|FreeBSD System Calls Manual
allocate memory, or map files or devices into memory
Standard C Library (libc, -lc)
*addr, size_t len,
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
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
- Pages may not be accessed.
- Pages may be read.
- Pages may be written.
- 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
This is accomplished by or'ing one or more
PROT_ values wrapped in the
PROT_MAX() macro into the prot
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
- Request a region in the first 2GB of the current process's address space.
If a suitable region cannot be found,
fail. This flag is only available on 64-bit platforms.
- Align the region on a requested boundary. If a suitable region cannot be
mmap() will fail. The
n argument specifies the binary logarithm of the
- 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 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.
- This flag is identical to
MAP_ANON and is provided
- This flag can only be used in combination with
MAP_FIXED. Please see the definition of
MAP_FIXED for the description of its effect.
- 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
is not specified, a successful
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.
- 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
Any memory access by a thread to the guarded range results in the
delivery of a
SIGSEGV signal to that thread.
- Region is not included in a core file.
- 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 (such as when you are using shared file-backed mmap regions
for IPC purposes). Dirty data will be flushed automatically when all
mappings of an object are removed and all descriptors referencing the
object are closed. Note that VM/file system coherency is maintained
whether you use
MAP_NOSYNC or not. This option is
not portable across UNIX platforms (yet), though
some may implement the same behavior by default.
WARNING! Extending a file with
thus creating a big hole, and then filling the hole by modifying a
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
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
to implement a file-based shared memory store. It is recommended that
you create the backing store by
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 “
if=filename of=/dev/null bs=32k”.
system call will flush all dirty data and metadata associated with a
file, including dirty NOSYNC VM data, to physical media. The
system call generally do not flush dirty NOSYNC VM data. The
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.
- 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.
- Modifications are private.
- Modifications are shared.
- 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 boundary between
the guard and the mapped region moves downward so that the access falls
within the enlarged mapped region. However, the guard will never shrink to
less than the number of pages specified by the sysctl
security.bsd.stack_guard_page, thereby ensuring
that a gap for detecting stack overflow always exists between the downward
growing mapped region and the closest mapped region beneath it.
MAP_ANON and offset of 0.
The fd argument must be -1 and
prot must include at least
PROT_WRITE. The size of the guard, in pages, is
specified by sysctl
system call does not unmap pages, see
for further information.
Although this implementation does not impose any alignment restrictions on the
offset argument, a portable program must only use
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
Upon successful completion,
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.
mmap() returns a pointer to
the mapped region. Otherwise, a value of
returned and errno is set to indicate the error.
mmap() system call will fail if:
- The flag
PROT_READ was specified as part of the
prot argument and fd was not
open for reading. The flags
PROT_WRITE were specified as part of the
flags and prot argument and
fd was not open for writing.
- The fd argument is not a valid open file
- An invalid (negative) value was passed in the offset
argument, when fd referenced a regular file or
- An invalid value was passed in the prot
- An undefined option was set in the flags
MAP_SHARED were specified.
- None of
was specified. At least one of these flags must be included.
was specified and len is less than or equal to the
was specified and the addr argument was not page
aligned, or part of the desired address space resides out of the valid
address space for a user process.
MAP_32BIT were specified and part of the desired
address space resides outside of the first 2GB of user address space.
- The len argument was equal to zero.
was specified and the desired alignment was either larger than the virtual
address size of the machine or smaller than a page.
was specified and the fd argument was not -1.
was specified and the offset argument was not
MAP_EXCL were specified, but the requested region
is already used by a mapping.
was specified, but
MAP_FIXED was not.
was specified, but the offset argument was not zero,
the fd argument was not -1, or the
prot argument was not
was specified together with one of the flags
has not been specified and fd did not reference a
regular or character special file.
was specified and the addr argument was not
MAP_ANON was specified and insufficient
memory was available.
- The prot argument contains protections which are not
a subset of the specified maximum protections.
mmap system call was first documented in
4.2BSD and implemented in
PROT_MAX functionality was introduced
in FreeBSD 13.
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