October 27, 2008 SHMGET 2

### NAME

shmget - get shared memory segment

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```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`.