shmop(2)                                                           shmop(2)




 NAME    [Toc]    [Back]
      shmat(), shmdt() - shared memory operations

 SYNOPSIS    [Toc]    [Back]
      #include <sys/shm.h>

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

      int shmdt(void *shmaddr);

 DESCRIPTION    [Toc]    [Back]
      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 for reading if (shmflg & SHM_RDONLY) is
      "true"; otherwise, it is attached for reading and writing.  It is not
      possible to attach a segment for write only.

      If the shared memory segment has never been attached to by any process
      prior to the current shmat() call, shmaddr must be specified as zero
      and the segment is attached at a location selected by the operating
      system.  That location is identical in all processes accessing that
      shared memory object.  Once the operating system selects a location
      for a shared memory segment, the same location will be used across any
      subsequent shmat() and shmdt() calls on the segment until it is
      removed by the IPC_RMID operation of shmctl().  See exceptions for
      MPAS processes below.

      If this is not the first shmat() call on the shared memory segment
      throughout the system, shmaddr must either be zero or contain a
      nonzero address that is identical to the one returned from previous
      shmat() calls for that segment.  Even if no processes are currently
      attached to the segment, as long as the segment has been attached
      before, the same rule applies. See exceptions for MPAS processes
      below.

      If the calling process is already attached to the shared memory
      segment, shmat() fails and returns SHM_FAILED regardless of what value
      is passed in shmaddr.  See exceptions for MPAS processes below.

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

    Exceptions for MPAS Processes    [Toc]    [Back]
      On Itanium(R)-based platforms, MPAS (Mostly Private Address Space)
      processes are not restricted to passing in zero or a fixed value to
      calls to shmat().  MPAS processes may pass in other addresses.  The
      shmat() call may fail or succeed due to implementation dependent
      reasons.  MGAS processes have all the restrictions outlined above.  In
      addition, an MGAS process may not assume that it can attach at the



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 shmop(2)                                                           shmop(2)




      address that an MPAS process can use to attach to the same segment.

      An MPAS process may be able to attach to the same shared memory
      segment multiple times.  Success or failure of such an operation is
      implementation dependent.  Failure will be indicated by a return value
      of SHM_FAILED.

      An MPAS process should specify IPC_SHARE32 or IPC_GLOBAL flags in the
      call to shmat().  These follow the same rules as such flags passed to
      shmget(2).

      See the Adaptive Address Space Whitepaper for details.

 RETURN VALUE    [Toc]    [Back]
      shmat() returns the following values:

            n   Successful completion.  n is the data segment start address
                of the attached shared memory segment.
           SHM_FAILED
                Failure.  The shared memory segment is not attached.  errno
                is set to indicate the error.  The symbol SHM_FAILED is
                defined in the header <sys/shm.h>.  No successful return
                from shmat() will return the value SHM_FAILED.

      shmdt() returns the following values:

            0   Successful completion.
           -1   Failure.  errno is set to indicate the error.

 ERRORS    [Toc]    [Back]
      If shmat() fails, errno is set to one of the following values.

           [EACCES]       Operation permission is denied to the calling
                          process.

           [EINVAL]       shmid is not a valid shared memory identifier,
                          (possibly because the shared memory segment was
                          already removed using shmctl(2) with IPC_RMID), or
                          the calling process is already attached to shmid.

           [EINVAL]       shmaddr is not zero and the machine does not
                          permit nonzero values, or shmaddr is not equal to
                          the current attach location for the shared memory
                          segment.

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

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



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 shmop(2)                                                           shmop(2)




      If shmdt() fails, errno is set to one of the following values.

           [EINVAL]       shmaddr is not the data segment start address of a
                          shared memory segment.

 EXAMPLES    [Toc]    [Back]
      The following call to shmat() attaches the shared memory segment to
      the process.  This example assumes the process has a valid shmid,
      which can be obtained by calling shmget(2).

           char *shmptr;
           shmptr = (char *) shmat(myshmid, 0, 0);

      The following call to shmdt() then detaches the shared memory segment.

           shmdt (shmptr);

 SEE ALSO    [Toc]    [Back]
      ipcs(1), exec(2), exit(2), fork(2), ftok(3C), shmctl(2), shmget(2),
      Adaptive Address Space Whitepaper.

 STANDARDS CONFORMANCE    [Toc]    [Back]
      shmat(): SVID2, SVID3, XPG2, XPG3, XPG4

      shmdt(): SVID2, SVID3, XPG2, XPG3, XPG4


 Hewlett-Packard Company            - 3 -   HP-UX 11i Version 2: August 2003