Difference between fork() and exec() in C

Here we will see the effect of fork() and exec() system call in C. The fork is used to create a new process by duplicating the calling process. The new process is the child process. See the following property.

• The child process has its own unique process id.
• The parent process id of the child process is same as the process id of the calling process.
• The child process does not inherit the parent’s memory lock and semaphores.

The fork() returns the PID of the child process. If the value is non-zero, then it is parent process’s id, and if this is 0, then this is child process’s id.

The exec() system call is used to replace the current process image with the new process image. It loads the program into the current space, and runs it from the entry point.

So the main difference between fork() and exec() is that fork starts new process which is a copy of the main process. the exec() replaces the current process image with new one, Both parent and child processes are executed simultaneously.

Example

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
int main() {
   pid_t process_id;
   int return_val = 1;
   int state;
   process_id = fork();
   if (process_id == -1) { //when process id is negative, there is an error, unable to fork
      printf("can't fork, error occured\n");
         exit(EXIT_FAILURE);
   } else if (process_id == 0) { //the child process is created
      printf("The child process is (%u)\n",getpid());
         char * argv_list[] = {"ls","-lart","/home",NULL};
      execv("ls",argv_list); // the execv() only return if error occured.
      exit(0);
   } else { //for the parent process
      printf("The parent process is (%u)\n",getppid());
      if (waitpid(process_id, &state, 0) > 0) { //wait untill the process change its state
         if (WIFEXITED(state) && !WEXITSTATUS(state))
            printf("program is executed successfully\n");
         else if (WIFEXITED(state) && WEXITSTATUS(state)) {
            if (WEXITSTATUS(state) == 127) {
               printf("Execution failed\n");
            } else
               printf("program terminated with non-zero status\n");
         } else
            printf("program didn't terminate normally\n");
      }
      else {
         printf("waitpid() function failed\n");
      }
      exit(0);
   }
   return 0;
}

Output

The parent process is (8627)
The child process is (8756)
program is executed successfully