Output redirection using fork() and execl()

Question

I want to use fork() to spawn off a new process in my program. The new process will have only one task: To redirect the mouse input to a serial port. I have tested the following command with success in a terminal window: hexdump /dev/input/mice > /dev/ttyS0 So far I have managed to use fork to create a child process, but my problem is that I can't seem to get my execl() method to work properly: ``` execl("/usr/bin/hexdump", "hexdump", "/dev/input/mice > /dev/ttyS0", (char*) NULL); ``` I have tried other variants also, like this: ``` execl("/usr/bin/hexdump", "hexdump", "/dev/input/mice", ">", "/dev/ttyS0", (char*) NULL); ``` But always with the same result, an exit value of 1 (general error). It's also worth mentioning that I have managed to make it work by using the popen() method, where you can type the command exactly like you would have done in a terminal. The problem with popen() is that I have not found a good way for terminating the process. With fork() I get the PID, and can t…

Accepted Answer

You cannot perform redirection in this way.

If you want to redirect `stdout` for a process you're about to `exec`, you need to `open` the path and `dup2` it onto the appropriate file descriptor.

Example:

```
 if (!(pid = fork()) 
 {
     int fd = open("/dev/ttyS0", O_WRONLY);
     dup2(fd, 1);  // redirect stdout
     execl("/usr/bin/hexdump", "hexdump", "/dev/input/mice", NULL);
 }
```

Output redirection using fork() and execl()

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