shmop(2)                                                              shmop(2)


NAME
     shmop: shmat, shmdt - shared memory operations

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

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

     int shmdt (void *shmaddr);

DESCRIPTION
     shmat 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 (void *) 0, the segment is attached at the
          first available address as selected by the system.  If
          (shmflg&SHM_SGI_ANYADDR) is true, the segment is attached at the
          first available address, otherwise the system selects an available
          address that is outside of the reserved area between 0x30000000 and
          0x40000000.  See mmap(2) and sgi_use_anyaddr(1) for more information
          on the reserved range.

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

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

     The actual argument corresponding to shmaddr is evaluated by setting an
     INTEGER*4 variable to the location of the process's data segment, in
     other words, the value returned by %loc(3F).

     Under certain conditions, performance can be greatly improved by sharing
     mapping resources among processes attached to the same SHM segment. In
     order to achieve sharing of mapping resources, the following conditions
     must hold:

     -   the size of the SHM segment is a multiple of the constant SHMSUS (SHM
         Segment Unit Size).

     -   the attaching address shmaddr is a multiple of SHMSUS.

     -   (shmflg&SHM_RDONLY) is false

     The caller authorizes the system to use shared mapping resources by
     setting the flag SHM_SHATTR in shmflg.


     If (shmflg&SHM_SHATTR) is true and the above conditions are met the
     system uses shared mapping resources.

     If (shmflg&SHM_SHATTR) is true and any of the above conditions are false
     the system call fails.

     Use of the SHM_SHATTR flag is discouraged in the following situations:

     -    if the calling process is likely to unmap (munmap(2)) or to re-map
          (mmap(2)) large portions of the SHM segment, or

     -    if the calling process is likely to change the mapping attributes
          (access permissions and cache policies) on large portions of the SHM
          segment (mprotect(2)).

     These behaviors cause the system to make private copies of mapping
     resources and stop sharing, thereby defeating the purpose of the
     SHM_SHATTR capability.

     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&SHM_RDONLY) 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 are true:

     EINVAL         shmid is not a valid shared memory identifier.

     EACCES         Operation permission is denied to the calling process [see
                    intro(2)].

     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 -
                    (shmaddr modulus SHMLBA)) is an illegal address.

     EINVAL         shmaddr is not equal to zero, (shmflg&SHM_RND) 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.

     EINVAL         shmaddr is not a multiple of SHMSUS and
                    (shmflg&SHM_SHATTR) is true, or the size of the SHM
                    segment is not a multiple of SHMSUS and
                    (shmflg&SHM_SHATTR) is true.


     EACCES         (shmflg&SHM_SHATTR) is true and (shmflg&SHM_RDONLY) is
                    true.

     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.

SEE ALSO
     sgi_use_anyaddr(1), exec(2), exit(2), fork(2), intro(2), shmctl(2),
     shmget(2), sproc(2).

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.


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