shmat, shmdt - System V shared memory operations
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);
shmat() attaches the System V shared memory segment identified by
shmid to the address space of the calling process. The attaching
address is specified by
shmaddr with one of the following criteria:
- •
- If shmaddr is NULL, the system chooses a suitable (unused)
page-aligned address to attach the segment.
- •
- If shmaddr isn't NULL and SHM_RND is specified in
shmflg, the attach occurs at the address equal to shmaddr
rounded down to the nearest multiple of SHMLBA.
- •
- Otherwise, shmaddr must be a page-aligned address at which the
attach occurs.
In addition to
SHM_RND, the following flags may be specified in the
shmflg bit-mask argument:
- SHM_EXEC (Linux-specific; since Linux 2.6.9)
- Allow the contents of the segment to be executed. The caller must have
execute permission on the segment.
- SHM_RDONLY
- Attach the segment for read-only access. The process must have read
permission for the segment. If this flag is not specified, the segment is
attached for read and write access, and the process must have read and
write permission for the segment. There is no notion of a write-only
shared memory segment.
- SHM_REMAP (Linux-specific)
- This flag specifies that the mapping of the segment should replace any
existing mapping in the range starting at shmaddr and continuing
for the size of the segment. (Normally, an EINVAL error would
result if a mapping already exists in this address range.) In this case,
shmaddr must not be NULL.
The
brk(2) value of the calling process is not altered by the attach. The
segment will automatically be detached at process exit. The same segment may
be attached as a read and as a read-write one, and more than once, in the
process's address space.
A successful
shmat() call updates the members of the
shmid_ds
structure (see
shmctl(2)) associated with the shared memory segment as
follows:
- •
- shm_atime is set to the current time.
- •
- shm_lpid is set to the process-ID of the calling process.
- •
- shm_nattch is incremented by one.
shmdt() detaches the shared memory segment located at the address
specified by
shmaddr from the address space of the calling process. The
to-be-detached segment must be currently attached with
shmaddr equal to
the value returned by the attaching
shmat() call.
On a successful
shmdt() call, the system updates the members of the
shmid_ds structure associated with the shared memory segment as
follows:
- •
- shm_dtime is set to the current time.
- •
- shm_lpid is set to the process-ID of the calling process.
- •
- shm_nattch is decremented by one. If it becomes 0 and the segment
is marked for deletion, the segment is deleted.
On success,
shmat() returns the address of the attached shared memory
segment; on error,
(void *) -1 is returned, and
errno is set to indicate the cause of the error.
On success,
shmdt() returns 0; on error -1 is returned, and
errno
is set to indicate the cause of the error.
When
shmat() fails,
errno is set to one of the following:
- EACCES
- The calling process does not have the required permissions for the
requested attach type, and does not have the CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
- EIDRM
- shmid points to a removed identifier.
- EINVAL
- Invalid shmid value, unaligned (i.e., not page-aligned and
SHM_RND was not specified) or invalid shmaddr value, or
can't attach segment at shmaddr, or SHM_REMAP was specified
and shmaddr was NULL.
- ENOMEM
- Could not allocate memory for the descriptor or for the page tables.
When
shmdt() fails,
errno is set as follows:
- EINVAL
- There is no shared memory segment attached at shmaddr; or,
shmaddr is not aligned on a page boundary.
POSIX.1-2001, POSIX.1-2008, SVr4.
In SVID 3 (or perhaps earlier), the type of the
shmaddr argument was
changed from
char * into
const void *, and the
returned type of
shmat() from
char * into
void *.
After a
fork(2), the child inherits the attached shared memory segments.
After an
execve(2), all attached shared memory segments are detached from
the process.
Upon
_exit(2), all attached shared memory segments are detached from the
process.
Using
shmat() with
shmaddr equal to NULL is the preferred,
portable way of attaching a shared memory segment. Be aware that the shared
memory segment attached in this way may be attached at different addresses in
different processes. Therefore, any pointers maintained within the shared
memory must be made relative (typically to the starting address of the
segment), rather than absolute.
On Linux, it is possible to attach a shared memory segment even if it is already
marked to be deleted. However, POSIX.1 does not specify this behavior and many
other implementations do not support it.
The following system parameter affects
shmat():
- SHMLBA
- Segment low boundary address multiple. When explicitly specifying an
attach address in a call to shmat(), the caller should ensure that
the address is a multiple of this value. This is necessary on some
architectures, in order either to ensure good CPU cache performance or to
ensure that different attaches of the same segment have consistent views
within the CPU cache. SHMLBA is normally some multiple of the
system page size. (On many Linux architectures, SHMLBA is the same
as the system page size.)
