NAME

       shmget — obtain a shared memory identifier

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/shm.h>

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

DESCRIPTION

       Based  on  the  values  of key and flag, shmget() returns the identifier of a newly created or previously
       existing shared memory segment.  The key is analogous to a filename: it provides a handle that  names  an
       IPC object.  There are three ways to specify a key:

       •   IPC_PRIVATE may be specified, in which case a new IPC object will be created.

       •   An  integer  constant  may  be specified.  If no IPC object corresponding to key is specified and the
           IPC_CREAT bit is set in flag, a new one will be created.

       •   The ftok(3) may be used to generate a key from a pathname.

       The mode of a newly created IPC object is determined by which are set by ORing these constants  into  the
       flag argument:

       0400
         Read access for owner.

       0200
         Write access for owner.

       0040
         Read access for group.

       0020
         Write access for group.

       0004
         Read access for other.

       0002
         Write access for other.

       When  creating  a new shared memory segment, size indicates the desired size of the new segment in bytes.
       The size of the segment may be rounded up to a multiple convenient to the kernel (i.e., the page size).

RETURN VALUES

       Upon successful completion, shmget() returns the positive integer identifier of a shared memory  segment.
       Otherwise, -1 is returned and errno set to indicate the error.

ERRORS

       The shmget() system call will fail if:

       [EINVAL]           Size  specified  is  greater  than  the size of the previously existing segment.  Size
                          specified is less than the system imposed minimum, or greater than the system  imposed
                          maximum.

       [ENOENT]           No shared memory segment was found matching key, and IPC_CREAT was not specified.

       [ENOSPC]           The kernel was unable to allocate enough memory to satisfy the request.

       [EEXIST]           IPC_CREAT  and  IPC_EXCL  were specified, and a shared memory segment corresponding to
                          key already exists.

SEE ALSO

       shmat(2), shmctl(2), shmdt(2), ftok(3)

Debian                                            March 4, 2018                                        SHMGET(2)