NAME

       shmat, shmdt - System V 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  System  V  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.

       After a fork(2) the child inherits the attached shared memory segments.

       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.

       EIDRM  shmid points to a removed identifier.

       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 *
       prototypes; 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 specify this behavior 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).

SEE ALSO

       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)

COLOPHON

       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                              2013-02-12                                           SHMOP(2)