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(libc.info)Memory-mapped I/O
Memory-mapped I/O
=================
On modern operating systems, it is possible to "mmap" (pronounced
"em-map") a file to a region of memory. When this is done, the file can
be accessed just like an array in the program.
This is more efficient than `read' or `write', as only the regions
of the file that a program actually accesses are loaded. Accesses to
not-yet-loaded parts of the mmapped region are handled in the same way
as swapped out pages.
Since mmapped pages can be stored back to their file when physical
memory is low, it is possible to mmap files orders of magnitude larger
than both the physical memory _and_ swap space. The only limit is
address space. The theoretical limit is 4GB on a 32-bit machine -
however, the actual limit will be smaller since some areas will be
reserved for other purposes. If the LFS interface is used the file size
on 32-bit systems is not limited to 2GB (offsets are signed which
reduces the addressable area of 4GB by half); the full 64-bit are
available.
Memory mapping only works on entire pages of memory. Thus, addresses
for mapping must be page-aligned, and length values will be rounded up.
To determine the size of a page the machine uses one should use
size_t page_size = (size_t) sysconf (_SC_PAGESIZE);
These functions are declared in `sys/mman.h'.
- Function: void * mmap (void *ADDRESS, size_t LENGTH,int PROTECT, int
FLAGS, int FILEDES, off_t OFFSET)
The `mmap' function creates a new mapping, connected to bytes
(OFFSET) to (OFFSET + LENGTH - 1) in the file open on FILEDES. A
new reference for the file specified by FILEDES is created, which
is not removed by closing the file.
ADDRESS gives a preferred starting address for the mapping.
`NULL' expresses no preference. Any previous mapping at that
address is automatically removed. The address you give may still be
changed, unless you use the `MAP_FIXED' flag.
PROTECT contains flags that control what kind of access is
permitted. They include `PROT_READ', `PROT_WRITE', and
`PROT_EXEC', which permit reading, writing, and execution,
respectively. Inappropriate access will cause a segfault (Note:
Program Error Signals).
Note that most hardware designs cannot support write permission
without read permission, and many do not distinguish read and
execute permission. Thus, you may receive wider permissions than
you ask for, and mappings of write-only files may be denied even
if you do not use `PROT_READ'.
FLAGS contains flags that control the nature of the map. One of
`MAP_SHARED' or `MAP_PRIVATE' must be specified.
They include:
`MAP_PRIVATE'
This specifies that writes to the region should never be
written back to the attached file. Instead, a copy is made
for the process, and the region will be swapped normally if
memory runs low. No other process will see the changes.
Since private mappings effectively revert to ordinary memory
when written to, you must have enough virtual memory for a
copy of the entire mmapped region if you use this mode with
`PROT_WRITE'.
`MAP_SHARED'
This specifies that writes to the region will be written back
to the file. Changes made will be shared immediately with
other processes mmaping the same file.
Note that actual writing may take place at any time. You
need to use `msync', described below, if it is important that
other processes using conventional I/O get a consistent view
of the file.
`MAP_FIXED'
This forces the system to use the exact mapping address
specified in ADDRESS and fail if it can't.
`MAP_ANONYMOUS'
`MAP_ANON'
This flag tells the system to create an anonymous mapping,
not connected to a file. FILEDES and OFF are ignored, and
the region is initialized with zeros.
Anonymous maps are used as the basic primitive to extend the
heap on some systems. They are also useful to share data
between multiple tasks without creating a file.
On some systems using private anonymous mmaps is more
efficient than using `malloc' for large blocks. This is not
an issue with the GNU C library, as the included `malloc'
automatically uses `mmap' where appropriate.
`mmap' returns the address of the new mapping, or -1 for an error.
Possible errors include:
`EINVAL'
Either ADDRESS was unusable, or inconsistent FLAGS were given.
`EACCES'
FILEDES was not open for the type of access specified in
PROTECT.
`ENOMEM'
Either there is not enough memory for the operation, or the
process is out of address space.
`ENODEV'
This file is of a type that doesn't support mapping.
`ENOEXEC'
The file is on a filesystem that doesn't support mapping.
- Function: void * mmap64 (void *ADDRESS, size_t LENGTH,int PROTECT,
int FLAGS, int FILEDES, off64_t OFFSET)
The `mmap64' function is equivalent to the `mmap' function but the
OFFSET parameter is of type `off64_t'. On 32-bit systems this
allows the file associated with the FILEDES descriptor to be
larger than 2GB. FILEDES must be a descriptor returned from a
call to `open64' or `fopen64' and `freopen64' where the descriptor
is retrieved with `fileno'.
When the sources are translated with `_FILE_OFFSET_BITS == 64' this
function is actually available under the name `mmap'. I.e., the
new, extended API using 64 bit file sizes and offsets transparently
replaces the old API.
