shmop(2)                                                           shmop(2)

NAME
     shmop: shmat, shmdt - shared memory operations

SYNOPSIS
     #include <sys/shm.h>

     void *shmat(int shmid, const void *shmaddr, int shmflg);

     int shmdt (const void *shmaddr);

DESCRIPTION
     shmat() attaches the shared memory segment associated with the shared
     memory identifier specified by shmid to the data segment of the cal-
     ling process. The segment is attached at the address specified by one
     of the following criteria:

     -  If shmaddr is equal to (void *) 0, the segment is attached at the
        first available address as selected by the system.

     -  If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is true,
        the segment is attached at the address given by
        (shmaddr - ((ptrdifft)shmaddr%SHMLBA)).

     -  If shmaddr is not equal to (void *) 0 and (shmflg&SHMRND) is
        false, the segment is attached at the address given by shmaddr.

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

     The segment is attached for reading if (shmflg&SHMRDONLY) is true
     otherwise it is attached for reading and writing.

     shmat() fails and does not attach the shared memory segment if one or
     more of the following apply:

     EINVAL    shmid is not a valid shared memory identifier.

     EACCES    Operation permission is denied to the calling process.

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

     EINVAL    shmaddr is not equal to zero, and the value of
               (shmaddr - ((ptrdifft)shmaddr%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.

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




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shmop(2)                                                           shmop(2)

     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.

     EINVAL    This executable is a specially preloaded binary. The argu-
               ment shmaddr lies illegally between the text and data
               regions.

RESULT
     Upon successful completion, the result 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.

     In the future there may be modifications to the interfaces for inter-
     process communication. You should design your applications so that in
     modules using shmop() this function can easily be replaced by others.

SEE ALSO
     exec(2), exit(2), fork(2), shmctl(2), shmget(2), preload(8).

























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