allocate memory, or map files or devices into
Standard C Library (libc, -lc)
() system call causes the pages
starting at addr
and continuing for at most
bytes to be mapped from the object
described by fd
, starting at byte offset
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.
references a regular file or a shared
memory object, the range of bytes starting at
and continuing for
bytes must be legitimate for the possible
(not necessarily current) offsets in the object. In particular, the
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.
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 restrictitions on the offset
value in this case, passing it unchanged to the driver.
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
is zero, an address will be selected by
the system. The actual starting address of the region is returned. A
deletes any previous mapping
in the allocated address range.
The protections (region accessibility) are specified in the
'ing the following values:
- Pages may not be accessed.
- Pages may be read.
- Pages may be written.
- Pages may be executed.
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
'ing the following values:
- Request a region in the first 2GB of the current process's address space.
If a suitable region cannot be found,
mmap() will 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 desired alignment.
- 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
must be -1. The offset argument must be
- This flag is identical to
is provided for compatibility.
- This flag can only be used in combination with
MAP_FIXED. Please see the definition of
MAP_FIXED for the description of its
- 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.
MAP_EXCL is not specified, a
MAP_FIXED request replaces
any previous mappings for the process' pages in the range from
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
SIGSEGV signal to that
- 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 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
pages, because pages may be flushed to disk in a totally random order.
The same applies when using
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
iostat 1” while reading a large
file sequentially, e.g., using “
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.
offset of 0. The
fd argument must be -1 and
prot must include at least
This option creates a memory region that grows to at most
len bytes in size, starting from the
stack top and growing down. The stack top is the starting address returned
by the call, plus len bytes. The bottom
of the stack at maximum growth is the starting address returned by the
Stacks created with
automatically grow. Guards prevent inadvertent use of the regions into
which those stacks can grow without requiring mapping the whole stack in
system call does not unmap pages, see
for further information.
Although this implementation does not impose any alignment restrictions on the
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
flag is an optimization
that will align the mapping request to the size of a large page similar to
, 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.
Upon successful completion,
() returns a
pointer to the mapped region. Otherwise, a value of
is returned and
is set to indicate the error.
() 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 shared
- An invalid value was passed in the prot
- An undefined option was set in the flags
MAP_SHARED were specified.
- None of
MAP_STACK was specified. At least one
of these flags must be included.
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
- 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
was specified and the offset argument was
MAP_EXCL were specified, but the
requested region is already used by a mapping.
was specified, but
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
MAP_ANON was specified
and insufficient memory was available.