SHMAT(2)                  FreeBSD System Calls Manual                 SHMAT(2)

NAME
     shmat, shmdt - attach or detach shared memory

LIBRARY
     Standard C Library (libc, -lc)

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

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

     int
     shmdt(const void *addr);

DESCRIPTION
     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 follows:

     •   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.

RETURN VALUES
     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.

ERRORS
     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
                        provided.

     [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.

SEE ALSO
     shmctl(2), shmget(2)

FreeBSD 13.1-RELEASE-p6        January 14, 2019        FreeBSD 13.1-RELEASE-p6