C - 重定向子进程的IO

Question

I am trying to redirect the IO of a child process (after fork() ) into a file, and I can't figure out why it isn't working.

Here's what I've done:

if(fork() == 0){
    execv(exe, (char*[]){ exe, "> temp.exe" });
    ...

And the executable runs, but it doesn't redirect to the file. I would appreciate it if anyone could explain what am I doing wrong, and how I should do it. I'm getting a feeling I need to redirect before the execv() but I have no idea how to.

Thanks in advance!

Answer 1

Shell redirections (like > file ) are implemented by the shell. By using execve() , you are bypassing the shell; the child process will see "> temp.exe" in argv , and will attempt to process it as an argument.

If you want to redirect output to a file, the easiest approach will be to implement that redirection yourself by opening the file after forking and using dup2() to move its file descriptor to standard output:

if (fork() == 0) {
    int fd = open("temp.exe", O_CREAT | O_WRONLY, 0666);
    if (fd < 0) { handle error... exit(255); }
    dup2(fd, 1);
    close(fd);
    execv(exe, ...);
}

Answer 2

The execX() family of calls does not have the same flexibility as, say system() or popen(). These latter methods call shell to do the interpretation of the command.

The arguments to the execX call are the exact path of the program you want to run and the arguments you want to give to that program. Any "shell" features such as redirection you have to implement yourself before calling execX.

Alternatively, you can let shell actually do the work, execp("sh","sh",myexe+" >test.txt"); , but that is lazy, and then why not just use system anyway?

Two very useful methods are pipe() and dup2(): pipe allows you to create pipes to your host program; dup2 lets you set a scenario where the program being executed thinks that it is writing to stdout (1), or reading from stdin (0), but is actually writing or reading to a file or pipe that you created.

You will get a long way by reading the man pages for pipe and dup2, or in google looking for exec pipe and dup2, so I won't take your enjoyment away by writing a full implementation here.