MMAP(2) | System Calls Manual | MMAP(2)

# NAME

`mmap` — map files or devices into memory

# SYNOPSIS

`#include <sys/mman.h>`

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

# DESCRIPTION

The `mmap`() function causes the contents of fd, starting at offset, to be mapped in memory at the given addr. The mapping will extend at least len bytes, subject to page alignment restrictions.

The addr argument describes the address where the system should place the mapping. If the `MAP_FIXED` flag is specified, the allocation will happen at the specified address, replacing any previously established mappings in its range. Otherwise, the mapping will be placed at the available spot at addr; failing that it will be placed "close by". If addr is `NULL`, the system can pick any address. Except for `MAP_FIXED` mappings, the system will never replace existing mappings.

The len argument describes the minimum amount of bytes the mapping will span. Since `mmap`() maps pages into memory, len may be rounded up to hit a page boundary. If len is 0, the mapping will fail with `EINVAL`.

If an fd and offset are specified, the resulting address may end up not on a page boundary, in order to align the page offset in the addr to the page offset in offset.

The protections (region accessibility) are specified in the prot argument. It should either be `PROT_NONE` (no permissions) or the bitwise OR of one or more of the following values:

`PROT_EXEC`
  Pages may be executed.
`PROT_READ`
  Pages may be read.
`PROT_WRITE`
  Pages may be written.

The flags parameter 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. Exactly one of the first two values must be specified:

`MAP_PRIVATE`
  Modifications are private.
`MAP_SHARED`
  Modifications are shared.

Any combination of the following flags may additionally be used:

`MAP_ANON`
  Map anonymous memory not associated with any specific file. The file descriptor used for creating `MAP_ANON` must currently be -1 indicating no name is associated with the region.
`MAP_ANONYMOUS`
  Synonym for `MAP_ANON`.
`MAP_FIXED`
  Demand that the mapping is placed at addr, rather than having the system select a location. addr, len and offset (in the case of fd mappings) must be multiples of the page size. Existing mappings in the address range will be replaced. Use of this option is discouraged.
`MAP_STACK`
  Indicate that the mapping is used as a stack. This flag must be used in combination with `MAP_ANON` and `MAP_PRIVATE`.
`MAP_CONCEAL`
  Exclude the mapping from core dumps.

Finally, the following flags are also provided for source compatibility with code written for other operating systems, but are not recommended for use in new OpenBSD code:

`MAP_COPY`
  Modifications are private and, unlike `MAP_PRIVATE`, modifications made by others are not visible. On OpenBSD this behaves just like `MAP_PRIVATE`.
`MAP_FILE`
  Mapped from a regular file, character special file, or block special file specified by file descriptor fd. On OpenBSD and all systems conforming to IEEE Std 1003.1-2008 (“POSIX.1”) this is the default mapping type and need not be specified.
`MAP_HASSEMAPHORE`
  Notify the kernel that the region may contain semaphores and that special handling may be necessary. On OpenBSD this flag is ignored.
`MAP_INHERIT`
  Permit regions to be inherited across execve(2) system calls. On OpenBSD this flag is ignored.
`MAP_TRYFIXED`
  Attempt to use the hint provided by addr. On OpenBSD this is the default behavior.

The close(2) function does not unmap pages; see munmap(2) for further information.

# RETURN VALUES

The `mmap`() function returns a pointer to the mapped region if successful; otherwise the value `MAP_FAILED` is returned and the global variable errno is set to indicate the error. A successful return from `mmap`() will never return the value `MAP_FAILED`.

# ERRORS

`mmap`() will fail if:

[`EACCES`]
  The flag `PROT_READ` was specified as part of the prot parameter and fd was not open for reading. The flags `MAP_SHARED` and `PROT_WRITE` were specified as part of the flags and prot parameters and fd was not open for writing.
[`EBADF`]
  fd is not a valid open file descriptor.
[`EINVAL`]
  `MAP_PRIVATE` and `MAP_SHARED` were both specified.
[`EINVAL`]
  `MAP_FIXED` was specified and the addr parameter was not page aligned.
[`EINVAL`]
  addr and len specified a region that would extend beyond the end of the address space.
[`EINVAL`]
  fd did not specify a regular, character special, or block special file.
[`EINVAL`]
  fd specified a character special or block special file and the underlying device does not support memory mapping.
[`EINVAL`]
  The allocation len was 0.
[`ENOMEM`]
  `MAP_FIXED` was specified and the addr parameter wasn't available. `MAP_ANON` was specified and insufficient memory was available.
[`ENOTSUP`]
  The accesses requested in the prot argument are not allowed. In particular, `PROT_WRITE` | `PROT_EXEC` mappings are not permitted unless the filesystem is mounted `wxallowed` and the process is link-time tagged with `wxneeded`. (See also `kern.wxabort` in sysctl(2) for a method to diagnose failure).
[`EPERM`]
  The addr and len parameters specify a region which contains at least one page marked immutable.

# SEE ALSO

madvise(2), mimmutable(2), mlock(2), mprotect(2), mquery(2), msync(2), munmap(2), getpagesize(3), core(5)

# STANDARDS

The `mmap`() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

# HISTORY

A fully functional `mmap`() system call first appeared in SunOS 4.0 and has been available since 4.3BSD-Net/2.

# CAVEATS

IEEE Std 1003.1-2008 (“POSIX.1”) specifies that references to pages beyond the end of a mapped object shall generate a `SIGBUS` signal; however, on some architectures OpenBSD generates a `SIGSEGV` signal in this case instead.

Additionally, IEEE Std 1003.1-2008 (“POSIX.1”) specifies that `mmap`() shall fail with `EINVAL` if neither `MAP_PRIVATE` nor `MAP_SHARED` is specified by flags; however, for compatibility with old programs, OpenBSD instead defaults to `MAP_SHARED` for mappings of character special files, and to `MAP_PRIVATE` for all other mappings. New programs should not rely on this behavior.

# BUGS

Due to a limitation of the current vm system (see uvm_init(9)), mapping descriptors `PROT_WRITE` without also specifying `PROT_READ` is useless (results in a segmentation fault when first accessing the mapping). This means that such descriptors must be opened with `O_RDWR`, which requires both read and write permissions on the underlying object.

October 7, 2022 | Debian