shmat man page on FreeBSD

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SHMAT(2)		    BSD System Calls Manual		      SHMAT(2)

     shmat, shmdt — attach or detach shared memory

     Standard C Library (libc, -lc)

     #include <machine/param.h>
     #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 seg‐
     ment is attached is determined as follows:

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

     ·	 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.

     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

     The shmat() system call will fail if:

     [EINVAL]		No shared memory segment was found corresponding to

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

     The shmdt() system call will fail if:

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

     shmctl(2), shmget(2)

BSD				August 2, 1995				   BSD

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