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 shmop(2)							    shmop(2)




 NAME
      shmat(), shmdt() - shared memory operations

 SYNOPSIS
      #include <&lt&lt&lt;sys/shm.h>&gt&gt&gt;

      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 for reading if (shmflg &&amp&amp&amp; 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().

      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.

      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.

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

 RETURN VALUE
      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



 Hewlett-Packard Company	    - 1 -   HP-UX Release 11i: November 2000






 shmop(2)							    shmop(2)




		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
      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.

      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
      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
      ipcs(1), exec(2), exit(2), fork(2), ftok(3C), shmctl(2), shmget(2).





 Hewlett-Packard Company	    - 2 -   HP-UX Release 11i: November 2000






 shmop(2)							    shmop(2)




 STANDARDS CONFORMANCE
      shmat(): SVID2, SVID3, XPG2, XPG3, XPG4

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


















































 Hewlett-Packard Company	    - 3 -   HP-UX Release 11i: November 2000