shmdt(2) - NetBSD Manual Pages

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


NAME

     shmat, shmdt - map/unmap shared memory


SYNOPSIS

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

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

     int
     shmdt(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, otherwise
     it returns NULL. 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)