SHMGET(2)                 FreeBSD System Calls Manual                SHMGET(2)

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)

FreeBSD 13.1-RELEASE-p6          March 4, 2018         FreeBSD 13.1-RELEASE-p6