Shared Memory is an efficeint means of passing data between programs. One program will create a memory portion which other processes (if permitted) can access.
A process creates a shared memory segment using shmget()
The original owner of a shared memory segment can assign ownership to another user with shmctl()
a shared segment can be attached to a process address space using shmat()
It can be detached using shmdt()
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Accessing a Shared Memory Segment
shmget() is used to obtain access to a shared memory segment. It is prottyped by:
int shmget(key_t key, size_t size, int shmflg);
The key argument is a access value associated with the semaphore ID. The size argument is the size in bytes of the requested shared memory. The shmflg argument specifies the initial access permissions and creation control flags.
Example:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
...
key_t key; /* key to be passed to shmget() */
int shmflg; /* shmflg to be passed to shmget() */
int shmid; /* return value from shmget() */
int size; /* size to be passed to shmget() */
...
key = ...
size = ...
shmflg) = ...
if ((shmid = shmget (key, size, shmflg)) == -1) {
perror("shmget: shmget failed"); exit(1); } else {
(void) fprintf(stderr, "shmget: shmget returned %d\n", shmid);
exit(0);
}
...
----------------------------------------------------------------------------------------------------------------------------------------------------------------------Controlling a Shared Memory Segment
shmctl() is used to alter the permissions and other characteristics of a shared memory segment.
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
cmd choice:
- SHM_LOCK
- -- Lock the specified shared memory segment in memory. The process must have the effective ID of superuser to perform this command.
- SHM_UNLOCK
- -- Unlock the shared memory segment. The process must have the effective ID of superuser to perform this command.
- IPC_STAT
- -- Return the status information contained in the control structure and place it in the buffer pointed to by buf. The process must have read permission on the segment to perform this command.
- IPC_SET
- -- Set the effective user and group identification and access permissions. The process must have an effective ID of owner, creator or superuser to perform this command.
- IPC_RMID
- -- Remove the shared memory segment.
- Example:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
...
int cmd; /* command code for shmctl() */
int shmid; /* segment ID */
struct shmid_ds shmid_ds; /* shared memory data structure to
hold results */
...
shmid = ...
cmd = ...
if ((rtrn = shmctl(shmid, cmd, shmid_ds)) == -1) {
perror("shmctl: shmctl failed");
exit(1);
}
...
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Attaching and Detaching a Shared Memory Segment
shmat() and shmdt() are used to attach and detach shared memory segments.
void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
shmat() returns a pointer, shmaddr, to the head of the shared segment associated with a valid shmid. shmdt() detaches the shared memory segment located at the address indicated by shmaddr.
Example:
#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("shmop: shmat failed");
nap--;
} else
(void) fprintf(stderr, "shmop: shmat returned %#8.8x\n",
p->shmaddr);
...
i = shmdt(addr);
if(i == -1) {
perror("shmop: 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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Example
shm_server.c
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#define SHMSZ 27
main()
{
char c;
int shmid;
key_t key;
char *shm, *s;
/*
* We'll name our shared memory segment
* "5678".
*/
key = 5678;
/*
* Create the segment.
*/
if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {
perror("shmget");
exit(1);
}
/*
* Now we attach the segment to our data space.
*/
if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
perror("shmat");
exit(1);
}
/*
* Now put some things into the memory for the
* other process to read.
*/
s = shm;
for (c = 'a'; c <= 'z'; c++)
*s++ = c;
*s = NULL;
/*
* Finally, we wait until the other process
* changes the first character of our memory
* to '*', indicating that it has read what
* we put there.
*/
while (*shm != '*')
sleep(1);
exit(0);
}
shm_client.c
/*
* shm-client - client program to demonstrate shared memory.
*/
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#define SHMSZ 27
main()
{
int shmid;
key_t key;
char *shm, *s;
/*
* We need to get the segment named
* "5678", created by the server.
*/
key = 5678;
/*
* Locate the segment.
*/
if ((shmid = shmget(key, SHMSZ, 0666)) < 0) {
perror("shmget");
exit(1);
}
/*
* Now we attach the segment to our data space.
*/
if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
perror("shmat");
exit(1);
}
/*
* Now read what the server put in the memory.
*/
for (s = shm; *s != NULL; s++)
putchar(*s);
putchar('\n');
/*
* Finally, change the first character of the
* segment to '*', indicating we have read
* the segment.
*/
*shm = '*';
exit(0);
}
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