shmget(2)

NAME

shmget() — get shared memory segment

SYNOPSIS

#include <sys/shm.h>

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

DESCRIPTION

shmget() returns the shared memory identifier associated with key.

A shared memory identifier and associated data structure and shared memory segment of size size bytes (see glossary(9)) are created for key if one of the following is true:

key is equal to IPC_PRIVATE. This call creates a new identifier, subject to available resources. The identifier will never be returned by another call to shmget() until it has been released by a call to shmctl(). The identifier should be used among the calling process and its descendents; however, it is not a requirement. The resource can be accessed by any process having the proper permissions.
key does not already have a shared memory identifier associated with it, and (shmflg & IPC_CREAT) is "true". If IPC_CREAT is set in shmflg the shared memory segment created can only be shared by processes of the same executable type. That is, an application compiled as a 32-bit process will be able to share the same memory segment with other 32-bit processes, and an application compiled as a 64-bit process will be able to share the same memory segment with other 64-bit processes. If a 64-bit bit process want to create a shared memory segment which can also be shared with 32-bit processes, the 64-bit process must specify IPC_SHARE32 in addition to IPC_CREAT in shmflg. The 32-bit process does not need to specify IPC_SHARE32. On Itanium(R)-based platforms, if the Adaptive Address Space feature is being used, then additional rules may apply. See the section below on Adaptive Address Space.

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

shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set equal to the effective user ID and effective group ID, respectively, of the calling process.
shm_perm.cuid, the low-order 9 bits of shm_perm.mode are set equal to the low-order 9 bits of shmflg. shm_segsz is set equal to the value of size.
shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal to 0.
shm_ctime is set equal to the current time.

Specifying IPC_GLOBAL in a call to shmget(), along with IPC_CREAT, forces the shared memory segment to be allocated from the global quadrant and not from the local quadrants. If this is not possible, shmget() will fail. Refer to setmemwindow(1M) for more information.

Specifying IPC_RELAXED_ISOLATION in a call to shmget(), along with IPC_CREAT, can improve the performance of co-operating processes in cases where these processes are accessing a large number of shared memory segments. This flag is treated as a hint.

The flags IPC_MEM_INTERLEAVED, IPC_MEM_LOCAL, and IPC_MEM_FIRST_TOUCH involve the placement of physical memory. This is important only on Cache Coherent Non-Uniform Memory Architecture (ccNUMA) systems. They will have no effect otherwise. For more information regarding ccNUMA and locality domains, see mpctl(2). These flags are hints to the system. If memory of the desired type is not available, the next most suitable memory is returned instead.

If IPC_MEM_INTERLEAVED is set in shmflg, physical memory will come from interleaved memory. Interleaved memory is a hardware construct which combines the memory from several locality domains into one region of memory. This memory is striped together with a very fine granularity. Interleaved memory is a good place to put shared objects, the kernel, and objects that could be accessed from any part of the system. This is the default behavior for shared memory segments created with this call.

If IPC_MEM_LOCAL is set in shmflg, physical memory for this mapping will always be allocated from the locality domain where the call to shmget() was made. Note that this flag implicitly uses the current locality domain, so the caller should be aware of locality domain configurations and bindings. See mpctl(2).

If IPC_MEM_FIRST_TOUCH is set in shmflg, physical memory will come from the locality domain of the first processor to read or write that memory. If the shared memory is paged out and then back in again, physical memory will come from the locality domain of the processor bringing in that memory.

System V Shared Memory Under Adaptive Address Space

Itanium-based platforms have the capability to create two types of processes: MGAS (Mostly Global Address Space), which is the default, and MPAS (Mostly Private Address Space). See ld(1) and chatr(1).

If 64-bit MPAS and 64-bit MGAS processes want to share a mapping among themselves without incurring the cost of aliasing, then (shmflg & IPC_GLOBAL) should be "true", but (shmflg & IPC_SHARE32) should be "false". 64-bit MPAS processes should also specify the IPC_GLOBAL flag to the shmat(2) call. Such a mapping cannot be shared with 32-bit MGAS processes.

See the Adaptive Address Space Whitepaper for details.

EXAMPLES

The following call to shmget() returns a unique shmid for the newly created shared memory segment of 4096 bytes:

int myshmid;
myshmid = shmget (IPC_PRIVATE, 4096, 0600);

RETURN VALUE

Upon successful completion, a non-negative integer, namely a shared memory identifier, is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS

shmget() fails if any of the following conditions are encountered:

EINVAL: size is less than the system-imposed minimum or greater than the system-imposed maximum.
EINVAL: A shared memory identifier exists for key but is in 64-bit address space and the process performing the request has been compiled as a 32-bit executable. In order to avoid receiving this error, both IPC_SHARE32 and IPC_CREAT must be set in shmflg by the 64-bit processes upon segment creation.
EACCES: A shared memory identifier exists for key but operation permission (see glossary(9)) as specified by the low-order 9 bits of shmflg would not be granted.
EINVAL: A shared memory identifier exists for key but the size of the segment associated with it is less than size and size is not equal to zero.
ENOENT: A shared memory identifier does not exist for key and (shmflg & IPC_CREAT) is "false".
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.
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.
ENOMEM: A shared memory segment with IPC_CREAT or IPC_GLOBAL specified was to be created; however, there is not enough space in the global quadrant.
EEXIST: A shared memory identifier exists for key but ((shmflg & IPC_CREAT) && (shmflg & IPC_EXCL)) is "true".

STANDARDS CONFORMANCE

shmget(): SVID2, SVID3, XPG2, XPG3, XPG4

shmget(2)

Technical documentation

» Table of Contents

» Index