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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 restrictitions 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.
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:
MAP_32BIT 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.
MAP_ALIGNED(n) Align the region on a requested boundary. If a suitable region cannot be found, mmap()
will fail. 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
(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 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 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.
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 call.
Stacks created with MAP_STACK automatically grow. Guards prevent inadvertent use of the
regions into which those stacks can grow without requiring mapping the whole stack in
advance.
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 flags must be included.
[EINVAL] MAP_FIXED 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.
[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.
SEE ALSO
madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3), getpagesizes(3)