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MMAP(2) Linux Programmer's Manual MMAP(2)
mmap, munmap - map or unmap files or devices into memory
#include <sys/mman.h>
void *mmap(void *addr, size_t length, int prot, int flags,
int fd, off_t offset);
int munmap(void *addr, size_t length);
See NOTES for information on feature test macro requirements.
mmap() creates a new mapping in the virtual address space of the
calling process. The starting address for the new mapping is
specified in addr. The length argument specifies the length of the
mapping (which must be greater than 0).
If addr is NULL, then the kernel chooses the address at which to
create the mapping; this is the most portable method of creating a
new mapping. If addr 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 a nearby 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
mapping (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
visible to other processes mapping the same region, and whether
updates are carried 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 mapping the same region, and (in the case of
file-backed mappings) are carried through to the underlying
file. (To precisely control when updates are carried through
to the underlying file requires the use of msync(2).)
MAP_PRIVATE
Create a private copy-on-write mapping. Updates to the
mapping 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 and POSIX.1-2008.
In addition, zero or more of the following values can be ORed in
flags:
MAP_32BIT (since Linux 2.4.20, 2.6)
Put the mapping into the first 2 Gigabytes of the process
address space. This flag is supported only on x86-64, for
64-bit programs. It was added to allow thread stacks to be
allocated somewhere in the first 2 GB of memory, so as to
improve context-switch performance on some early 64-bit
processors. Modern x86-64 processors no longer have this
performance problem, so use of this flag is not required on
those systems. The MAP_32BIT flag is ignored when MAP_FIXED
is set.
MAP_ANON
Synonym for MAP_ANONYMOUS. Deprecated.
MAP_ANONYMOUS
The mapping is not backed by any file; its contents are
initialized to zero. The fd argument is ignored; however,
some implementations require fd to be -1 if MAP_ANONYMOUS (or
MAP_ANON) is specified, and portable applications should
ensure this. The offset argument should be zero. The use of
MAP_ANONYMOUS in conjunction with MAP_SHARED is supported on
Linux only 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 addr as a hint: place the mapping at exactly
that address. addr must be suitably aligned: for most
architectures a multiple of the page size is sufficient;
however, some architectures may impose additional
restrictions. If the memory region specified by addr and len
overlaps pages of any existing mapping(s), then the overlapped
part of the existing mapping(s) will be discarded. If the
specified address cannot be used, mmap() will fail. Software
that aspires to be portable should use this option with care,
keeping in mind that the exact layout of a process's memory
mappings is allowed to change significantly between kernel
versions, C library versions, and operating system releases.
Furthermore, this option is extremely hazardous (when used on
its own), because it forcibly removes preexisting mappings,
making it easy for a multithreaded process to corrupt its own
address space.
For example, thread A looks through /proc/<pid>/maps and
locates an available address range, while thread B
simultaneously acquires part or all of that same address
range. Thread A then calls mmap(MAP_FIXED), effectively
overwriting the mapping that thread B created.
Thread B need not create a mapping directly; simply making a
library call that, internally, uses dlopen(3) to load some
other shared library, will suffice. The dlopen(3) call will
map the library into the process's address space.
Furthermore, almost any library call may be implemented in a
way that adds memory mappings to the address space, either
with this technique, or by simply allocating memory. Examples
include brk(2), malloc(3), pthread_create(3), and the PAM
libraries ⟨http://www.linux-pam.org⟩.
MAP_GROWSDOWN
This flag is used for stacks. It indicates to the kernel vir‐
tual memory system that the mapping should extend downward in
memory. The return address is one page lower than the memory
area that is actually created in the process's virtual address
space. Touching an address in the "guard" page below the map‐
ping will cause the mapping to grow by a page. This growth
can be repeated until the mapping grows to within a page of
the high end of the next lower mapping, at which point touch‐
ing the "guard" page will result in a SIGSEGV signal.
MAP_HUGETLB (since Linux 2.6.32)
Allocate the mapping using "huge pages." See the Linux kernel
source file Documentation/vm/hugetlbpage.txt for further
information, as well as NOTES, below.
MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux 3.8)
Used in conjunction with MAP_HUGETLB to select alternative
hugetlb page sizes (respectively, 2 MB and 1 GB) on systems
that support multiple hugetlb page sizes.
More generally, the desired huge page size can be configured
by encoding the base-2 logarithm of the desired page size in
the six bits at the offset MAP_HUGE_SHIFT. (A value of zero
in this bit field provides the default huge page size; the
default huge page size can be discovered vie the Hugepagesize
field exposed by /proc/meminfo.) Thus, the above two con‐
stants are defined as:
#define MAP_HUGE_2MB (21 << MAP_HUGE_SHIFT)
#define MAP_HUGE_1GB (30 << MAP_HUGE_SHIFT)
The range of huge page sizes that are supported by the system
can be discovered by listing the subdirectories in /sys/ker‐
nel/mm/hugepages.
