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     shmat, shmdt — attach or detach shared memory


     Standard C Library (libc, -lc)


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

     void *
     shmat(int shmid, const void *addr, int flag);

     shmdt(const void *addr);


     The shmat() system call attaches the shared memory segment identified by shmid to the
     calling process's address space.  The address where the segment is attached is determined as

        If addr is 0, the segment is attached at an address selected by the kernel.

        If addr is nonzero and SHM_RND is not specified in flag, the segment is attached the
         specified address.

        If addr is specified and SHM_RND is specified, addr is rounded down to the nearest
         multiple of SHMLBA.

     If the SHM_REMAP flag is specified and the passed addr is not NULL, any existing mappings in
     the virtual addresses range are cleared before the segment is attached.  If the flag is not
     specified, addr is not NULL, and the virtual address range contains some pre-existing
     mappings, the shmat() call fails.

     The shmdt() system call detaches the shared memory segment at the address specified by addr
     from the calling process's address space.


     Upon success, shmat() returns the address where the segment is attached; otherwise, -1 is
     returned and errno is set to indicate the error.

     The shmdt() function returns the value 0 if successful; otherwise the value -1 is returned
     and the global variable errno is set to indicate the error.


     The shmat() system call will fail if:

     [EINVAL]           No shared memory segment was found corresponding to shmid.

     [EINVAL]           The addr argument was not an acceptable address.

     [ENOMEM]           The specified addr cannot be used for mapping, for instance due to the
                        amount of available space being smaller than the segment size, or because
                        pre-existing mappings are in the range and no SHM_REMAP flag was

     [EMFILE]           Failed to attach the shared memory segment because the per-process
                        kern.ipc.shmseg sysctl(3) limit was reached.

     The shmdt() system call will fail if:

     [EINVAL]           The addr argument does not point to a shared memory segment.


     shmctl(2), shmget(2)