使用posix而不是fork/execv运行bash

问题描述

我有一个CLI，其中一个命令正在输入Linux bash shell.这是使用fork&执行的代码. execv:

I have a CLI, one of the commands is entering into Linux bash shell.This is the code which does it using fork & execv:

if ((pid = fork()) < 0) {
    syslog_debug(LOG_ERR, "Could not fork");
}
if (pid == 0) {
    /* In child, open the child's side of the tty.  */
    int i;
    for(i = 0; i <= maxfd; i++)
    {
        close(i);
    }
    /* make new process group */
    setsid();
    if ((fd[0] = open(tty_name, O_RDWR /*| O_NOCTTY*/)) < 0) {
        syslog_debug(LOG_ERR, "Could not open tty");
        exit(1);
    }
    fd[1] = dup(0);
    fd[2] = dup(0);
    /* exec shell, with correct argv and env */
    execv("/bin/sh", (char *const *)argv_init);
    exit(1);
}

我想替换fork/execv并改用posix_spawn:

I want to replace the fork/execv and to use posix_spawn instead:

ret = posix_spawn_file_actions_init(&action);
pipe(fd);
for(i = 0; i <= maxfd; i++)
{
   ret = posix_spawn_file_actions_addclose (&action, i);
}
ret = posix_spawn_file_actions_adddup2 (&action, fd[1], 0);
ret = posix_spawn_file_actions_adddup2 (&action, fd[2], 0);
ret = posix_spawn_file_actions_addopen (&action, STDOUT_FILENO, tty_name, O_RDWR, 0);
char* argv[] = { "/bin/sh", "-c", "bash", NULL };
int status;
extern char **environ;
posix_spawnattr_t attr = { 0 };

snprintf( cmd, sizeof(cmd), "bash");
status = posix_spawn( &pid,
                    argv[0],
                    action /*__file_actions*/,
                    &attr,
                    argv,
                    environ );
posix_spawn_file_actions_destroy(&action);

但是它不起作用.有帮助吗?

But it doesn't work.Any help?

推荐答案

让我们看一下原始fork/exec中的代码是做什么的:

Let's look at what the code in the original fork/exec does:

• 关闭所有文件描述符
• 打开tty，它会同时获得fd 0，即它是stdin
• 将此文件复制两次(dup(0))，将其放入stdout和stderr
• 执行shell命令

• close all file descriptors
• open the tty, which will coincidentally get an fd of 0, i.e. it's stdin
• duplicate this fd twice (dup(0)), which puts them into stdout and stderr
• exec the shell command

您的新代码完全没有遵循相同的模式.您想要做的就是重复该过程，但要更加明确:

Your new code doesn't follow the same pattern at all. What you want to do is repeat the process, but be more explicit:

关闭所有FD:

for(i = 0; i <= maxfd; i++)
{
   ret = posix_spawn_file_actions_addclose (&action, i);
}

将tty打开到STDIN_FILENO:

ret = posix_spawn_file_actions_addopen (&action, STDIN_FILENO, tty_name, O_RDWR, 0);

将STDIN_FILENO复制为STDOUT_FILENO和STDERR_FILENO:

ret = posix_spawn_file_actions_adddup2 (&action, STDIN_FILENO, STDOUT_FILENO);
ret = posix_spawn_file_actions_adddup2 (&action, STDIN_FILENO, STDERR_FILENO);

然后posix_spawn应该在正确的上下文中发生.

Then the posix_spawn should take place in the correct context.

对于旧的fork/exec流程，您应该执行以下操作:

For the old fork/exec process, you should have done something like:

int fd = open(tty_name, O_RDWR /*| O_NOCTTY*/);
if (fd != STDIN_FILENO) dup2(fd, STDIN_FILENO);
if (fd != STDOUT_FILENO) dup2(fd, STDOUT_FILENO);
if (fd != STDERR_FILENO) dup2(fd, STDERR_FILENO);

意图更明确

要使用echo something而不是调用bash将此代码组合成一小段代码，我们可以:

To combine this into a small piece of code, with echo something rather than an invocation of bash we have:

#include <stdio.h>
#include <spawn.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>

void
do_spawn()
{
  int ret;
  posix_spawn_file_actions_t action;
  int i;
  pid_t pid;
  int maxfd = 1024;
  char *tty_name = ttyname (0);

  ret = posix_spawn_file_actions_init (&action);
  for (i = 0; i <= maxfd; i++) {
      ret = posix_spawn_file_actions_addclose (&action, i);
  }
  ret = posix_spawn_file_actions_addopen (&action, STDIN_FILENO, tty_name, O_RDWR, 0);
  ret = posix_spawn_file_actions_adddup2 (&action, STDIN_FILENO, STDOUT_FILENO);
  ret = posix_spawn_file_actions_adddup2 (&action, STDIN_FILENO, STDERR_FILENO);
  char *argv[] = { "/bin/sh", "-c", "echo something", NULL };
  int status;
  extern char **environ;
  posix_spawnattr_t attr = { 0 };

  posix_spawnattr_init(&attr);
  posix_spawnattr_setflags(&attr, POSIX_SPAWN_USEVFORK);
  status = posix_spawn(&pid, argv[0], &action /*__file_actions*/ , &attr, argv, environ);
  printf ("%d %ld\n", status, pid);

  wait (0);

  posix_spawn_file_actions_destroy (&action);

}

int
main(int argc, char **argv)
{
  do_spawn();
}

这需要用-D_GNU_SOURCE进行编译，否则未定义POSIX_SPAWN_USEVFORK.

This needs to be compiled with -D_GNU_SOURCE, as otherwise POSIX_SPAWN_USEVFORK is not defined.

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