#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SHM_SIZE 1024 /* make it a 1K shared memory segment */
int main(int argc, char *argv[])
{
key_t key;
int shmid;
char *data;
int mode;
if (argc > 2) {
fprintf(stderr, "usage: shmdemo [data_to_write]\n");
exit(1);
}
/* make the key: */
if ((key = ftok("hello.txt", 'R')) == -1) /*Here the file must exist */
{
perror("ftok");
exit(1);
}
/* create the segment: */
if ((shmid = shmget(key, SHM_SIZE, 0644 | IPC_CREAT)) == -1) {
perror("shmget");
exit(1);
}
/* attach to the segment to get a pointer to it: */
if ((data = shmat(shmid, NULL, 0)) == (void *)-1) {
perror("shmat");
exit(1);
}
/* read or modify the segment, based on the command line: */
if (argc == 2) {
printf("writing to segment: \"%s\"\n", argv[1]);
strncpy(data, argv[1], SHM_SIZE);
} else
printf("segment contains: \"%s\"\n", data);
/* detach from the segment: */
if (shmdt(data) == -1) {
perror("shmdt");
exit(1);
}
return 0;
}
Use ftok to convert a pathname and a project identifier to a System V IPC key
Use shmget which allocates a shared memory segment
Use shmat to attache the shared memory segment identified by shmid to the address space of the calling process
Do the operations on the memory area
Detach using shmdt