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SHMGET(2)                DragonFly System Calls Manual               SHMGET(2)


NAME

     shmget -- obtain a shared memory identifier


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <machine/param.h>
     #include <sys/types.h>
     #include <sys/ipc.h>
     #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:

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

     o   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.

     o   ftok() may be used to generate a key from a pathname.  See ftok(3).

     The mode of a newly created IPC object is determined by OR'ing the
     following constants into the flag parameter:

     SHM_R         Read access for user.

     SHM_W         Write access for user.

     (SHM_R>>3)    Read access for group.

     (SHM_W>>3)    Write access for group.

     (SHM_R>>6)    Read access for other.

     (SHM_W>>6)    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.


ENVIRONMENT

     The XSI Interprocess Communication family of functions is also available
     as an implementation in userspace.  To use it, the sysvipcd(8) daemon has
     to be running.

     If the USR_SYSVIPC variable is set in a process' environment, the process
     and its children will use the userspace implementation.


ERRORS

     Shmget() 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)


AUTHORS

     The DragonFly specific userspace implementation (see ENVIRONMENT) was
     written by Larisa Grigore.

DragonFly 4.3                   January 4, 2014                  DragonFly 4.3

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