How to share memory between processes created by fork()?
CForkC Problem Overview
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.
}
C Solutions
Solution 1 - C
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;
}
Solution 2 - C
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 pthreads
are sharing the global
data and the changes in global variable is reflected in parent.
Then read some Pthreads tutorial.
Solution 3 - C
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