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SHM_MAP(9)		    Kernel Developer's Manual		      SHM_MAP(9)

NAME
     shm_map,  shm_unmap  -- map shared memory objects into the kernel's address
     space

SYNOPSIS
     #include <sys/types.h>
     #include <sys/mman.h>

     int
     shm_map(struct file *fp, size_t size, off_t offset, void **memp);

     int
     shm_unmap(struct file *fp, void *mem, size_t size);

DESCRIPTION
     The shm_map() and shm_unmap() functions provide an API for  mapping  shared
     memory  objects  into  the  kernel.   Shared  memory objects are created by
     shm_open(2).  These objects can then be passed into the kernel via file de-
     scriptors.

     A shared memory object cannot be shrunk while it is mapped into the kernel.
     This is to avoid invalidating any pages that may be wired into the kernel's
     address space.  Shared memory objects can still be grown while mapped  into
     the kernel.

     To simplify the accounting needed to enforce the above requirement, callers
     of  this  API  are  required to unmap the entire region mapped by shm_map()
     when calling shm_unmap().	Unmapping only a portion of the  region  is  not
     permitted.

     The shm_map() function locates the shared memory object associated with the
     open  file  fp.   It maps the region of that object described by offset and
     size into the kernel's address space.  If it succeeds, *memp will be set to
     the start of the mapping.	All pages for the range will be wired into  mem-
     ory upon successful return.

     The  shm_unmap()  function  unmaps a region previously mapped by shm_map().
     The mem argument should match the value previously returned in  *memp,  and
     the size argument should match the value passed to shm_map().

     Note  that  shm_map()  will not hold an extra reference on the open file fp
     for the lifetime of the mapping.  Instead, the calling code is required  to
     do this if it wishes to use shm_unmap() on the region in the future.

RETURN VALUES
     The  shm_map() and shm_unmap() functions return zero on success or an error
     on failure.

EXAMPLES
     The following function accepts a file descriptor for a  shared  memory  ob-
     ject.   It  maps the first sixteen kilobytes of the object into the kernel,
     performs some work on that address, and then unmaps the address before  re-
     turning.

	   int
	   shm_example(int fd)
	   {
		   struct file *fp;
		   void *mem;
		   int error;

		   error = fget(curthread, fd, CAP_MMAP, &fp);
		   if (error)
			   return (error);
		   error = shm_map(fp, 16384, 0, &mem);
		   if (error) {
			   fdrop(fp, curthread);
			   return (error);
		   }

		   /* Do something with 'mem'. */

		   error = shm_unmap(fp, mem, 16384);
		   fdrop(fp, curthread);
		   return (error);
	   }

ERRORS
     The shm_map() function returns the following errors on failure:

     [EINVAL]		The open file fp is not a shared memory object.

     [EINVAL]		The  requested	region	described by offset and size ex-
			tends beyond the end of the shared memory object.

     [ENOMEM]		Insufficient address space was available.

     [EACCES]		The shared memory object could not be mapped  due  to  a
			protection error.

     [EINVAL]		The shared memory object could not be mapped due to some
			other VM error.

     The shm_unmap() function returns the following errors on failure:

     [EINVAL]		The open file fp is not a shared memory object.

     [EINVAL]		The  address  range  described	by mem and size is not a
			valid address range.

     [EINVAL]		The address range described  by  mem  and  size  is  not
			backed	by  the shared memory object associated with the
			open file fp, or the address range does  not  cover  the
			entire mapping of the object.

SEE ALSO
     shm_open(2)

HISTORY
     This API was first introduced in FreeBSD 10.0.

FreeBSD 8.4-stable		December 14, 2011		      SHM_MAP(9)
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