destroy(&fa);
                                f->pipe_pid = pid;
                                if (!strchr(mode, 'e'))
                                        fcntl(p[op], F_SETFD, 0);
                                __syscall(SYS_close, p[1-op]);
                                FUNLOCK(f);
                                return f;
                        }
                }
                posix_spawn_file_actions_destroy(&fa);
        }
fail:
        fclose(f);
        __syscall(SYS_close, p[1-op]);

        errno = e;
        return 0;
}