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MMAP(2) FreeBSD System Calls Manual MMAP(2)
NAME
mmap - allocate memory, or map files or devices into memory
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);
DESCRIPTION
The mmap() system call causes the pages starting at addr and continuing
for at most len bytes to be mapped from the object described by fd,
starting at byte offset offset. If len is not a multiple of the page
size, the mapped region may extend past the specified range. Any such
extension beyond the end of the mapped object will be zero-filled.
If fd references a regular file or a shared memory object, the range of
bytes starting at offset and continuing for len bytes must be legitimate
for the possible (not necessarily current) offsets in the object. In
particular, the offset value cannot be negative. If the object is
truncated and the process later accesses a page that is wholly within the
truncated region, the access is aborted and a SIGBUS signal is delivered
to the process.
If fd references a device file, the interpretation of the offset value is
device specific and defined by the device driver. The virtual memory
subsystem does not impose any restrictions on the offset value in this
case, passing it unchanged to the driver.
If addr is non-zero, it is used as a hint to the system. (As a
convenience to the system, the actual address of the region may differ
from the address supplied.) If addr is zero, an address will be selected
by the system. The actual starting address of the region is returned. A
successful mmap deletes any previous mapping in the allocated address
range.
The protections (region accessibility) are specified in the prot argument
by or'ing the following values:
PROT_NONE Pages may not be accessed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_EXEC Pages may be executed.
In addition to these protection flags, FreeBSD provides the ability to
set the maximum protection of a region allocated by mmap and later
altered by mprotect(2). This is accomplished by or'ing one or more PROT_
values wrapped in the PROT_MAX() macro into the prot argument.
The flags argument specifies the type of the mapped object, mapping
options and whether modifications made to the mapped copy of the page are
private to the process or are to be shared with other references.
Sharing, mapping type and options are specified in the flags argument by
or'ing the following values:
The n argument specifies the binary logarithm of the
desired alignment.
MAP_ALIGNED_SUPER Align the region to maximize the potential use of
large ("super") pages. If a suitable region cannot be
found, mmap() will fail. The system will choose a
suitable page size based on the size of mapping. The
page size used as well as the alignment of the region
may both be affected by properties of the file being
mapped. In particular, the physical address of
existing pages of a file may require a specific
alignment. The region is not guaranteed to be aligned
on any specific boundary.
MAP_ANON Map anonymous memory not associated with any specific
file. The file descriptor used for creating MAP_ANON
must be -1. The offset argument must be 0.
MAP_ANONYMOUS This flag is identical to MAP_ANON and is provided for
compatibility.
MAP_EXCL This flag can only be used in combination with
MAP_FIXED. Please see the definition of MAP_FIXED for
the description of its effect.
MAP_FIXED Do not permit the system to select a different address
than the one specified. If the specified address
cannot be used, mmap() will fail. If MAP_FIXED is
specified, addr must be a multiple of the page size.
If MAP_EXCL is not specified, a successful MAP_FIXED
request replaces any previous mappings for the
process' pages in the range from addr to addr + len.
In contrast, if MAP_EXCL is specified, the request
will fail if a mapping already exists within the
range.
MAP_GUARD Instead of a mapping, create a guard of the specified
size. Guards allow a process to create reservations
in its address space, which can later be replaced by
actual mappings.
mmap will not create mappings in the address range of
a guard unless the request specifies MAP_FIXED.
Guards can be destroyed with munmap(2). Any memory
access by a thread to the guarded range results in the
delivery of a SIGSEGV signal to that thread.
MAP_NOCORE Region is not included in a core file.
MAP_NOSYNC Causes data dirtied via this VM map to be flushed to
physical media only when necessary (usually by the
pager) rather than gratuitously. Typically this
prevents the update daemons from flushing pages
dirtied through such maps and thus allows efficient
sharing of memory across unassociated processes using
a file-backed shared memory map. Without this option
any VM pages you dirty may be flushed to disk every so
often (every 30-60 seconds usually) which can create
performance problems if you do not need that to occur
though some may implement the same behavior by
default.
WARNING! Extending a file with ftruncate(2), thus
creating a big hole, and then filling the hole by
modifying a shared mmap() can lead to severe file
fragmentation. In order to avoid such fragmentation
you should always pre-allocate the file's backing
store by write()ing zero's into the newly extended
area prior to modifying the area via your mmap(). The
fragmentation problem is especially sensitive to
MAP_NOSYNC pages, because pages may be flushed to disk
in a totally random order.
The same applies when using MAP_NOSYNC to implement a
file-based shared memory store. It is recommended
that you create the backing store by write()ing zero's
to the backing file rather than ftruncate()ing it.
You can test file fragmentation by observing the KB/t
(kilobytes per transfer) results from an "iostat 1"
while reading a large file sequentially, e.g., using
"dd if=filename of=/dev/null bs=32k".
