NAME

shmget — get shared memory segment

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/shm.h>

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

DESCRIPTION

shmget() returns the shared memory identifier associated with the key key.

A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated with it, and the IPC_CREAT bit is set in shmflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in shmflg, and key has a shared memory segment identifier associated with it already, the operation will fail.

If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:

• shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
• shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
• shm_perm.mode is set to the lower 9 bits of shmflg.
• shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
• shm_ctime is set to the current time.
• shm_segsz is set to the value of size.

RETURN VALUES

Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS

[EACCES]

A shared memory segment is already associated with key and the caller has no permission to access it.

[EEXIST]

Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.

[EINVAL]

No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associated with it is less than size, which is non-zero.

A shared memory segment is to be created, and size is less than the system imposed minimum, or greater than the system imposed maximum (refer to the kern.ipc values in sysctl(7)).

[ENOENT]

IPC_CREAT is not set in shmflg and no shared memory segment associated with key was found.

[ENOMEM]

There is not enough memory left to create a shared memory segment of the requested size.

[ENOSPC]

A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.

SEE ALSO

ipcrm(1), ipcs(1), mmap(2), shmat(2), shmctl(2), ftok(3), sysctl(7)

STANDARDS

The shmget system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).

HISTORY

Shared memory segments appeared in the first release of AT&T System V UNIX.