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man pages section 2: System Calls

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Updated: Thursday, June 13, 2019
 
 

shmat(2)

Name

shmop, shmat, shmdt - shared memory operations

Synopsis

#include <sys/types.h>
#include <sys/shm.h>

void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(const void *shmaddr);

Description

The shmat() function attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of the calling process.

The permission required for a shared memory control operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

00400    READ by user
00200    WRITE by user
00040    READ by group
00020    WRITE by group
00004    READ by others
00002    WRITE by others

See the Shared Memory Operation Permissions section of Intro(2) for more information.

For shared memory segments created with the SHM_SHARE_MMU or SHM_PAGEABLE flags, the default protections cannot be changed so as to prevent a single process from affecting other processes sharing the same shared segment.

When (shmflg & SHM_SHARE_MMU) is true, virtual memory resources in addition to shared memory itself are shared among processes that use the same shared memory.

When (shmflg & SHM_PAGEABLE) is true, virtual memory resources are shared and the dynamic shared memory (DISM) framework is created. The dynamic shared memory can be resized dynamically within the specified size in shmget(2). The DISM shared memory is pageable unless it is locked.

Segments created using shmget_osm(2) may not be attached with shmflag flags of SHM_SHARE_MMU, SHM_PAGEABLE, or SHM_RND.

    The shared memory segment is attached to the data segment of the calling process at the address specified based on one of the following criteria:

  • If shmaddr is equal to (void *) 0, the segment is attached to the first available address as selected by the system.

  • If shmaddr is equal to (void *) 0 and ( shmflg & SHM_SHARE_MMU) or (shmflg & SHM_PAGEABLE) is true, then the segment is attached to the first available suitably aligned address. When (shmflg & SHM_SHARE_MMU) or (shmflg & SHM_PAGEABLE) is set, however, the permission given by shmget() determines whether the segment is attached for reading or reading and writing.

  • If shmaddr is not equal to (void *) 0 and (shmflg & SHM_RND) is true, the segment is attached to the address given by (shmaddr- (shmaddr modulus SHMLBA)).

  • If shmaddr is not equal to (void *) 0 and (shmflg & SHM_RND) is false, the segment is attached to the address given by shmaddr.

  • The segment is attached for reading if (shmflg & SHM_RDONLY) is true {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

The shmdt() function detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.

Shared memory segments must be explicitly removed after the last reference to them has been removed.

Return Values

Upon successful completion, shmat() returns the data segment start address of the attached shared memory segment; shmdt() returns 0. Otherwise, −1 is returned, the shared memory segment is not attached, and errno is set to indicate the error.

Errors

The shmat() function will fail if:

EACCES

Operation permission is denied to the calling process (see Intro(2)).

EINVAL

The shmid argument is not a valid shared memory identifier.

The shmaddr argument is not equal to 0, and the value of (shmaddr- (shmaddr modulus SHMLBA)) is an illegal address.

The shmaddr argument is not equal to 0, is an illegal address, and (shmflg & SHM_RND) is false.

The shmaddr argument is not equal to 0, is not properly aligned, and (shmflg & SHM_SHARE_MMU) is true.

SHM_SHARE_MMU is not supported in certain architectures.

Both (shmflg & SHM_SHARE_MMU) and (shmflg & SHM_PAGEABLE) are true.

(shmflg & SHM_SHARE_MMU) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg & SHM_PAGEABLE) was true.

(shmflg & SHM_PAGEABLE) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg & SHM_SHARE_MMU) was true.

The segment was created using shmget_osm(2), and one of (shmflg & SHM_PAGEABLE), (shmflg & SHM_SHARE_MMU), or (shmflg & SHM_RND) is true.

The segment was created using shmget_osm(2), and the shmaddr argument is not equal to 0, and shmaddr is not aligned to the segment's granule size.

EMFILE

The number of shared memory segments attached to the calling process would exceed the system-imposed limit.

ENOMEM

The available data space is not large enough to accommodate the shared memory segment.

The shmdt() function will fail if:

EINVAL

The shmaddr argument is not the data segment start address of a shared memory segment.

ENOMEM

(shmflg & SHM_SHARE_MMU) is true and attaching to the shared memory segment would exceed a limit or resource control on locked memory.

Warnings

Using a fixed value for the shmaddr argument can adversely affect performance on certain platforms due to D-cache aliasing.

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Async-Signal-Safe
Standard

See Also

exec(2), exit(2), fork(2), Intro(2), shmctl(2), shmget(2), shmget_osm(2), attributes(7), standards(7)