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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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SHMOP(2) System Calls Manual SHMOP(2)
shmat, shmdt - System V shared memory operations
Standard C library (libc, -lc)
#include <sys/shm.h>
void *shmat(int shmid, const void *_Nullable shmaddr, int shmflg);
int shmdt(const void *shmaddr);
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 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()
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 error.
On success, shmdt() returns 0; on error -1 is returned, and errno
is set to indicate the error.
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 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.
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.
POSIX.1-2008.
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 *.
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
Program source: svshm_string.h
The following header file is included by the "reader" and "writer"
programs:
/* 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. 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".
/* 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 size;
struct sembuf sop;
if (argc != 4) {
fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
exit(EXIT_FAILURE);
}
size = strlen(argv[3]) + 1; /* +1 to include trailing '\0' */
if (size > 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], size);
/* 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);
}
brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7),
shm_overview(7), sysvipc(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 SHMOP(2)
Pages that refer to this page: ipcrm(1), ipcs(1), lsipc(1), pcp-ipcs(1), execve(2), futex(2), ipc(2), mbind(2), mmap(2), open(2), PR_SET_TAGGED_ADDR_CTRL(2const), semctl(2), semop(2), shmctl(2), shmget(2), syscalls(2), numa(3), systemd.exec(5), shm_overview(7), sysvipc(7), migratepages(8), numactl(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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