The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


shmget - get shared memory segment


#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);


The shmget() function returns 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 is initialised as follows:

When the shared memory segment is created, it will be initialised with all zero values.


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


The shmget() function will 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 IPC.
A shared memory identifier exists for the argument key but (shmflg&IPC_CREAT)&&(shmflg&IPC_EXCL) is non-zero.
The value of size is less than the system-imposed minimum or greater than the system-imposed maximum, or a shared memory identifier exists for the argument key but the size of the segment associated with it is less than size and size is not 0.
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 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 IPC can be easily modified to use the alternative interfaces.




shmat(), shmctl(), shmdt(), shm_open(), shm_unlink(), <sys/shm.h>, IPC.


Derived from Issue 2 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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