Updated: 2021/Apr/14

SHMAT(2)                      System Calls Manual                     SHMAT(2)

     shmat, shmdt - map/unmap shared memory

     Standard C Library (libc, -lc)

     #include <sys/shm.h>

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

     shmdt(const void *shmaddr);

     shmat() maps the shared memory segment associated with the shared memory
     identifier shmid into the address space of the calling process.  The
     address at which the segment is mapped is determined by the shmaddr
     parameter.  If it is equal to 0, the system will pick an address itself.
     Otherwise, 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

     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.

     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
                        system-wide limit.

     shmdt() will fail if:

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

     ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)

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

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

NetBSD 9.99                      June 17, 2002                     NetBSD 9.99