Example
This program demonstrates how to run another process using fork() and wait its termination using waitpid():
-
fork()creates an identical copy of the current process. The original process is the parent process, while the newly created one is the child process. Both processes continue exactly after thefork(). -
waitpid()blocks until child process exits or terminates and returns its exit code and termination reason.
#include <unistd.h> /* for fork(), getpid() */
#include <sys/types.h> /* for waitpid() */
#include <sys/wait.h> /* for waitpid() */
#include <stdlib.h> /* for exit() */
#include <stdio.h> /* for printf(), perror() */
int
main(int argc, char *argv[])
{
/* Create child process.
*
* On success, fork() returns the process ID of the child (> 0) to the
* parent and 0 to the child. On error, -1 is returned.
*/
pid_t child_pid = fork();
if (child_pid < 0) {
perror("fork() failed");
exit(EXIT_FAILURE);
} else if (child_pid == 0) {
/* Print message from child process.
*
* getpid() returns the PID (process identifier) of current process,
* which is typically int but doesn't have to be, so we cast it.
*
* getppid() returns the PID of the parent process.
*/
printf("from child: pid=%d, parent_pid=%d\n",
(int)getpid(), (int)getppid());
/* We can do something here, e.g. load another program using exec().
*/
exit(33);
} else if (child_pid > 0) {
/* Print message from parent process.
*/
printf("from parent: pid=%d child_pid=%d\n",
(int)getpid(), (int)child_pid);
/* Wait until child process exits or terminates.
*
* The return value of waitpid() is PID of the child process, while
* its argument is filled with exit code and termination reason.
*/
int status;
pid_t waited_pid = waitpid(child_pid, &status, 0);
if (waited_pid < 0) {
perror("waitpid() failed");
exit(EXIT_FAILURE);
} else if (waited_pid == child_pid) {
if (WIFEXITED(status)) {
/* WIFEXITED(status) returns true if the child has terminated
* normally. In this case WEXITSTATUS(status) returns child's
* exit code.
*/
printf("from parent: child exited with code %d\n",
WEXITSTATUS(status));
}
}
}
exit(EXIT_SUCCESS);
}
Example output:
from parent: pid=2486 child_pid=2487
from child: pid=2487, parent_pid=2486
from parent: child exited with code 33
Copied from here, originally created by M.Geiger.
