Shared memory

Shared memory

Example1:

#include <stdio.h>
#include <sys/shm.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char** argv) {
    int fd;
    int* p;
    int i;
    int k;

    /* If run with one argument create the shared memory segment
    otherwise just connect to it */
    if(argc > 1) {
            fd = shmget(1234, 5*sizeof(int), IPC_CREAT|0600);
    }
    else {
            fd = shmget(1234, 0, 0);
    }

    /* Attach to the shared memory segment */
     p = shmat(fd, 0, 0);

     /* Repeat 20 times */
     for(k=0; k<20; k++) {
              /* Write the PID on all 5 positions of the shared memory segment.
            Add sleep to give the user time for starting other instances */
            for(i=0; i<5; i++) {
                  p[i] = getpid();
                  sleep(1);
             }

             /* Print the content of the shared memory segment. It will most
             likely contain a mix of PIDs belonging to all the instances  of this program */ 
             printf("%d: ", getpid());
             for(i=0; i<5; i++) {
                   printf("%5d ", p[i]); 
             }
             printf("\n");
     }

     /* Detach from the shared memory segment */
     shmdt(p);

     /* If run with one argument, then it created the shared memory
     segment, so now it must destory it */
     if(argc > 1) {
          shmctl(fd, IPC_RMID, NULL);
     }

     return 0;
}

Example 2 (server/client):

First run server, then client.  Commands useful: ipcs -m (to see the create memory location) ; ipcrm -M 9876 (to delete it)

Server.c----------------------------------

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

#define SMSIZE 100

int main(){

int shmid;
key_t key;
char *shm;
char *s;

key=9876;

shmid=shmget(key, SMSIZE, IPC_CREAT | 0666);
if (shmid<0){
      perror("shmget");
      exit(1);
}

shm=shmat(shmid, NULL, 0);

if (shm==(char *) -1){
     perror("shmat");
     exit(1);
}

memcpy(shm, "Hello world", 11);

s=shm;
s+=11;
*s=0;

while (*shm != '*')
      sleep(1);



return 0;

}

Client.c----------------------

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

#define SMSIZE 100

int main(){

int shmid;
key_t key;
char *shm;
char *s;

key=9876;

shmid=shmget(key, SMSIZE, 0666);
if (shmid<0){
     perror("shmget");
     exit(1);
}

shm=shmat(shmid, NULL, 0);

if (shm==(char *) -1){
     perror("shmat");
     exit(1);
}

for (s=shm; *s!=0; s++ ){
     printf("%c", *s);
}
printf("\n");

*shm='*';


return 0;

}

Other examples: https://users.cs.cf.ac.uk/Dave.Marshall/C/node27.html