C/C++: Pass value from parent to child after fork() via a pipe()


The following code will create a pipe, fork the process and then send from the parent to the child an int value (the id we want to give to the child), finally the child will read the value and terminate.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
	int fd[2];
	int childID = 0;

	// create pipe descriptors
	pipe(fd);

	// fork() returns 0 for child process, child-pid for parent process.
	if (fork() != 0) {
		// parent: writing only, so close read-descriptor.
		close(fd[0]);

		// send the childID on the write-descriptor.
		childID = 1;
		write(fd[1], &childID, sizeof(childID));
		printf("Parent(%d) send childID: %d\n", getpid(), childID);

		// close the write descriptor
		close(fd[1]);
	} else {
		// child: reading only, so close the write-descriptor
		close(fd[1]);

		// now read the data (will block until it succeeds)
		read(fd[0], &childID, sizeof(childID));
		printf("Child(%d) received childID: %d\n", getpid(), childID);

		// close the read-descriptor
		close(fd[0]);
	}
	return 0;
}

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