shmop(S)                       6 January 1993                       shmop(S)


 Name

    shmop: shmat, shmdt - shared memory operations

 Syntax


    cc  . . .  -lc


    #include  <sys/types.h>
    #include  <sys/ipc.h>
    #include  <sys/shm.h>

    char *shmat (shmid, shmaddr, shmflg)
    int shmid;
    char *shmaddr;
    int shmflg;

    int shmdt (shmaddr)
    char *shmaddr;


 Description

    The shmat system call attaches the shared memory segment associated with
    the shared memory identifier specified by shmid to the data segment of
    the calling process.  The segment is attached at the address specified by
    one of the following criteria:

    +  If shmaddr is equal to zero, the segment is attached at the first
       available address as selected by the system.

    +  If shmaddr is not equal to zero and (shmflg & SHMRND) is ``true'',
       the segment is attached at the address given by (shmaddr - (shmaddr
       modulus SHMLBA)).

    +  If shmaddr is not equal to zero and (shmflg & SHMRND) is ``false'',
       the segment is attached at the address given by shmaddr.

    shmdt detaches from the calling process's data segment the shared memory
    segment located at the address specified by shmaddr.

    The segment is attached for reading if (shmflg & SHMRDONLY) is ``true''
    {READ}; otherwise it is attached for reading and writing {READ/WRITE}.

    shmat fails and does not attach the shared memory segment if one or more
    of the following is true:

    [EACCES]    Operation permission is denied to the calling process (see
                Intro(S)).

    [EINVAL]    shmid is not a valid shared memory identifier.

    [EINVAL]    shmaddr is not equal to zero, and the value of (shmaddr -
                (shmaddr modulus SHMLBA)) is an illegal address.

    [EINVAL]    shmaddr is not equal to zero, (shmflg & SHMRND) is
                ``false'', and the value of shmaddr is an illegal address.

    [EINVAL]    shmdt fails and does not detach the shared memory segment if
                shmaddr is not the data segment start address of a shared
                memory segment.

    [EMFILE]    The number of shared memory segments attached to the calling
                process would exceed the system-imposed limit.

    [ENOMEM]    The available data space is not large enough to accommodate
                the shared memory segment.


 Diagnostics

    Upon successful completion, the return value is as follows:

    +  shmat returns the data segment start address of the attached shared
       memory segment.

    +  shmdt returns a value of 0.

    Otherwise, a value of -1 is returned, and errno is set to indicate the
    error.

 Notes

    The user must explicitly remove shared memory segments after the last
    reference to them has been removed.

 See also

    exec(S), exit(S), fork(S), shmctl(S), shmget(S)

 Standards conformance

    shmat and shmdt are conformant with:
    AT&T SVID Issue 2;
    and X/Open Portability Guide, Issue 3, 1989.