shmop


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  speci-
       fied 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 mul-
       tiple 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 read-
       ing and the process must have read permission for the segment.   Other-
       wise  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 indi-
       cate that the mapping of the segment should replace any  existing  map-
       ping  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 spec-
       ified  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.


       After an execve(2) all attached shared  memory  segments  are  detached
       from the process.

       Upon _exit(2) 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  can't
              attach  segment  at  shmaddr,  or  SHM_REMAP  was  specified and
              shmaddr was NULL.

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

       When shmdt() fails, errno is set as follows:

       EINVAL There  is  no  shared  memory  segment  attached at shmaddr; or,
              shmaddr is not aligned on a page boundary.

CONFORMING TO
       SVr4, POSIX.1-2001.

       In SVID 3 (or perhaps earlier) 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 * proto-
       types; glibc2 has void *.)

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 spec-
       ify this behavior and many other implementations do not support it.

COLOPHON
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2008-06-03                          SHMOP(2)
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