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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 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  per-
       mission  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 seg-
       ment.  (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  differ-
       ent 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.   How-
       ever, 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  (SHM-
       SEG).

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-03                          SHMOP(2)