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 length 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. The mmap()
call doesn't fail if the mapping cannot be populated (for
example, due to limitations on the number of mapped huge
pages when using MAP_HUGETLB). 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.
