How to share memory between processes created by fork()?

Question

In fork child, if we modify a global variable, it will not get changed in the main program.

Is there a way to change a global variable in child fork?

#include <stdio.h> #include <stdlib.h> #include <unistd.h>  int glob_var;  main (int ac, char **av) {   int pid;    glob_var = 1;    if ((pid = fork()) == 0) {     /* child */     glob_var = 5;   }   else {     /* Error */     perror ("fork");     exit (1);   }    int status;   while (wait(&status) != pid) {   }    printf("%d\n",glob_var); // this will display 1 and not 5. } 

Answer 1

You can use shared memory (shm_open(), shm_unlink(), mmap(), etc.).

#include <stdio.h> #include <stdlib.h> #include <sys/mman.h> #include <sys/types.h> #include <sys/wait.h> #include <unistd.h>  static int *glob_var;  int main(void) {     glob_var = mmap(NULL, sizeof *glob_var, PROT_READ | PROT_WRITE,                      MAP_SHARED | MAP_ANONYMOUS, -1, 0);      *glob_var = 1;      if (fork() == 0) {         *glob_var = 5;         exit(EXIT_SUCCESS);     } else {         wait(NULL);         printf("%d\n", *glob_var);         munmap(glob_var, sizeof *glob_var);     }     return 0; } 

Answer 2

Changing a global variable is not possible because the new created process (child)is having it's own address space.

So it's better to use shmget(),shmat() from POSIX api

Or You can use pthread , since pthreadsare sharing the globaldata and the changes in global variable is reflected in parent.

Then read some Pthreads tutorial.