Question

So I’ve got a problem with my code and it’s driving me nuts since I can’t seem to figure out what’s causing this. Basically, I’m trying to write to a file that I have open, using multiple child processes with fork(). Before I start forking, I can write to it just fine, but once I fork and THEN do an if statement to see if it’s the child process, it won’t write.

Basically, what I have is this:

FILE *output = NULL;
output = fopen(...); // Done successfully
fprintf(output, "This writes okay\n");
// Fork n processes
for (i = 0; i <= n; n++)
{
    pid[i] = fork();
    fprintf(output, "We can still write\n");
    if (pid[i] == 0) // Child process
    {
        fprintf(output, "This won't write to output\n");
        printf("I can still write and calculate stuff otherwise\n");
        ...
    }
}

Can anyone figure out why it’s not able to write after checking to see if it’s a child process? There’s definitely no error with forking from what I’ve seen.

Answer 1

Opening a file descriptor and using it in more than one process (for example in both a parent and child process) is a race condition and will most likely cause problems and unpredictable behavior.