Ubuntu Manpage: mmap — allocate memory, or map files or devices into memory

Provided by: freebsd-manpages_10.1~RC1-1_all

NAME

       mmap — allocate memory, or map files or devices into memory

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/mman.h>

       void *
       mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);

DESCRIPTION

The mmap() system call causes the pages starting at addr and continuing for at most len bytes to be
mapped from the object described by fd, starting at byte offset offset. If 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.

If 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 addr is zero, an address will be
selected by the system. The actual starting address of the region is returned. A successful mmap
deletes any previous mapping in the allocated address range.

The protections (region accessibility) are specified in the prot argument by or'ing the following values:

PROT_NONE Pages may not be accessed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_EXEC Pages may be executed.

The 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 flags argument by or'ing the following values:

MAP_32BIT Request a region in the first 2GB of the current process's address space. If a
suitable region cannot be found, mmap() will fail. This flag is only available on
64-bit platforms.

MAP_ALIGNED(n) Align the region on a requested boundary. If a suitable region cannot be found,
mmap() will fail. The n argument specifies the binary logarithm of the desired
alignment.

MAP_ALIGNED_SUPER Align the region to maximize the potential use of large (“super”) pages. If a
suitable region cannot be found, 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.

MAP_ANON Map anonymous memory not associated with any specific file. The file descriptor used
for creating MAP_ANON must be -1. The offset argument must be 0.

MAP_ANONYMOUS This flag is identical to MAP_ANON and is provided for compatibility.

MAP_EXCL This flag can only be used in combination with MAP_FIXED. Please see the definition
of MAP_FIXED for the description of its effect.

MAP_FIXED Do not permit the system to select a different address than the one specified. If the
specified address cannot be used, mmap() will fail. If MAP_FIXED is specified, addr
must be a multiple of the pagesize. If MAP_EXCL is not specified, a successfull
MAP_FIXED request replaces any previous mappings for the process' pages in the range
from addr to addr + len. In contrast, if MAP_EXCL is specified, the request will fail
if a mapping already exists within the range.

MAP_HASSEMAPHORE Notify the kernel that the region may contain semaphores and that special handling may
be necessary.

MAP_INHERIT This flag never operated as advertised and is no longer supported. Please refer to
minherit(2) for further information.

MAP_NOCORE Region is not included in a core file.

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
MAP_NOSYNC or not. This option is not portable across Unix platforms (yet), though
some may implement the same behavior by default.

WARNING! Extending a file with ftruncate(2), thus creating a big hole, and then
filling the hole by modifying a shared mmap() can lead to severe file fragmentation.
In order to avoid such fragmentation you should always pre-allocate the file's backing
store by write()ing zero's into the newly extended area prior to modifying the area
via your mmap(). The fragmentation problem is especially sensitive to MAP_NOSYNC
pages, because pages may be flushed to disk in a totally random order.

The same applies when using MAP_NOSYNC to implement a file-based shared memory store.
It is recommended that you create the backing store by write()ing zero's to the
backing file rather than ftruncate()ing it. You can test file fragmentation by
observing the KB/t (kilobytes per transfer) results from an “iostat 1” while reading a
large file sequentially, e.g. using “dd if=filename of=/dev/null bs=32k”.

The fsync(2) system call will flush all dirty data and metadata associated with a
file, including dirty NOSYNC VM data, to physical media. The sync(8) command and
sync(2) system call generally do not flush dirty NOSYNC VM data. The msync(2) system
call is usually not needed since BSD 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.

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
prot from including PROT_WRITE, it does not eliminate soft faults on the initial write
accesses to the region.

MAP_PRIVATE Modifications are private.

MAP_SHARED Modifications are shared.

MAP_STACK MAP_STACK implies MAP_ANON, and offset of 0. The fd argument must be -1 and prot must
include at least PROT_READ and PROT_WRITE. This option creates a memory region that
grows to at most len bytes in size, starting from the stack top and growing down. The
stack top is the starting address returned by the call, plus len bytes. The bottom of
the stack at maximum growth is the starting address returned by the call.

The close(2) system call does not unmap pages, see munmap(2) for further information.

The current design does not allow a process to specify the location of swap space. In the future we may
define an additional mapping type, MAP_SWAP, in which the file descriptor argument specifies a file or
device to which swapping should be done.

NOTES

       Although this implementation does not impose  any  alignment  restrictions  on  the  offset  argument,  a
       portable program must only use page-aligned values.

       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
       MAP_ALIGNED_SUPER flag is an optimization that will align the mapping request to the size of a large page
       similar to 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.

RETURN VALUES

       Upon  successful  completion,  mmap()  returns  a  pointer  to  the mapped region.  Otherwise, a value of
       MAP_FAILED is returned and errno is set to indicate the error.

ERRORS

The mmap() system call will fail if:

[EACCES] The flag PROT_READ was specified as part of the prot argument and fd was not open for
reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and
prot argument and fd was not open for writing.

[EBADF] The fd argument is not a valid open file descriptor.

[EINVAL] MAP_FIXED was specified and the addr argument was not page aligned, or part of the
desired address space resides out of the valid address space for a user process.

[EINVAL] Both MAP_FIXED and MAP_32BIT were specified and part of the desired address space
resides outside of the first 2GB of user address space.

[EINVAL] The len argument was equal to zero.

[EINVAL] MAP_ALIGNED was specified and the desired alignment was either larger than the virtual
address size of the machine or smaller than a page.

[EINVAL] MAP_ANON was specified and the fd argument was not -1.

[EINVAL] MAP_ANON was specified and the offset argument was not 0.

[EINVAL] Both MAP_FIXED and MAP_EXCL were specified, but the requested region is already used
by a mapping.

[EINVAL] MAP_EXCL was specified, but MAP_FIXED was not.

[ENODEV] MAP_ANON has not been specified and fd did not reference a regular or character
special file.

[ENOMEM] MAP_FIXED was specified and the addr argument was not available. MAP_ANON was
specified and insufficient memory was available.

BUGS

       The 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.

       The previous documented limit of 2GB  was  a  documentation  bug.   That  limit  has  not  existed  since
       FreeBSD 2.2.

Debian                                            June 19, 2014                                          MMAP(2)