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

       shmat, shmdt - System V shared memory operations

       #include <sys/types.h>
       #include <sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

       int shmdt(const void *shmaddr);

       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.

       In  addition to SHM_RND, the following flags may be specified in the shmflg bit-mask argu‐

       SHM_EXEC (Linux-specific; since Linux 2.6.9)
              Allow the contents of the segment to be executed.  The  caller  must  have  execute
              permission on the segment.

              Attach the segment for read-only access.  The process must have read permission for
              the segment.  If this flag is not specified, the segment is attached for  read  and
              write  access, and the process must have read and write permission for the segment.
              There is no notion of a write-only shared memory segment.

       SHM_REMAP (Linux-specific)
              This flag specifies 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 cur‐
       rently 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.

       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.

       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 speci‐
              fied) 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.

       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 *.

       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.

       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 main‐
       tained 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 implemen‐
       tations do not support it.

       The following system parameter affects shmat():

       SHMLBA Segment  low  boundary  address  multiple.   When  explicitly  specifying an attach
              address in a call to shmat(), the caller should ensure that the address is a multi‐
              ple  of  this  value.   This is necessary on some architectures, in order either to
              ensure good CPU cache performance or to ensure that different attaches of the  same
              segment have consistent views within the CPU cache.  SHMLBA is normally some multi‐
              ple of the system page size (on many Linux architectures, it is  the  same  as  the
              system page size).

       The  implementation  places  no intrinsic per-process limit on the number of shared memory
       segments (SHMSEG).

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

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-07-08                                   SHMOP(2)

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