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?

Answer 1

Here is an alternative solution.

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

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