SHMOP(2) System Calls Manual SHMOP(2)
NAME
shmat, shmdt - System V shared memory operations
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/shm.h>
void *shmat(int shmid, const void *_Nullable shmaddr, int shmflg);
int shmdt(const void *shmaddr);
DESCRIPTION
shmat()
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 shm-
flg 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 re-
place any existing mapping in the range starting at shmaddr and
continuing for the size of the segment. (Normally, an EINVAL er-
ror 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()
shmdt() detaches the shared memory segment located at the address speci-
fied 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.
RETURN VALUE
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 error.
On success, shmdt() returns 0; on error -1 is returned, and errno is set
to indicate the error.
ERRORS
shmat() can fail with one of the following errors:
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 at-
tach segment at shmaddr, or SHM_REMAP was specified and shmaddr
was NULL.
ENOMEM Could not allocate memory for the descriptor or for the page ta-
bles.
shmdt() can fail with one of the following errors:
EINVAL There is no shared memory segment attached at shmaddr; or,
shmaddr is not aligned on a page boundary.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 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 *.
NOTES
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 specify-
ing an attach address in a call to shmat(), the caller should en-
sure that the address is a multiple of this value. This is nec-
essary 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).
EXAMPLES
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 pro-
gram 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 at-
taches 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
Program source: svshm_string.h
The following header file is included by the "reader" and "writer" pro-
grams:
/* svshm_string.h
Licensed under GNU General Public License v2 or later.
*/
#ifndef SVSHM_STRING_H
#define SVSHM_STRING_H
#include <stdio.h>
#include <stdlib.h>
#include <sys/sem.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
#endif // include guard
Program source: svshm_string_read.c
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. Fi-
nally, the program waits for the semaphore value to become 0, and after-
wards prints the string that has been copied into the shared memory seg-
ment by the "writer".
/* svshm_string_read.c
Licensed under GNU General Public License v2 or later.
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include "svshm_string.h"
int
main(void)
{
int semid, shmid;
char *addr;
union semun arg, dummy;
struct sembuf sop;
/* 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 (semid == -1)
errExit("semget");
/* 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);
}
Program source: svshm_string_write.c
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.
/* svshm_string_write.c
Licensed under GNU General Public License v2 or later.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include "svshm_string.h"
int
main(int argc, char *argv[])
{
int semid, shmid;
char *addr;
size_t len;
struct sembuf sop;
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 to 0. */
sop.sem_num = 0;
sop.sem_op = -1;
sop.sem_flg = 0;
if (semop(semid, &sop, 1) == -1)
errExit("semop");
exit(EXIT_SUCCESS);
}
SEE ALSO
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7),
sysvipc(7)
Linux man-pages 6.9.1 2024-06-15 SHMOP(2)
Generated by dwww version 1.16 on Tue Dec 16 04:05:01 CET 2025.