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Name

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.

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:

key
is equal to IPC_PRIVATE .
key
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:

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

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 one or more of the following are true:

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_CREATE ) and (shmflg&IPC_EXCL ) are true.
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 the size of the segment associated with it is less than size and size is not equal to 0.
ENOENT
A shared memory identifier does not exist for key and (shmflg&IPC_CREATE ) 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 Also

intro(2) , shmctl(2) , shmop(2) , stdipc(3C)

Notes

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

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