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@(#)mmap.2 8.4 (Berkeley) 5/11/95
$FreeBSD: releng/10.2/lib/libc/sys/mmap.2 275837 2014-12-16 20:05:10Z jhb $
.Dd September 17, 2014 .Dt MMAP 2 .Os .Sh NAME .Nm mmap .Nd allocate memory, or map files or devices into memory .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/mman.h .Ft void * .Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset" .Sh DESCRIPTION The .Fn mmap system call causes the pages starting at .Fa addr and continuing for at most .Fa len bytes to be mapped from the object described by .Fa fd , starting at byte offset .Fa offset . If .Fa len is not a multiple of the pagesize, the mapped region may extend past the specified range. Any such extension beyond the end of the mapped object will be zero-filled.
p If .Fa addr is non-zero, it is used as a hint to the system. (As a convenience to the system, the actual address of the region may differ from the address supplied.) If .Fa addr is zero, an address will be selected by the system. The actual starting address of the region is returned. A successful .Fa mmap deletes any previous mapping in the allocated address range.
p The protections (region accessibility) are specified in the .Fa prot argument by .Em or Ns 'ing the following values:
p l -tag -width PROT_WRITE -compact t Dv PROT_NONE Pages may not be accessed. t Dv PROT_READ Pages may be read. t Dv PROT_WRITE Pages may be written. t Dv PROT_EXEC Pages may be executed. .El
p The .Fa flags argument specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Sharing, mapping type and options are specified in the .Fa flags argument by .Em or Ns 'ing the following values: l -tag -width MAP_PREFAULT_READ t Dv MAP_32BIT Request a region in the first 2GB of the current process's address space. If a suitable region cannot be found, .Fn mmap will fail. This flag is only available on 64-bit platforms. t Dv MAP_ALIGNED Ns Pq Fa n Align the region on a requested boundary. If a suitable region cannot be found, .Fn mmap will fail. The .Fa n argument specifies the binary logarithm of the desired alignment. t Dv MAP_ALIGNED_SUPER Align the region to maximize the potential use of large
q Dq super
pages.
If a suitable region cannot be found,
.Fn mmap
will fail.
The system will choose a suitable page size based on the size of
mapping.
The page size used as well as the alignment of the region may both be
affected by properties of the file being mapped.
In particular,
the physical address of existing pages of a file may require a specific
alignment.
The region is not guaranteed to be aligned on any specific boundary.
t Dv MAP_ANON Map anonymous memory not associated with any specific file.
The file descriptor used for creating
.Dv MAP_ANON
must be -1.
The
.Fa offset
argument must be 0.
.It Dv MAP_FILE
Mapped from a regular file or character-special device memory.
t Dv MAP_ANONYMOUS This flag is identical to
.Dv MAP_ANON
and is provided for compatibility.
t Dv MAP_EXCL This flag can only be used in combination with
.Dv MAP_FIXED .
Please see the definition of
.Dv MAP_FIXED
for the description of its effect.
t Dv MAP_FIXED Do not permit the system to select a different address than the one
specified.
If the specified address cannot be used,
.Fn mmap
will fail.
If
.Dv MAP_FIXED
is specified,
.Fa addr
must be a multiple of the pagesize.
If
.Dv MAP_EXCL
is not specified, a successfull
.Dv MAP_FIXED
request replaces any previous mappings for the process'
pages in the range from
.Fa addr
to
.Fa addr
+
.Fa len .
In contrast, if
.Dv MAP_EXCL
is specified, the request will fail if a mapping
already exists within the range.
t Dv MAP_HASSEMAPHORE Notify the kernel that the region may contain semaphores and that special
handling may be necessary.
t Dv MAP_NOCORE Region is not included in a core file.
t Dv MAP_NOSYNC Causes data dirtied via this VM map to be flushed to physical media
only when necessary (usually by the pager) rather than gratuitously.
Typically this prevents the update daemons from flushing pages dirtied
through such maps and thus allows efficient sharing of memory across
unassociated processes using a file-backed shared memory map.
Without
this option any VM pages you dirty may be flushed to disk every so often
(every 30-60 seconds usually) which can create performance problems if you
do not need that to occur (such as when you are using shared file-backed
mmap regions for IPC purposes).
Note that VM/file system coherency is
maintained whether you use
.Dv MAP_NOSYNC
or not.
This option is not portable
across
x platforms (yet), though some may implement the same behavior
by default.
p .Em WARNING ! Extending a file with .Xr ftruncate 2 , thus creating a big hole, and then filling the hole by modifying a shared .Fn mmap can lead to severe file fragmentation. In order to avoid such fragmentation you should always pre-allocate the file's backing store by .Fn write Ns ing zero's into the newly extended area prior to modifying the area via your .Fn mmap . The fragmentation problem is especially sensitive to .Dv MAP_NOSYNC pages, because pages may be flushed to disk in a totally random order.
p The same applies when using .Dv MAP_NOSYNC to implement a file-based shared memory store. It is recommended that you create the backing store by .Fn write Ns ing zero's to the backing file rather than .Fn ftruncate Ns ing it. You can test file fragmentation by observing the KB/t (kilobytes per transfer) results from an .Dq Li iostat 1 while reading a large file sequentially, e.g. using .Dq Li dd if=filename of=/dev/null bs=32k .
