The mmap function asks to map length bytes starting at offset
offset from the file (or other object) specified by the file descriptor
fd into memory, preferably at address start. This latter address
is a hint only, and is usually specified as 0. The actual place where the
object is mapped is returned by mmap, and is never 0.
The prot argument describes the desired memory protection (and must not
conflict with the open mode of the file). It is either PROT_NONE or is
the bitwise OR of one or more of the other PROT_* flags.
Pages may be executed.
Pages may be read.
Pages may be written.
Pages may not be accessed.
The flags parameter 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. It has bits
Do not select a different address than the one specified.
If the specified address cannot be used, mmap will fail. If
MAP_FIXED is specified, start must be a multiple of the pagesize.
Use of this option is discouraged.
Share this mapping with all other processes that map this
object. Storing to the region is equivalent to writing to the file. The
file may not actually be updated until msync(2) or munmap(2)
Create a private copy-on-write mapping. Stores to the
region do not affect the original file. It is unspecified whether changes
made to the file after the mmap call are visible in the mapped
You must specify exactly one of MAP_SHARED and MAP_PRIVATE.
The above three flags are described in POSIX.1b (formerly POSIX.4) and SUSv2.
Linux also knows about the following non-standard flags:
This flag is ignored. (Long ago, it signalled that attempts
to write to the underlying file should fail with ETXTBUSY. But this was a
source of denial-of-service attacks.)
This flag is ignored.
(Used together with MAP_PRIVATE.) Do not reserve swap space
pages for this mapping. When swap space is reserved, one has the guarantee
that it is possible to modify this private copy-on-write region. When it
is not reserved one might get SIGSEGV upon a write when no memory is
MAP_LOCKED (since Linux 2.5.37)
Lock the pages of the mapped region into memory in the
manner of mlock(). This flag is ignored in older kernels.
Used for stacks. Indicates to the kernel VM system that the
mapping should extend downwards in memory.
The mapping is not backed by any file; the fd and
offset arguments are ignored. This flag in conjunction with
MAP_SHARED is implemented since Linux 2.4.
Alias for MAP_ANONYMOUS. Deprecated.
Compatibility flag. Ignored.
Put the mapping into the first 2GB of the process address
space. Ignored when MAP_FIXED is set. This flag is currently only
supported on x86-64 for 64bit programs.
MAP_POPULATE (since Linux 2.5.46)
Populate (prefault) pagetables.
MAP_NONBLOCK (since Linux 2.5.46)
Do not block on IO.
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
MAP_COPY, and MAP_LOCAL.
fd should be a valid file descriptor, unless MAP_ANONYMOUS is set, in
which case the argument is ignored.
offset should be a multiple of the page size as returned by
Memory mapped by mmap is preserved across fork(2), with the same
A file is mapped in multiples of the page size. For a file that is not a
multiple of the page size, the remaining memory is zeroed when mapped, and
writes to that region are not written out to the file. The effect of changing
the size of the underlying file of a mapping on the pages that correspond to
added or removed regions of the file is unspecified.
The munmap system call deletes the mappings for the specified address
range, and causes further references to addresses within the range to generate
invalid memory references. The region is also automatically unmapped when the
process is terminated. On the other hand, closing the file descriptor does not
unmap the region.
The address start must be a multiple of the page size. All pages
containing a part of the indicated range are unmapped, and subsequent
references to these pages will generate SIGSEGV. It is not an error if the
indicated range does not contain any mapped pages.
For file-backed mappings, the st_atime field for the mapped file may be
updated at any time between the mmap() and the corresponding unmapping;
the first reference to a mapped page will update the field if it has not been
The st_ctime and st_mtime field for a file mapped with PROT_WRITE
and MAP_SHARED will be updated after a write to the mapped region, and before
a subsequent msync() with the MS_SYNC or MS_ASYNC flag, if one occurs.
On success, mmap returns a pointer to the mapped area. On error, the
value MAP_FAILED (that is, (void *) -1) is returned, and errno
is set appropriately. On success, munmap returns 0, on failure -1, and
errno is set (probably to EINVAL).
A file descriptor refers to a non-regular file. Or
MAP_PRIVATE was requested, but fd is not open for reading. Or
MAP_SHARED was requested and PROT_WRITE is set, but fd is not open
in read/write (O_RDWR) mode. Or PROT_WRITE is set, but the file is
The file has been locked, or too much memory has been
fd is not a valid file descriptor (and MAP_ANONYMOUS
was not set).
We don't like start or length or
offset. (E.g., they are too large, or not aligned on a PAGESIZE
The system limit on the total number of open files has been
The underlying filesystem of the specified file does not
support memory mapping.
No memory is available, or the process's maximum number of
mappings would have been exceeded.
The prot argument asks for PROT_EXEC but the
mapped area belongs to a file on a filesystem that was mounted
MAP_DENYWRITE was set but the object specified by fd
is open for writing.
Use of a mapped region can result in these signals:
Attempted write into a region specified to mmap as
Attempted access to a portion of the buffer that does not
correspond to the file (for example, beyond the end of the file, including
the case where another process has truncated the file).