The fsync(2) system call will flush all dirty data and
metadata associated with a file, including dirty
NOSYNC VM data, to physical media. The sync(8)
command and sync(2) system call generally do not flush
dirty NOSYNC VM data. The msync(2) system call is
usually not needed since BSD implements a coherent
file system buffer cache. However, it may be used to
associate dirty VM pages with file system buffers and
thus cause them to be flushed to physical media sooner
rather than later.
MAP_PREFAULT_READ Immediately update the calling process's lowest-level
virtual address translation structures, such as its
page table, so that every memory resident page within
the region is mapped for read access. Ordinarily
these structures are updated lazily. The effect of
this option is to eliminate any soft faults that would
otherwise occur on the initial read accesses to the
region. Although this option does not preclude prot
from including PROT_WRITE, it does not eliminate soft
faults on the initial write accesses to the region.
MAP_PRIVATE Modifications are private.
MAP_SHARED Modifications are shared.
MAP_STACK Creates both a mapped region that grows downward on
demand and an adjoining guard that both reserves
address space for the mapped region to grow into and
limits the mapped region's growth. Together, the
mapped region and the guard occupy len bytes of the
address space. The guard starts at the returned
address, and the mapped region ends at the returned
address plus len bytes. Upon access to the guard, the
mapped region automatically grows in size, and the
guard shrinks by an equal amount. Essentially, the
closest mapped region beneath it.
MAP_STACK implies MAP_ANON and offset of 0. The fd
argument must be -1 and prot must include at least
PROT_READ and PROT_WRITE. The size of the guard, in
pages, is specified by sysctl
security.bsd.stack_guard_page.
The close(2) system call does not unmap pages, see munmap(2) for further
information.
NOTES
Although this implementation does not impose any alignment restrictions
on the offset argument, a portable program must only use page-aligned
values.
Large page mappings require that the pages backing an object be aligned
in matching blocks in both the virtual address space and RAM. The system
will automatically attempt to use large page mappings when mapping an
object that is already backed by large pages in RAM by aligning the
mapping request in the virtual address space to match the alignment of
the large physical pages. The system may also use large page mappings
when mapping portions of an object that are not yet backed by pages in
RAM. The MAP_ALIGNED_SUPER flag is an optimization that will align the
mapping request to the size of a large page similar to MAP_ALIGNED,
except that the system will override this alignment if an object already
uses large pages so that the mapping will be consistent with the existing
large pages. This flag is mostly useful for maximizing the use of large
pages on the first mapping of objects that do not yet have pages present
in RAM.
RETURN VALUES
Upon successful completion, mmap() returns a pointer to the mapped
region. Otherwise, a value of MAP_FAILED is returned and errno is set to
indicate the error.
ERRORS
The mmap() system call will fail if:
[EACCES] The flag PROT_READ was specified as part of the prot
argument and fd was not open for reading. The flags
MAP_SHARED and PROT_WRITE were specified as part of
the flags and prot argument and fd was not open for
writing.
[EBADF] The fd argument is not a valid open file descriptor.
[EINVAL] An invalid (negative) value was passed in the offset
argument, when fd referenced a regular file or shared
memory.
[EINVAL] An invalid value was passed in the prot argument.
[EINVAL] An undefined option was set in the flags argument.
[EINVAL] Both MAP_PRIVATE and MAP_SHARED were specified.
[EINVAL] None of MAP_ANON, MAP_GUARD, MAP_PRIVATE, MAP_SHARED,
or MAP_STACK was specified. At least one of these
resides out of the valid address space for a user
process.
[EINVAL] Both MAP_FIXED and MAP_32BIT were specified and part
of the desired address space resides outside of the
first 2GB of user address space.
[EINVAL] The len argument was equal to zero.
[EINVAL] MAP_ALIGNED was specified and the desired alignment
was either larger than the virtual address size of the
machine or smaller than a page.
[EINVAL] MAP_ANON was specified and the fd argument was not -1.
[EINVAL] MAP_ANON was specified and the offset argument was not
0.
[EINVAL] Both MAP_FIXED and MAP_EXCL were specified, but the
requested region is already used by a mapping.
[EINVAL] MAP_EXCL was specified, but MAP_FIXED was not.
[EINVAL] MAP_GUARD was specified, but the offset argument was
not zero, the fd argument was not -1, or the prot
argument was not PROT_NONE.
[EINVAL] MAP_GUARD was specified together with one of the flags
MAP_ANON, MAP_PREFAULT, MAP_PREFAULT_READ,
MAP_PRIVATE, MAP_SHARED, MAP_STACK.
[ENODEV] MAP_ANON has not been specified and fd did not
reference a regular or character special file.
[ENOMEM] MAP_FIXED was specified and the addr argument was not
available. MAP_ANON was specified and insufficient
memory was available.
[ENOTSUP] The prot argument contains protections which are not a
subset of the specified maximum protections.
SEE ALSO
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
munlock(2), munmap(2), getpagesize(3), getpagesizes(3)
HISTORY
The mmap system call was first documented in 4.2BSD and implemented in
4.4BSD.
The PROT_MAX functionality was introduced in FreeBSD 13.
FreeBSD 14.0-RELEASE-p11 August 14, 2023 FreeBSD 14.0-RELEASE-p11