p The .Xr fsync 2 system call will flush all dirty data and metadata associated with a file, including dirty NOSYNC VM data, to physical media. The .Xr sync 8 command and .Xr sync 2 system call generally do not flush dirty NOSYNC VM data. The .Xr msync 2 system call is usually not needed since x implements a coherent file system buffer cache. However, it may be used to associate dirty VM pages with file system buffers and thus cause them to be flushed to physical media sooner rather than later. t Dv MAP_PREFAULT_READ Immediately update the calling process's lowest-level virtual address translation structures, such as its page table, so that every memory resident page within the region is mapped for read access. Ordinarily these structures are updated lazily. The effect of this option is to eliminate any soft faults that would otherwise occur on the initial read accesses to the region. Although this option does not preclude .Fa prot from including .Dv PROT_WRITE , it does not eliminate soft faults on the initial write accesses to the region. t Dv MAP_PRIVATE Modifications are private. t Dv MAP_SHARED Modifications are shared. t Dv MAP_STACK .Dv MAP_STACK implies .Dv MAP_ANON , and .Fa offset of 0. The .Fa fd argument must be -1 and .Fa prot must include at least .Dv PROT_READ and .Dv PROT_WRITE . This option creates a memory region that grows to at most .Fa len bytes in size, starting from the stack top and growing down. The stack top is the starting address returned by the call, plus .Fa len bytes. The bottom of the stack at maximum growth is the starting address returned by the call. .El
p The .Xr close 2 system call does not unmap pages, see .Xr munmap 2 for further information. .Sh NOTES Although this implementation does not impose any alignment restrictions on the .Fa offset argument, a portable program must only use page-aligned values.
p Large page mappings require that the pages backing an object be aligned in matching blocks in both the virtual address space and RAM. The system will automatically attempt to use large page mappings when mapping an object that is already backed by large pages in RAM by aligning the mapping request in the virtual address space to match the alignment of the large physical pages. The system may also use large page mappings when mapping portions of an object that are not yet backed by pages in RAM. The .Dv MAP_ALIGNED_SUPER flag is an optimization that will align the mapping request to the size of a large page similar to .Dv MAP_ALIGNED , except that the system will override this alignment if an object already uses large pages so that the mapping will be consistent with the existing large pages. This flag is mostly useful for maximizing the use of large pages on the first mapping of objects that do not yet have pages present in RAM. .Sh RETURN VALUES Upon successful completion, .Fn mmap returns a pointer to the mapped region. Otherwise, a value of .Dv MAP_FAILED is returned and .Va errno is set to indicate the error. .Sh ERRORS The .Fn mmap system call will fail if: l -tag -width Er t Bq Er EACCES The flag .Dv PROT_READ was specified as part of the .Fa prot argument and .Fa fd was not open for reading. The flags .Dv MAP_SHARED and .Dv PROT_WRITE were specified as part of the .Fa flags and .Fa prot argument and .Fa fd was not open for writing. t Bq Er EBADF The .Fa fd argument is not a valid open file descriptor. t Bq Er EINVAL An invalid value was passed in the .Fa prot argument. t Bq Er EINVAL An undefined option was set in the .Fa flags argument. t Bq Er EINVAL Both .Dv MAP_PRIVATE and .Dv MAP_SHARED were specified. t Bq Er EINVAL None of .Dv MAP_ANON , .Dv MAP_PRIVATE , .Dv MAP_SHARED , or .Dv MAP_STACK was specified. At least one of these flags must be included. t Bq Er EINVAL .Dv MAP_FIXED was specified and the .Fa addr argument was not page aligned, or part of the desired address space resides out of the valid address space for a user process. t Bq Er EINVAL Both .Dv MAP_FIXED and .Dv MAP_32BIT were specified and part of the desired address space resides outside of the first 2GB of user address space. t Bq Er EINVAL The .Fa len argument was equal to zero. t Bq Er EINVAL .Dv MAP_ALIGNED was specified and the desired alignment was either larger than the virtual address size of the machine or smaller than a page. t Bq Er EINVAL .Dv MAP_ANON was specified and the .Fa fd argument was not -1. t Bq Er EINVAL .Dv MAP_ANON was specified and the .Fa offset argument was not 0. t Bq Er EINVAL Both .Dv MAP_FIXED and .Dv MAP_EXCL were specified, but the requested region is already used by a mapping. t Bq Er EINVAL .Dv MAP_EXCL was specified, but .Dv MAP_FIXED was not. t Bq Er ENODEV .Dv MAP_ANON has not been specified and .Fa fd did not reference a regular or character special file. t Bq Er ENOMEM .Dv MAP_FIXED was specified and the .Fa addr argument was not available. .Dv MAP_ANON was specified and insufficient memory was available. .El .Sh SEE ALSO .Xr madvise 2 , .Xr mincore 2 , .Xr minherit 2 , .Xr mlock 2 , .Xr mprotect 2 , .Xr msync 2 , .Xr munlock 2 , .Xr munmap 2 , .Xr getpagesize 3 , .Xr getpagesizes 3 .Sh BUGS The .Fa len argument is limited to the maximum file size or available userland address space. Files may not be able to be made more than 1TB large on 32 bit systems due to file systems restrictions and bugs, but address space is far more restrictive. Larger files may be possible on 64 bit systems.
p The previous documented limit of 2GB was a documentation bug. That limit has not existed since .Fx 2.2 .