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SHMAT(2)		       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  seg-
     ment 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 ex-
     isting  mappings in the virtual addresses range are cleared before the seg-
     ment 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  in-
			stance	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 pro-
			vided.

     [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 seg-
			ment.

SEE ALSO
     shmctl(2), shmget(2)

FreeBSD ports 15.quarterly	January 14, 2019			SHMAT(2)

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