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Manual Pages  — MMAP


mmap – allocate memory, or map files or devices into memory



Standard C Library (libc, -lc)


#include <sys/mman.h>

void *
mmap(void *addr, size_t len, int prot, int flags, int fd, off_t offset);


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.
  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.
  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.
  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.,amp; 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.

  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 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.


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.


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.


The 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 MAP_SHARED and 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 descriptor.
  An invalid (negative) value was passed in the offset argument, when fd referenced a regular file or shared memory.
  An invalid value was passed in the prot argument.
  An undefined option was set in the flags argument.
  Both MAP_PRIVATE and MAP_SHARED were specified.
  None of MAP_ANON, MAP_GUARD, MAP_PRIVATE, MAP_SHARED, or MAP_STACK was specified. At least one of these flags must be included.
  MAP_STACK was specified and len is less than or equal to the guard size.
  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.
  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.
  The len argument was equal to zero.
  MAP_ALIGNED was specified and the desired alignment was either larger than the virtual address size of the machine or smaller than a page.
  MAP_ANON was specified and the fd argument was not -1.
  MAP_ANON was specified and the offset argument was not 0.
  Both MAP_FIXED and MAP_EXCL were specified, but the requested region is already used by a mapping.
  MAP_EXCL was specified, but MAP_FIXED was not.
  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.
  MAP_ANON has not been specified and fd did not reference a regular or character special file.
  MAP_FIXED was specified and the addr argument was not available. MAP_ANON was specified and insufficient memory was available.


madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2), munlock(2), munmap(2), getpagesize(3), getpagesizes(3)

MMAP (2) December 24, 2020

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