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NAME
mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#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.
DESCRIPTION
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 (page-aligned) 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 kernel
will pick a nearby page boundary (but always above or equal to the value specified by
/proc/sys/vm/mmap_min_addr) and attempt to create the mapping there. If another mapping
already exists there, the kernel picks a new address that may or may not depend on the
hint. 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).
After the mmap() call has returned, the file descriptor, fd, can be closed immediately
without invalidating the mapping.
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
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_SHARED_VALIDATE (since Linux 4.15)
This flag provides the same behavior as MAP_SHARED except that MAP_SHARED mappings
ignore unknown flags in flags. By contrast, when creating a mapping using
MAP_SHARED_VALIDATE, the kernel verifies all passed flags are known and fails the
mapping with the error EOPNOTSUPP for unknown flags. This mapping type is also
required to be able to use some mapping flags (e.g., MAP_SYNC).
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 MAP_SHARED and MAP_PRIVATE are described in POSIX.1-2001 and POSIX.1-2008.
MAP_SHARED_VALIDATE is a Linux extension.
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; provided for compatibility with other implementations.
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—Linux 2.0 and earlier—it signaled that attempts to
write to the underlying file should fail with ETXTBSY. 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 the MAP_FIXED flag 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. Carefully read the discussion of this flag in NOTES!
MAP_FIXED_NOREPLACE (since Linux 4.17)
This flag provides behavior that is similar to MAP_FIXED with respect to the addr
enforcement, but differs in that MAP_FIXED_NOREPLACE never clobbers a preexisting
mapped range. If the requested range would collide with an existing mapping, then
this call fails with the error EEXIST. This flag can therefore be used as a way to
atomically (with respect to other threads) attempt to map an address range: one
thread will succeed; all others will report failure.
Note that older kernels which do not recognize the MAP_FIXED_NOREPLACE flag will
typically (upon detecting a collision with a preexisting mapping) fall back to a
"non-MAP_FIXED" type of behavior: they will return an address that is different
from the requested address. Therefore, backward-compatible software should check
the returned address against the requested address.
MAP_GROWSDOWN
This flag is used for stacks. It indicates to the kernel virtual 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 mapping 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
touching 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/admin-guide/mm/hugetlbpage.rst 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 via the Hugepagesize field exposed by
/proc/meminfo.) Thus, the above two constants 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/kernel/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 reimplemented.
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 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 supported 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 on Linux. However, by employing this flag,
applications can ensure that they transparently obtain support if the flag is
implemented in the future. Thus, it is used in the glibc threading implementation
to allow for the fact that some architectures may (later) require special treatment
for stack allocations. A further reason to employ this flag is portability:
MAP_STACK exists (and has an effect) on some other systems (e.g., some of the
BSDs).
MAP_SYNC (since Linux 4.15)
This flag is available only with the MAP_SHARED_VALIDATE mapping type; mappings of
type MAP_SHARED will silently ignore this flag. This flag is supported only for
files supporting DAX (direct mapping of persistent memory). For other files,
creating a mapping with this flag results in an EOPNOTSUPP error.
Shared file mappings with this flag provide the guarantee that while some memory is
mapped writable in the address space of the process, it will be visible in the same
file at the same offset even after the system crashes or is rebooted. In
conjunction with the use of appropriate CPU instructions, this provides users of
such mappings with a more efficient way of making data modifications persistent.
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_UNINITIALIZED 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).
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.
RETURN VALUE
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).
ERRORS
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).
EEXIST MAP_FIXED_NOREPLACE was specified in flags, and the range covered by addr and
length clashes with an existing mapping.
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 none of MAP_PRIVATE, MAP_SHARED or MAP_SHARED_VALIDATE.
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 page of the buffer that lies beyond the end of the mapped
file. For an explanation of the treatment of the bytes in the page that
corresponds to the end of a mapped file that is not a multiple of the page size,
see NOTES.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────┼───────────────┼─────────┤
│mmap(), munmap() │ Thread safety │ MT-Safe │
└───────────────────┴───────────────┴─────────┘
CONFORMING TO
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).)
NOTES
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 bytes in the partial page at the end of the mapping are zeroed
when mapped, and modifications 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.
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).
Using MAP_FIXED safely
The only safe use for MAP_FIXED is where the address range specified by addr and length
was previously reserved using another mapping; otherwise, the use of MAP_FIXED is
hazardous because it forcibly removes preexisting mappings, making it easy for a
multithreaded process to corrupt its own address space.
For example, suppose that thread A looks through /proc/<pid>/maps in order to locate an
unused address range that it can map using MAP_FIXED, while thread B simultaneously
acquires part or all of that same address range. When thread A subsequently employs
mmap(MAP_FIXED), it will effectively clobber the mapping that thread B created. In this
scenario, 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⟩.
Since Linux 4.17, a multithreaded program can use the MAP_FIXED_NOREPLACE flag to avoid
the hazard described above when attempting to create a mapping at a fixed address that has
not been reserved by a preexisting mapping.
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.
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 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)).
EXAMPLES
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);
}
SEE ALSO
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.
COLOPHON
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