The implementation places no intrinsic per-process limit on the number of shared
memory segments (
SHMSEG).
The two programs shown below exchange a string using a shared memory segment.
Further details about the programs are given below. First, we show a shell
session demonstrating their use.
In one terminal window, we run the "reader" program, which creates a
System V shared memory segment and a System V semaphore set. The program
prints out the IDs of the created objects, and then waits for the semaphore to
change value.
$ ./svshm_string_read
shmid = 1114194; semid = 15
In another terminal window, we run the "writer" program. The
"writer" program takes three command-line arguments: the IDs of the
shared memory segment and semaphore set created by the "reader", and
a string. It attaches the existing shared memory segment, copies the string to
the shared memory, and modifies the semaphore value.
$ ./svshm_string_write 1114194 15 'Hello, world'
Returning to the terminal where the "reader" is running, we see that
the program has ceased waiting on the semaphore and has printed the string
that was copied into the shared memory segment by the writer:
Hello, world
The following header file is included by the "reader" and
"writer" programs.
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
union semun { /* Used in calls to semctl() */
int val;
struct semid_ds * buf;
unsigned short * array;
#if defined(__linux__)
struct seminfo * __buf;
#endif
};
#define MEM_SIZE 4096
The "reader" program creates a shared memory segment and a semaphore
set containing one semaphore. It then attaches the shared memory object into
its address space and initializes the semaphore value to 1. Finally, the
program waits for the semaphore value to become 0, and afterwards prints the
string that has been copied into the shared memory segment by the
"writer".
#include "svshm_string.h"
int
main(int argc, char *argv[])
{
int semid, shmid;
union semun arg, dummy;
struct sembuf sop;
char *addr;
/* Create shared memory and semaphore set containing one
semaphore */
shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
if (shmid == -1)
errExit("shmget");
semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
if (shmid == -1)
errExit("shmget");
/* Attach shared memory into our address space */
addr = shmat(shmid, NULL, SHM_RDONLY);
if (addr == (void *) -1)
errExit("shmat");
/* Initialize semaphore 0 in set with value 1 */
arg.val = 1;
if (semctl(semid, 0, SETVAL, arg) == -1)
errExit("semctl");
printf("shmid = %d; semid = %d\n", shmid, semid);
/* Wait for semaphore value to become 0 */
sop.sem_num = 0;
sop.sem_op = 0;
sop.sem_flg = 0;
if (semop(semid, &sop, 1) == -1)
errExit("semop");
/* Print the string from shared memory */
printf("%s\n", addr);
/* Remove shared memory and semaphore set */
if (shmctl(shmid, IPC_RMID, NULL) == -1)
errExit("shmctl");
if (semctl(semid, 0, IPC_RMID, dummy) == -1)
errExit("semctl");
exit(EXIT_SUCCESS);
}
The writer program takes three command-line arguments: the IDs of the shared
memory segment and semaphore set that have already been created by the
"reader", and a string. It attaches the shared memory segment into
its address space, and then decrements the semaphore value to 0 in order to
inform the "reader" that it can now examine the contents of the
shared memory.
#include "svshm_string.h"
int
main(int argc, char *argv[])
{
int semid, shmid;
struct sembuf sop;
char *addr;
size_t len;
if (argc != 4) {
fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
exit(EXIT_FAILURE);
}
len = strlen(argv[3]) + 1; /* +1 to include trailing '\0' */
if (len > MEM_SIZE) {
fprintf(stderr, "String is too big!\n");
exit(EXIT_FAILURE);
}
/* Get object IDs from command-line */
shmid = atoi(argv[1]);
semid = atoi(argv[2]);
/* Attach shared memory into our address space and copy string
(including trailing null byte) into memory. */
addr = shmat(shmid, NULL, 0);
if (addr == (void *) -1)
errExit("shmat");
memcpy(addr, argv[3], len);
/* Decrement semaphore t0 0 */
sop.sem_num = 0;
sop.sem_op = -1;
sop.sem_flg = 0;
if (semop(semid, &sop, 1) == -1)
errExit("semop");
exit(EXIT_SUCCESS);
}
brk(2),
mmap(2),
shmctl(2),
shmget(2),
capabilities(7),
shm_overview(7),
sysvipc(7)