munmap(2)
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.
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 2GB 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 a multiple of the page size. 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. Because requiring a fixed address
for a mapping is less portable, the use of this option is
discouraged.
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/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 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 mmaped 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, but is used in
the glibc threading implementation so that if some architectures
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_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).
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.
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).
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(2), 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.
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).
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.
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
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. 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 POSIX shared memory interface documented in shm_overview(7)).
EXAMPLE
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
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), 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.
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