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RedHat 9 (Linux i386) - man page for shmdt (redhat section 2)

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SHMOP(2)			    Linux Programmer's Manual				 SHMOP(2)

NAME
       shmop - shared memory operations

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

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

       int shmdt(const void *shmaddr);

DESCRIPTION
       The  function  shmat attaches the shared memory segment identified by shmid to the address
       space of the calling process.  The attaching address is specified by shmaddr with  one  of
       the following criteria:

       If  shmaddr is NULL, the system chooses a suitable (unused) address at which to attach the
       segment.

       If shmaddr isn't NULL and SHM_RND is asserted in shmflg, the attach occurs at the  address
       equal  to  shmaddr rounded down to the nearest multiple of SHMLBA.  Otherwise shmaddr must
       be a page aligned address at which the attach occurs.

       If SHM_RDONLY is asserted in shmflg, the segment is attached for reading and  the  process
       must have read permission for the segment.  Otherwise the segment is attached for read and
       write and the process must have read and write permission for the segment.   There  is  no
       notion of a write-only shared memory segment.

       The brk value of the calling process is not altered by the attach.  The segment will auto-
       matically be detached at process exit.  The same segment may be attached as a read and  as
       a read-write one, and more than once, in the process's address space.

       On  a successful shmat call the system updates the members of the shmid_ds structure asso-
       ciated to the shared memory segment as follows:

	      shm_atime is set to the current time.

	      shm_lpid is set to the process-ID of the calling process.

	      shm_nattch is incremented by one.

       Note that the attach succeeds also if the shared memory segment is marked to be deleted.

       The function shmdt detaches the shared memory segment located at the address specified  by
       shmaddr from the address space of the calling process.  The to-be-detached segment must be
       currently attached with shmaddr equal to the value returned by  the  its  attaching  shmat
       call.

       On  a successful shmdt call the system updates the members of the shmid_ds structure asso-
       ciated with the shared memory segment as follows:

	      shm_dtime is set to the current time.

	      shm_lpid is set to the process-ID of the calling process.

	      shm_nattch is decremented by one.  If it becomes 0 and the segment  is  marked  for
	      deletion, the segment is deleted.

       The occupied region in the user space of the calling process is unmapped.

SYSTEM CALLS
       fork() After a fork() the child inherits the attached shared memory segments.

       exec() After an exec() all attached shared memory segments are detached from the process.

       exit() Upon exit() all attached shared memory segments are detached from the process.

RETURN VALUE
       On  failure  both  functions  return -1 with errno indicating the error.  On success shmat
       returns the address of the attached shared memory segment, and shmdt returns 0.

ERRORS
       When shmat fails, errno is set to one of the following:

       EACCES	  The calling process has no access permissions for the requested attach type.

       EINVAL	  Invalid shmid value, unaligned (i.e., not  page-aligned  and	SHM_RND  was  not
		  specified) or invalid shmaddr value, or failing attach at brk.

       ENOMEM	  Could not allocate memory for the descriptor or for the page tables.

       The function shmdt can fail only if there is no shared memory segment attached at shmaddr,
       in such a case at return errno will be set to EINVAL.

NOTES
       Using shmat with shmaddr equal to NULL is the  preferred,  portable  way  of  attaching	a
       shared  memory  segment.  Be aware that the shared memory segment attached in this way may
       be attached at different addresses in different processes.  Therefore, any pointers  main-
       tained  within  the shared memory must be made relative (typically to the starting address
       of the segment), rather than absolute.

       The following system parameter affects a shmat system call:

       SHMLBA	  Segment low boundary address multiple.  Must be page aligned.  For the  current
		  implementation the SHMBLA value is PAGE_SIZE.

       The implementation has no intrinsic limit to the per-process maximum number of shared mem-
       ory segments (SHMSEG).

CONFORMING TO
       SVr4, SVID.  SVr4 documents an additional error condition EMFILE.  In SVID-v4 the type  of
       the  shmaddr  argument was changed from char * into const void *, and the returned type of
       shmat() from char * into void *.  (Linux libc4 and  libc5  have	the  char  *  prototypes;
       glibc2 has void *.)

SEE ALSO
       ipc(5), shmctl(2), shmget(2)

Linux 2.5				    2002-01-05					 SHMOP(2)
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