The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


shmget - get an XSI shared memory segment


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

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


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


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.


The shmget() function shall fail if:

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 XSI Interprocess Communication .
A shared memory identifier exists for the argument key but (shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.
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.
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.
A shared memory identifier does not exist for the argument key and (shmflg &IPC_CREAT) is 0.
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.
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.




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 XSI Interprocess Communication can be easily modified to use the alternative interfaces.






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

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


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

Issue 5


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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