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.
}

possible duplicate of [How to use shared memory with Linux in C](http://stackoverflow.com/questions/5656530/how-to-use-shared-memory-with-linux-in-c) — alk, Nov 07 '12 at 17:26

md5 · Accepted Answer · 2012-11-07T17:45:04.493

109

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;
}

edited Nov 07 '12 at 17:45

answered Nov 07 '12 at 17:17

md5

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4

could you provide a simple example – MOHAMED Nov 07 '12 at 17:18
You'll need to look up shared memory; lots of examples on google. This allows two separate applications to use the same memory to store and read variables. When you `fork()` a process it creates a child process with a separate memory heap from the parent. Your parent will maintain its global variables while the child will allocate its own copies. – Grambot Nov 07 '12 at 17:20
You can find some examples with a very good explanation [here](http://www.cs.cf.ac.uk/Dave/C/node27.html) – iabdalkader Nov 07 '12 at 17:26
@md5: In the mmap function call the file descriptor handle is "-1". What does -1 signify ? – TheLoneJoker Jun 20 '14 at 20:09
1

@TheLoneJoker. From man page of `mmap`:`however, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this` – D3Hunter Jan 03 '15 at 00:34

score 12 · Answer 2 · edited Jun 18 '20 at 14:08

12

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.

edited Jun 18 '20 at 14:08

Basile Starynkevitch

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answered Nov 07 '12 at 17:47

Omkant

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score 3 · Answer 3 · edited Sep 21 '20 at 20:10

Here is an alternative solution.

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>

typedef struct
{
  int id;
  size_t size;
} shm_t;

shm_t *shm_new(size_t size)
{
  shm_t *shm = calloc(1, sizeof *shm);
  shm->size = size;

  if ((shm->id = shmget(IPC_PRIVATE, size, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR)) < 0)
  {
    perror("shmget");
    free(shm);
    return NULL;
  }

  return shm;
}

void shm_write(shm_t *shm, void *data)
{
  void *shm_data;

  if ((shm_data = shmat(shm->id, NULL, 0)) == (void *) -1)
  {
    perror("write");
    return;
  }

  memcpy(shm_data, data, shm->size);
  shmdt(shm_data);
}

void shm_read(void *data, shm_t *shm)
{
  void *shm_data;

  if ((shm_data = shmat(shm->id, NULL, 0)) == (void *) -1)
  {
    perror("read");
    return;
  }
  memcpy(data, shm_data, shm->size);
  shmdt(shm_data);
}

void shm_del(shm_t *shm)
{
  shmctl(shm->id, IPC_RMID, 0);
  free(shm);
}

int main()
{
  int var = 1;
  shm_t *shm = shm_new(sizeof var);

  int pid;
  if ((pid = fork()) == 0)
  { /* child */
    var = 5;
    shm_write(shm, &var);
    printf("child: %d\n", var);
    return 0;
  }
  /* Wait for child to return */
  int status;
  while (wait(&status) != pid);
  /* */
  shm_read(&var, shm);
  /* Parent is updated by child */
  printf("parent: %d\n", var);
  shm_del(shm);
  return 0;
}

Build with:

$ gcc shm.c -o shm && ./shm

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

3 Answers3

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