fork

Learn to create processes with fork.

What is fork?

fork creates an exact copy of the running process. The original is the parent, the new copy is the child. Both run the same code but are different processes.

fork returns a pid_t value: 0 to the child, the child's ID (positive) to the parent, -1 on error. You check the return value to know which process you are in. This is the foundation of concurrent programming.

Parent and child

After fork, parent and child continue from the same point. Use if (pid == 0) for the child branch and else for the parent branch.

The child copies the parent's file descriptors and memory but does not share memory. Descriptors point to the same table; both see the same files. When the child finishes, the parent must wait for it; otherwise the child becomes a zombie. You will see wait in the next lesson.

Exercise Create a process with fork

Create a process with pid_t pid = fork();. In the if (pid == 0) branch, have the child exit with _exit(0). In the parent branch, print with printf("fork done\n");.

Show example solution

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(void) {
    pid_t pid = fork();
    if (pid == 0) {
        _exit(0);
    }
    printf("fork done\n");
    return 0;
}

Test yourself

What does fork return to the child process?
- -1
- The parent's PID
- 0
- 1
Show answer

0 — The child gets 0; the parent gets the child's PID.
fork call
```
pid_t pid = ____();
```
Show answer
```
pid_t pid = fork();
```
Match process functions

fork

Creates process
exec

Replaces program
wait

Waits for child
getpid

Returns PID

Next lesson: exec

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