MAP_LOCKED (since Linux 2.5.37)
Mark the mapped region to be locked in the same way as
mlock(2). This implementation will try to populate (prefault)
the whole range but the mmap() call doesn't fail with ENOMEM
if this fails. Therefore major faults might happen later on.
So the semantic is not as strong as mlock(2). One should use
mmap() plus mlock(2) when major faults are not acceptable
after the initialization of the mapping. The MAP_LOCKED flag
is ignored in older kernels.
MAP_NONBLOCK (since Linux 2.5.46)
This flag is meaningful only in conjunction with MAP_POPULATE.
Don't perform read-ahead: create page tables entries only for
pages that are already present in RAM. Since Linux 2.6.23,
this flag causes MAP_POPULATE to do nothing. One day, the
combination of MAP_POPULATE and MAP_NONBLOCK may be reimple‐
mented.
MAP_NORESERVE
Do not reserve swap space for this mapping. When swap space
is reserved, one has the guarantee that it is possible to mod‐
ify 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/overcom‐
mit_memory in proc(5). In kernels before 2.6, this flag had
effect only for private writable mappings.
MAP_POPULATE (since Linux 2.5.46)
Populate (prefault) page tables for a mapping. For a file
mapping, this causes read-ahead on the file. This will help
to reduce blocking on page faults later. MAP_POPULATE is sup‐
ported for private mappings only since Linux 2.6.23.
MAP_STACK (since Linux 2.6.27)
Allocate the mapping at an address suitable for a process or
thread stack. This flag is currently a no-op, but is used in
the glibc threading implementation so that if some architec‐
tures require special treatment for stack allocations, support
can later be transparently implemented for glibc.
MAP_UNINITIALIZED (since Linux 2.6.33)
Don't clear anonymous pages. This flag is intended to improve
performance on embedded devices. This flag is honored only if
the kernel was configured with the CONFIG_MMAP_ALLOW_UNINI‐
TIALIZED option. Because of the security implications, that
option is normally enabled only on embedded devices (i.e.,
devices where one has complete control of the contents of user
memory).
Of the above flags, only MAP_FIXED is specified in POSIX.1-2001 and
POSIX.1-2008. However, most systems also support MAP_ANONYMOUS (or
its synonym MAP_ANON).
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.
munmap()
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 addr must be a multiple of the page size (but length need
not be). 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.
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 to indicate the cause of the error.
On success, munmap() returns 0. On failure, it returns -1, and errno
is set to indicate the cause of the error (probably to EINVAL).
EACCES A file descriptor refers to a non-regular file. Or a file
mapping 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 addr, 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-wide 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.
ENOMEM The process's maximum number of mappings would have been
exceeded. This error can also occur for munmap(), when
unmapping a region in the middle of an existing mapping, since
this results in two smaller mappings on either side of the
region being unmapped.
ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described
in getrlimit(2), would have been exceeded.
EOVERFLOW
On 32-bit architecture together with the large file extension
(i.e., using 64-bit off_t): the number of pages used for
length plus number of pages used for offset would overflow
unsigned long (32 bits).
EPERM The prot argument asks for PROT_EXEC but the mapped area
belongs to a file on a filesystem that was mounted no-exec.
EPERM The operation was prevented by a file seal; see fcntl(2).
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
correspond to the file (for example, beyond the end of the
file, including the case where another process has truncated
the file).
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────┼───────────────┼─────────┤
│mmap(), munmap() │ Thread safety │ MT-Safe │
└───────────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.
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).)
On some hardware architectures (e.g., i386), PROT_WRITE implies
PROT_READ. 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.
The portable way to create a mapping is to specify addr as 0 (NULL),
and omit MAP_FIXED from flags. In this case, the system chooses the
address for the mapping; the address is chosen so as not to conflict
with any existing mapping, and will not be 0. If the MAP_FIXED flag
is specified, and addr is 0 (NULL), then the mapped address will be 0
(NULL).
Certain flags constants are defined only if suitable feature test
macros are defined (possibly by default): _DEFAULT_SOURCE with glibc
2.19 or later; or _BSD_SOURCE or _SVID_SOURCE in glibc 2.19 and
earlier. (Employing _GNU_SOURCE also suffices, and requiring that
macro specifically would have been more logical, since these flags
are all Linux-specific.) The relevant flags are: MAP_32BIT,
MAP_ANONYMOUS (and the synonym MAP_ANON), MAP_DENYWRITE,
MAP_EXECUTABLE, MAP_FILE, MAP_GROWSDOWN, MAP_HUGETLB, MAP_LOCKED,
MAP_NONBLOCK, MAP_NORESERVE, MAP_POPULATE, and MAP_STACK.
