The Open Group Base Specifications Issue 8
IEEE Std 1003.1-2024
Copyright © 2001-2024 The IEEE and The Open Group

NAME

shmget — get an XSI shared memory segment

SYNOPSIS

[XSI] [Option Start] #include <sys/shm.h>

int shmget(key_t
key, size_t size, int shmflg); [Option End]

DESCRIPTION

The shmget() function operates on XSI shared memory (see XBD 3.332 Shared Memory Object ). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 2.8 Realtime .

The shmget() function shall return the shared memory identifier associated with key.

A shared memory identifier, associated data structure, and shared memory segment of at least size bytes (see <sys/shm.h>) are created for key if one of the following is true:

Upon creation, the data structure associated with the new shared memory identifier shall be initialized as follows:

When the shared memory segment is created, it shall be initialized with all zero values.

RETURN VALUE

Upon successful completion, shmget() shall return a non-negative integer, namely a shared memory identifier; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

The shmget() function shall fail if:

[EACCES]
A shared memory identifier exists for key but operation permission as specified by the low-order nine bits of shmflg would not be granted; see 2.7 XSI Interprocess Communication .
[EEXIST]
A shared memory identifier exists for the argument key but (shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.
[EINVAL]
A shared memory segment is to be created and the value of size is less than the system-imposed minimum or greater than the system-imposed maximum.
[EINVAL]
No shared memory segment is to be created and a shared memory segment exists for key but the size of the segment associated with it is less than size.
[ENOENT]
A shared memory identifier does not exist for the argument key and (shmflg &IPC_CREAT) is 0.
[ENOMEM]
A shared memory identifier and associated shared memory segment are to be created, but the amount of available physical memory is not sufficient to fill the request.
[ENOSPC]
A shared memory identifier is to be created, but the system-imposed limit on the maximum number of allowed shared memory identifiers system-wide would be exceeded.

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication , 2.8 Realtime , ftok , shmat , shmctl , shmdt , shm_open , shm_unlink

XBD 3.332 Shared Memory Object , <sys/shm.h>

CHANGE HISTORY

First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

Moved from SHARED MEMORY to BASE.

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0575 [345], XSH/TC1-2008/0576 [363], and XSH/TC1-2008/0577 [344] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0328 [640] is applied.

End of informative text.

 

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