Attaching and Detaching a Shared Memory Segment
shmat() and shmdt() are used to attach and detach shared memory segments (see the shmop(2) man page). shmat(2) returns a pointer to the head of the shared segment. shmdt(2) detaches the shared memory segment located at the address indicated by shmaddr. The following code illustrates calls to shmat(2) and shmdt(2)
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
static struct state { /* Internal record of attached segments. */
int shmid; /* shmid of attached segment */
char *shmaddr; /* attach point */
int shmflg; /* flags used on attach */
} ap[MAXnap]; /* State of current attached segments. */
int nap; /* Number of currently attached segments. */
...
char *addr; /* address work variable */
register int i; /* work area */
register struct state *p; /* ptr to current state entry */
...
p = &ap[nap++];
p->shmid = ...
p->shmaddr = ...
p->shmflg = ...
p->shmaddr = shmat(p->shmid, p->shmaddr, p->shmflg);
if(p->shmaddr == (char *)-1) {
perror("shmat failed");
nap--;
} else
(void) fprintf(stderr, "shmop: shmat returned %p\n",
p->shmaddr);
...
i = shmdt(addr);
if(i == -1) {
perror("shmdt failed");
} else {
(void) fprintf(stderr, "shmop: shmdt returned %d\n", i);
for (p = ap, i = nap; i--; p++) {
if (p->shmaddr == addr) *p = ap[--nap];
}
}
...
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