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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

       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 specified 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  specified  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  (Linux-specific)  SHM_REMAP  flag  may  be  specified in shmflg to
       indicate that the mapping of the segment should  replace  any  existing
       mapping in the range starting at shmaddr and continuing for the size of
       the segment.  (Normally an EINVAL  error  would  result  if  a  mapping
       already  exists in this address range.)  In this case, shmaddr must not
       be NULL.

       The brk(2) value of the calling process is not altered by  the  attach.
       The  segment  will automatically 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.

       A successful shmat() call updates the members of the shmid_ds structure
       (see shmctl(2)) associated with 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.

       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 attaching shmat() call.

       On  a  successful  shmdt()  call  the system updates the members of the
       shmid_ds  structure  associated  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.

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  success  shmat()  returns the address of the attached shared memory
       segment; on error (void *) -1 is returned, and errno is set to indicate
       the cause of the error.

       On success shmdt() returns 0; on error -1 is returned, and errno is set
       to indicate the cause of the error.

ERRORS

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

       EACCES The calling process does not have the required  permissions  for
              the  requested  attach type, and does not have the CAP_IPC_OWNER
              capability.

       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(), or SHM_REMAP  was  specified  and  shmaddr  was
              NULL.

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

       shmdt() can fail only if there is no shared memory segment attached  at
       shmaddr; in this case, 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  maintained  within  the
       shared  memory must be made relative (typically to the starting address
       of the segment), rather than absolute.

       On Linux, it is possible to attach a shared memory segment even  if  it
       is  already  marked  to  be  deleted.   However,  POSIX.1-2001 does not
       specify this behaviour and many other implementations  do  not  support
       it.

       The following system parameter affects shmat():

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

       The implementation places no intrinsic limit on the per-process maximum
       number of shared memory 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

       brk(2), mmap(2), shmctl(2), shmget(2), ipc(5), capabilities(7)