shmat(2) - NetBSD Manual Pages

SHMAT(2)                  NetBSD Programmer's Manual                  SHMAT(2)


NAME

     shmat, shmdt - map/unmap shared memory


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/shm.h>

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

     int
     shmdt(const void *shmaddr);


DESCRIPTION

     shmat() maps the shared memory segment associated with the shared memory
     identifier shmid into the address space of the calling process. The ad-
     dress at which the segment is mapped is determined by the shmaddr parame-
     ter. If it is equal to 0, the system will pick an address itself.  Other-
     wise, an attempt is made to map the shared memory segment at the address
     shmaddr specifies. If SHM_RND is set in shmflg, the system will round the
     address down to a multiple of SHMLBA bytes (SHMLBA is defined in
     <sys/shm.h>).

     A shared memory segment can be mapped read-only by specifying the
     SHM_RDONLY flag in shmflg.

     shmdt() unmaps the shared memory segment that is currently mapped at
     shmaddr from the calling process' address space.  shmaddr must be a value
     returned by a prior shmat() call. A shared memory segment will remain in
     existence until it is removed by a call to shmctl(2) with the IPC_RMID
     command.


RETURN VALUES

     shmat() returns the address at which the shared memory segment has been
     mapped into the calling process' address space when successful, shmdt()
     returns 0 on successful completion. Otherwise, a value of -1 is returned,
     and the global variable errno is set to indicate the error.


ERRORS

     shmat() will fail if:

     [EACCES]      The calling process has no permission to access this shared
                   memory segment.

     [ENOMEM]      There is not enough available data space for the calling
                   process to map the shared memory segment.

     [EINVAL]      shmid is not a valid shared memory identifier.

                   shmaddr specifies an illegal address.

     [EMFILE]      The number of shared memory segments has reached the sys-
                   tem-wide limit.

     shmdt() will fail if:

     [EINVAL]      shmaddr is not the start address of a mapped shared memory
                   segment.


SEE ALSO

     mmap(2), shmctl(2), shmget(2)


STANDARDS

     The shmat and shmdt system calls conform to X/Open System Interfaces and
     Headers Issue 5 (``XSH5'').


HISTORY

     Shared memory segments appeared in the first release of AT&T System V
     UNIX.

NetBSD 1.6                      August 25, 1999                              2