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man pages section 2: System Calls

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Updated: Thursday, June 13, 2019
 
 

shmget_osm(2)

Name

shmget, shmget_osm - get shared memory segment identifier

Synopsis

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

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

Description

The shmget() and shmget_osm() functions return the shared memory identifier associated with key.

A shared memory identifier and associated data structure and shared memory segment of at least size bytes (see Intro(2)) are created for key if one of the following are true:

  • The key argument is equal to IPC_PRIVATE.

  • The key argument does not already have a shared memory identifier associated with it, and (shmflg&IPC_CREAT) is true.

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

  • The values of 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.

  • The access permission bits of shm_perm.mode are set equal to the access permission bits of shmflg. shm_segsz is set equal to the value of size.

  • The values of shm_lpid, shm_nattch shm_atime, and shm_dtime are set equal to 0.

  • The shm_ctime is set equal to the current time.

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

The shmget_osm() system call is a variant of shmget() which creates an optimized shared memory (OSM) segment. The new fourth argument, granule_size, specifies the segment's granule size. This value must be a power of two which is atleast as large as the value returned by sysconf(_SC_OSM_PAGESIZE_MIN). A segment created with shmget_osm() starts out with all granules unlocked. Unlocked granules have PROT_NONE permissions, and have no memory allocated to back them. The memcntl() MC_LOCK_GRANULE operation can be used to lock granules in a shmget_osm(2) created segment which has been mapped into the address space with shmat(2).

Return Values

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

Errors

The shmget() and shmget_osm() functions will fail if:

EACCES

A shared memory identifier exists for key but operation permission (see Intro(2)) as specified by the low-order 9 bits of shmflg would not be granted.

EEXIST

A shared memory identifier exists for key but both (shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.

EINVAL

The size argument is less than the system-imposed minimum or greater than the system-imposed maximum. See NOTES.

For shmget(), 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 0.

For shmget_osm(), size and granule_size are not both zero, and there is an existing segment whose size and granule_size do not match the passed in values.

For shmget_osm(), either size or granule_size are zero, and there is no existing segment.

For shmget_osm(), a new segment is going to be created, size is not a multiple of granule_size, or granule_size is not a power-of-2 greater than the value returned by sysconf(_SC_OSM_PAGESIZE_MIN).

ENOENT

A shared memory identifier does not exist for key and (shmflg&IPC_CREAT) is false.

ENOMEM

A shared memory identifier and associated shared memory segment are to be created but the amount of available 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. See NOTES.

Attributes

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

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
Standard

See Also

Intro(2), setrctl(2), shmctl(2), shmop(2), getpagesize(3C), ftok(3C), attributes(7), standards(7), rctladm(8)

Notes

The project.max-shm-memory resource control restricts the total amount of shared memory a project can allocate. The zone.max-shm-memory resource control restricts the total amount of shared memory that can be allocated by a zone. The system-imposed maximum on the size of a shared memory segment is therefore a function of the sizes of any other shared memory segments the calling project might have allocated that are still in use, as well as any other shared memory segments allocated and still in use by processes in the zone. For accounting purposes, segment sizes are rounded up to the nearest multiple of the system page size. See getpagesize(3C).

The system-imposed limit on the number of shared memory identifiers is maintained on a per-project basis using the project.max-shm-ids resource control. The zone.max-shm-ids resource control restricts the total number of shared memory identifiers that can be allocated by a zone.

See rctladm(8) and setrctl(2) for information about using resource controls.