SHMGET(2) | System Calls Manual | SHMGET(2)

# 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.

October 27, 2008 | NetBSD 11.0