SHMGET(2) — SYSTEM CALLS

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.

A shared memory identifier (shmid) is a unique positive integer created by a shmget(2) system call. Each shmid has a segment of memory (referred to as a shared memory segment) and a data structure associated with it. The data structure is referred to as shmid_ds and contains the following members:

structipc_perm shm_perm; /∗ operation permission struct ∗/
intshm_segsz;/∗ size of segment ∗/
ushortshm_cpid;/∗ creator pid ∗/
ushortshm_lpid;/∗ pid of last operation ∗/
shortshm_nattch;/∗ number of current attaches ∗/
time_tshm_atime;/∗ last attach time ∗/
time_tshm_dtime;/∗ last detach time ∗/
time_tshm_ctime;/∗ last change time ∗/
/∗ Times measured in secs since ∗/
/∗ 00:00:00 GMT, Jan. 1, 1970 ∗/

shm_perm is an ipc_perm structure that specifies the shared memory operation permission (see below). This structure includes the following members:

ushortcuid;/∗ creator user id ∗/
ushortcgid;/∗ creator group id ∗/
ushortuid;/∗ user id ∗/
ushortgid;/∗ group id ∗/
ushortmode;/∗ r/w permission ∗/

shm_segsz specifies the size of the shared memory segment. shm_cpid is the process ID of the process that created the shared memory identifier. shm_lpid is the process ID of the last process that performed a shmop(2) operation. shm_nattch is the number of processes that currently have this segment attached. shm_atime is the time of the last shmat operation, shm_dtime is the time of the last shmdt operation, and shm_ctime is the time of the last shmctl(2) operation that changed one of the members of the above structure.

RETURN VALUES

shmget() returns a non-negative shared memory identifier on success. On failure, it returns −1 and sets errno to indicate the error.

ERRORS

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 ( (shmflg & IPC_CREAT) && ( shmflg & IPC_EXCL) ) is “true”.

EINVAL size 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 zero.

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 physical 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.

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