NAME

shmget − get shared memory segment identifier

SYNOPSIS

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

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

shmget fails if one or more of the following are true:

­[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(2)] 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 memory is not sufficient to fill the request. 

­[EEXIST] A shared memory identifier exists for key but ((shmflg & IPC_CREAT) and (shmflg& IPC_EXCL)) is “true”. 

SEE ALSO

intro(2), shmctl(2), shmop(2). 

DIAGNOSTICS

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. 

NOTES

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

September 29, 2021