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



NAME
       shmat, shmdt - 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 attach-
       ing 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.

       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 3.25 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                                                      2008-06-03                                                   SHMOP(2)