SHM_MAP(9)             FreeBSD 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
     descriptors.

     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 memory 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
     object.  It maps the first sixteen kilobytes of the object into the
     kernel, performs some work on that address, and then unmaps the address
     before returning.

           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
                        extends 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 13.1-RELEASE-p6        December 14, 2011       FreeBSD 13.1-RELEASE-p6