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NAME
mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#include <sys/mman.h>
void *mmap(void *start, size_t length, int prot, int flags,
int fd, off_t offset);
int munmap(void *start, size_t length);
DESCRIPTION
mmap() creates a new mapping in the virtual address space of the call‐
ing process. The starting address for the new mapping is specified in
start. The length argument specifies the length of the mapping.
If start is NULL, then the kernel chooses the address at which to cre‐
ate the mapping; this is the most portable method of creating a new
mapping. If start is not NULL, then the kernel takes it as a hint
about where to place the mapping; on Linux, the mapping will be created
at the next higher page boundary. The address of the new mapping is
returned as the result of the call.
The contents of a file mapping (as opposed to an anonymous mapping; see
MAP_ANONYMOUS below), are initialized using length bytes starting at
offset offset in the file (or other object) referred to by the file
descriptor fd. offset must be a multiple of the page size as returned
by sysconf(_SC_PAGE_SIZE).
The prot argument describes the desired memory protection of the map‐
ping (and must not conflict with the open mode of the file). It is
either PROT_NONE or the bitwise OR of one or more of the following
flags:
PROT_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_NONE Pages may not be accessed.
The flags argument determines whether updates to the mapping are visi‐
ble to other processes mapping the same region, and whether updates are
caried through to the underlying file. This behavior is determined by
including exactly one of the following values in flags:
MAP_SHARED Share this mapping. Updates to the mapping are visible to
other processes that map this file, and are carried through
to the underlying file. The file may not actually be
updated until msync(2) or munmap(2) is called.
MAP_PRIVATE
Create a private copy-on-write mapping. Updates to the map‐
ping are not visible to other processes mapping the same
file, and are not carried through to the underlying file.
It is unspecified whether changes made to the file after the
mmap() call are visible in the mapped region.
Both of these flags are described in POSIX.1-2001.
In addition, zero or more of the following values can be ORed in flags:
MAP_32BIT
Put the mapping into the first 2GB of the process address space.
Ignored when MAP_FIXED is set. This flag is currently only sup‐
ported on x86-64 for 64bit programs.
MAP_ANON
Synonym for MAP_ANONYMOUS. Deprecated.
MAP_ANONYMOUS
The mapping is not backed by any file; its contents are initial‐
ized to zero. The fd and offset arguments are ignored; however,
some implementations require fd to be -1 if MAP_ANONYMOUS (or
MAP_ANON) is specified, and portable applications should ensure
this. The use of MAP_ANONYMOUS in conjunction with MAP_SHARED
is only supported on Linux since kernel 2.4.
MAP_DENYWRITE
This flag is ignored. (Long ago, it signaled that attempts to
write to the underlying file should fail with ETXTBUSY. But
this was a source of denial-of-service attacks.)
MAP_EXECUTABLE
This flag is ignored.
MAP_FILE
Compatibility flag. Ignored.
MAP_FIXED
Don’t interpret start as a hint: place the mapping at exactly
that address. start must be a multiple of the page size. If
the memory region specified by start and len overlaps pages of
any existing mapping(s), then the overlapped part of the exist‐
ing mapping(s) will be discarded. If the specified address can‐
not be used, mmap() will fail. Because requiring a fixed
address for a mapping is less portable, the use of this option
is discouraged.
MAP_GROWSDOWN
Used for stacks. Indicates to the kernel virtual memory system
that the mapping should extend downwards in memory.
MAP_LOCKED (since Linux 2.5.37)
Lock the pages of the mapped region into memory in the manner of
mlock(2). This flag is ignored in older kernels.
MAP_NONBLOCK (since Linux 2.5.46)
Only meaningful in conjunction with MAP_POPULATE. Don’t perform
read-ahead: only create page tables entries for pages that are
already present in RAM.
MAP_NORESERVE
Do not reserve swap space for this mapping. When swap space is
reserved, one has the guarantee that it is possible to modify
the mapping. When swap space is not reserved one might get
SIGSEGV upon a write if no physical memory is available. See
also the discussion of the file /proc/sys/vm/overcommit_memory
in proc(5). In kernels before 2.6, this flag only had effect
for private writable mappings.
MAP_POPULATE (since Linux 2.5.46)
Populate (prefault) page tables for a file mapping, by perform‐
ing read-ahead on the file. Later accesses to the mapping will
not be blocked by page faults.
Of the above flags, only MAP_FIXED is specified in POSIX.1-2001.
However, most systems also support MAP_ANONYMOUS (or its synonym
MAP_ANON).
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
MAP_COPY, and MAP_LOCAL.
Memory mapped by mmap() is preserved across fork(2), with the same
attributes.
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 con‐
taining a part of the indicated range are unmapped, and subsequent ref‐
erences 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 already.
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(2) 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).
ERRORS
EACCES 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
append-only.
EAGAIN The file has been locked, or too much memory has been locked
(see setrlimit(2)).
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not
set).
EINVAL We don’t like start, length, or offset (e.g., they are too
large, or not aligned on a page boundary).
EINVAL (since Linux 2.6.12), length was 0.
EINVAL flags contained neither MAP_PRIVATE or MAP_SHARED, or contained
both of these values.
ENFILE The system limit on the total number of open files has been
reached.
ENODEV The underlying filesystem of the specified file does not support
memory mapping.
ENOMEM No memory is available, or the process’s maximum number of map‐
pings would have been exceeded.
EPERM The prot argument asks for PROT_EXEC but the mapped area belongs
to a file on a filesystem that was mounted no-exec.
ETXTBSY
MAP_DENYWRITE was set but the object specified by fd is open for
writing.
Use of a mapped region can result in these signals:
SIGSEGV
Attempted write into a region mapped as read-only.
SIGBUS Attempted access to a portion of the buffer that does not corre‐
spond to the file (for example, beyond the end of the file,
including the case where another process has truncated the
file).
SVr4, 4.4BSD, POSIX.1-2001.
AVAILABILITY
On POSIX systems on which mmap(), msync(2) and munmap() are available,
_POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0.
(See also sysconf(3).)
NOTES
It is architecture dependent whether PROT_READ implies PROT_EXEC or
not. Portable programs should always set PROT_EXEC if they intend to
execute code in the new mapping.
BUGS
On Linux there are no guarantees like those suggested above under
MAP_NORESERVE. By default, any process can be killed at any moment
when the system runs out of memory.
In kernels before 2.6.7, the MAP_POPULATE flag only has effect if prot
is specified as PROT_NONE.
SUSv3 specifies that mmap() should fail if length is 0. However, in
kernels before 2.6.12, mmap() succeeded in this case: no mapping was
created and the call returned start. Since kernel 2.6.12, mmap() fails
with the error EINVAL for this case.
getpagesize(2), mincore(2), mlock(2), mmap2(2), mremap(2), msync(2),
remap_file_pages(2), setrlimit(2), shm_open(3)
B.O. Gallmeister, POSIX.4, O’Reilly, pp. 128-129 and 389-391.