- Function: int munmap (void *ADDR, size_t LENGTH)
`munmap' removes any memory maps from (ADDR) to (ADDR + LENGTH).
LENGTH should be the length of the mapping.
It is safe to unmap multiple mappings in one command, or include
unmapped space in the range. It is also possible to unmap only
part of an existing mapping. However, only entire pages can be
removed. If LENGTH is not an even number of pages, it will be
rounded up.
It returns 0 for success and -1 for an error.
One error is possible:
`EINVAL'
The memory range given was outside the user mmap range or
wasn't page aligned.
- Function: int msync (void *ADDRESS, size_t LENGTH, int FLAGS)
When using shared mappings, the kernel can write the file at any
time before the mapping is removed. To be certain data has
actually been written to the file and will be accessible to
non-memory-mapped I/O, it is necessary to use this function.
It operates on the region ADDRESS to (ADDRESS + LENGTH). It may
be used on part of a mapping or multiple mappings, however the
region given should not contain any unmapped space.
FLAGS can contain some options:
`MS_SYNC'
This flag makes sure the data is actually written _to disk_.
Normally `msync' only makes sure that accesses to a file with
conventional I/O reflect the recent changes.
`MS_ASYNC'
This tells `msync' to begin the synchronization, but not to
wait for it to complete.
`msync' returns 0 for success and -1 for error. Errors include:
`EINVAL'
An invalid region was given, or the FLAGS were invalid.
`EFAULT'
There is no existing mapping in at least part of the given
region.
- Function: void * mremap (void *ADDRESS, size_t LENGTH, size_t
NEW_LENGTH, int FLAG)
This function can be used to change the size of an existing memory
area. ADDRESS and LENGTH must cover a region entirely mapped in
the same `mmap' statement. A new mapping with the same
characteristics will be returned with the length NEW_LENGTH.
One option is possible, `MREMAP_MAYMOVE'. If it is given in FLAGS,
the system may remove the existing mapping and create a new one of
the desired length in another location.
The address of the resulting mapping is returned, or -1. Possible
error codes include:
`EFAULT'
There is no existing mapping in at least part of the original
region, or the region covers two or more distinct mappings.
`EINVAL'
The address given is misaligned or inappropriate.
`EAGAIN'
The region has pages locked, and if extended it would exceed
the process's resource limit for locked pages. Note: Limits
on Resources.
`ENOMEM'
The region is private writable, and insufficient virtual
memory is available to extend it. Also, this error will
occur if `MREMAP_MAYMOVE' is not given and the extension
would collide with another mapped region.
This function is only available on a few systems. Except for
performing optional optimizations one should not rely on this function.
Not all file descriptors may be mapped. Sockets, pipes, and most
devices only allow sequential access and do not fit into the mapping
abstraction. In addition, some regular files may not be mmapable, and
older kernels may not support mapping at all. Thus, programs using
`mmap' should have a fallback method to use should it fail. *Note Mmap:
(standards)Mmap.
- Function: int madvise (void *ADDR, size_t LENGTH, int ADVICE)
This function can be used to provide the system with ADVICE about
the intended usage patterns of the memory region starting at ADDR
and extending LENGTH bytes.
The valid BSD values for ADVICE are:
`MADV_NORMAL'
The region should receive no further special treatment.
`MADV_RANDOM'
The region will be accessed via random page references. The
kernel should page-in the minimal number of pages for each
page fault.
`MADV_SEQUENTIAL'
The region will be accessed via sequential page references.
This may cause the kernel to aggressively read-ahead,
expecting further sequential references after any page fault
within this region.
`MADV_WILLNEED'
The region will be needed. The pages within this region may
be pre-faulted in by the kernel.
`MADV_DONTNEED'
The region is no longer needed. The kernel may free these
pages, causing any changes to the pages to be lost, as well
as swapped out pages to be discarded.
The POSIX names are slightly different, but with the same meanings:
`POSIX_MADV_NORMAL'
This corresponds with BSD's `MADV_NORMAL'.
`POSIX_MADV_RANDOM'
This corresponds with BSD's `MADV_RANDOM'.
`POSIX_MADV_SEQUENTIAL'
This corresponds with BSD's `MADV_SEQUENTIAL'.
`POSIX_MADV_WILLNEED'
This corresponds with BSD's `MADV_WILLNEED'.
`POSIX_MADV_DONTNEED'
This corresponds with BSD's `MADV_DONTNEED'.
`msync' returns 0 for success and -1 for error. Errors include:
`EINVAL'
An invalid region was given, or the ADVICE was invalid.
`EFAULT'
There is no existing mapping in at least part of the given
region.
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