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

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;}

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.

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