shmget(S)

shmget -- 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);

Description

shmget returns the shared memory identifier associated with key. This identifier is accessible by any process in the system, subject to normal access restrictions and the permissions set with shmflg.

Permitted values for shmflg are:

IPC_CREAT: Create the segment if it does not already exist in the kernel.
IPC_EXCL: When used with IPC_CREAT, fail if segment already exists.
If IPC_CREAT is used alone, shmget either returns the segment identifier for a newly created segment, or returns the identifier for a segment which exists with the same key value. If IPC_CREAT is used with IPC_EXCL, a new segment is created unless the segment exists, in which case, the call fails with -1. IPC_EXCL guarantees that no existing segment is opened for access.
IPC_SPECIAL: IPC_SPECIAL must be set if any of SHM_BALANCED, SHM_CPUGROUP, SHM_FINEGRAIN or SHM_FIRSTUSAGE is set. The calling process must have the P_PLOCK privilege to use this flag.
SHM_R: Specifies that the shared memory will have read access only.
SHM_W: Specifies that the shared memory will have write access only.
SHM_BALANCED: Specifies that shared memory will be accessed read/write, or readonly but such that replication at each CPU-group in a partitioned system is not desired. It is expected that the memory will be accessed from many CPU-groups, so it is appropriate to allocate physical memory so as to spread it evenly across these CPU-groups.
SHM_BALANCED may not be specified if either of SHM_CPUGROUP and SHM_FIRSTUSAGE is specified.
SHM_CPUGROUP: Specifies that the memory for the shared memory should be allocated from the CPU-Group where the caller is currently executing.
SHM_CPUGROUP may not be specified if either of SHM_BALANCED and SHM_FIRSTUSAGE is specified.
SHM_FINEGRAIN: Advises the system that shmctl(S) may be used to control the location of portions of the shared memory segment at a finer degree of granularity. If this flag is not set the system may choose to optimize access to the shared memory segment in such a way that it is not possible to control physical memory allocation on a page granular basis.
SHM_FIRSTUSAGE: Specifies that physical pages for the shared memory segment are to be allocated on whichever CPU-Group is executing the thread that makes the initial access to the shared memory.
SHM_FIRSTUSAGE may not be specified if either of SHM_BALANCED and SHM_CPUGROUP is specified.

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

key is equal to IPC_PRIVATE.
key does not already have a shared memory identifier associated with it, and (shmflg&IPC_CREAT) is true.

On 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.
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.
shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal to 0.
shm_ctime is set equal to the current time.

Return values

On success, shmget returns a non-negative integer, namely a shared memory identifier. On failure, shmget returns -1 and sets errno to identify the error.

Errors

In the following conditions, shmget fails and sets errno to:

EINVAL: size is less than the system-imposed minimum or greater than the system-imposed maximum.
EACCES: A shared memory identifier exists for key but operation permission (see intro(S)) 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 and associated shared memory segment are to be created but the system-imposed limit on the maximum number of shared memory segments system-wide would be exceeded.
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.
EEXIST: A shared memory identifier exists for key but both (shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.

References

intro(S), shmctl(S), shmop(S), stdipc(S)

Notices

The user must explicitly remove shared memory segments after the last reference to them has been removed.