shmget - get shared memory segment identifier
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 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:
o The key argument is equal to IPC_PRIVATE.
o The key argument does not already have a shared memory
identifier associated with it, and (shmflg&IPC_CREAT)
Upon creation, the data structure associated with the new
shared memory identifier is initialized as follows:
o 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, respec-
tively, of the calling process.
o 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.
o The values of shm_lpid, shm_nattch shm_atime, and
shm_dtime are set equal to 0.
o 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.
Upon successful completion, a non-negative integer
representing a shared memory identifier is returned. Other-
wise, -1 is returned and errno is set to indicate the error.
The shmget() function will fail if:
A shared memory identifier exists for key but opera-
tion permission (see intro(2)) as specified by the
low-order 9 bits of shmflg would not be granted.
A shared memory identifier exists for key but both
(shmflg&IPC_CREAT) and (shmflg&IPC_EXCL) are true.
The size argument is less than the system-imposed
minimum or greater than the system-imposed maximum.
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.
A shared memory identifier does not exist for key and
(shmflg&IPC_CREAT) is false.
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
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
When a UFS file system is mounted with logging enabled, file
system transactions that free blocks from files might not
actually add those freed blocks to the file system's free
list until some unspecified time in the future. This
behavior improves file system performance but does not con-
form to the POSIX, Single UNIX Specification, SPARC Confor-
mance Definition, System V Application Binary Interface,
System V Interface Definition, and X/Open Portability Guide
Standards, which require that freed space be available
immediately. To enable standards conformance regarding file
deletions or to address the problem of not being able to
grow files on a relatively full UFS file system even after
files have been deleted, disable UFS logging (see
mount_ufs(1M), intro(2), shmctl(2), shmop(2), ftok(3C)
Man(1) output converted with