An application can determine which pages of a mapping are currently
resident in the buffer/page cache using mincore(2).
Timestamps changes for file-backed mappings
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.
Huge page (Huge TLB) mappings
For mappings that employ huge pages, the requirements for the
arguments of mmap() and munmap() differ somewhat from the
requirements for mappings that use the native system page size.
For mmap(), offset must be a multiple of the underlying huge page
size. The system automatically aligns length to be a multiple of the
underlying huge page size.
For munmap(), addr and length must both be a multiple of the
underlying huge page size.
C library/kernel differences
This page describes the interface provided by the glibc mmap()
wrapper function. Originally, this function invoked a system call of
the same name. Since kernel 2.4, that system call has been
superseded by mmap2(2), and nowadays the glibc mmap() wrapper
function invokes mmap2(2) with a suitably adjusted value for offset.
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 has effect only 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 addr. Since kernel 2.6.12, mmap()
fails with the error EINVAL for this case.
POSIX specifies that the system shall always zero fill any partial
page at the end of the object and that system will never write any
modification of the object beyond its end. On Linux, when you write
data to such partial page after the end of the object, the data stays
in the page cache even after the file is closed and unmapped and even
though the data is never written to the file itself, subsequent
mappings may see the modified content. In some cases, this could be
fixed by calling msync(2) before the unmap takes place; however, this
doesn't work on tmpfs(5) (for example, when using the POSIX shared
memory interface documented in shm_overview(7)).
The following program prints part of the file specified in its first
command-line argument to standard output. The range of bytes to be
printed is specified via offset and length values in the second and
third command-line arguments. The program creates a memory mapping
of the required pages of the file and then uses write(2) to output
the desired bytes.
Program source
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
char *addr;
int fd;
struct stat sb;
off_t offset, pa_offset;
size_t length;
ssize_t s;
if (argc < 3 || argc > 4) {
fprintf(stderr, "%s file offset [length]\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_RDONLY);
if (fd == -1)
handle_error("open");
if (fstat(fd, &sb) == -1) /* To obtain file size */
handle_error("fstat");
offset = atoi(argv[2]);
pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
/* offset for mmap() must be page aligned */
if (offset >= sb.st_size) {
fprintf(stderr, "offset is past end of file\n");
exit(EXIT_FAILURE);
}
if (argc == 4) {
length = atoi(argv[3]);
if (offset + length > sb.st_size)
length = sb.st_size - offset;
/* Can't display bytes past end of file */
} else { /* No length arg ==> display to end of file */
length = sb.st_size - offset;
}
addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
MAP_PRIVATE, fd, pa_offset);
if (addr == MAP_FAILED)
handle_error("mmap");
s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
if (s != length) {
if (s == -1)
handle_error("write");
fprintf(stderr, "partial write");
exit(EXIT_FAILURE);
}
munmap(addr, length + offset - pa_offset);
close(fd);
exit(EXIT_SUCCESS);
}
ftruncate(2), getpagesize(2), memfd_create(2), mincore(2), mlock(2),
mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2),
setrlimit(2), shmat(2), userfaultfd(2), shm_open(3), shm_overview(7)
The descriptions of the following files in proc(5): /proc/[pid]/maps,
/proc/[pid]/map_files, and /proc/[pid]/smaps.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-12-18 MMAP(2)
Pages that refer to this page: memusage(1), alloc_hugepages(2), arch_prctl(2), clone(2), execve(2), fcntl(2), fork(2), futex(2), get_mempolicy(2), getpagesize(2), getrlimit(2), ioctl_userfaultfd(2), madvise(2), mbind(2), memfd_create(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2), open(2), perf_event_open(2), personality(2), posix_fadvise(2), prctl(2), readahead(2), remap_file_pages(2), seccomp(2), sendfile(2), set_mempolicy(2), shmget(2), shmop(2), syscalls(2), uselib(2), userfaultfd(2), vfork(2), fopen(3), mallinfo(3), malloc(3), malloc_stats(3), mallopt(3), numa(3), pthread_attr_setguardsize(3), pthread_attr_setstack(3), selinux_status_open(3), sem_init(3), shm_open(3), core(5), proc(5), systemd.exec(5), tmpfs(5), capabilities(7), fanotify(7), futex(7), inode(7), inotify(7), pkeys(7), shm_overview(7), spufs(7), ld.so(8), xfs_io(8)
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