MMAP(2)

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

# NAME

`mmap` —
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` function 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 pagesize, the mapped region may extend past the specified range. Any such extension beyond the end of the mapped object will be zero-filled.

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_EXEC`
  Pages may be executed.
`PROT_READ`
  Pages may be read.
`PROT_WRITE`
  Pages may be written.
`PROT_NONE`
  Placeholder when requesting no access permission.

As a NetBSD extension, the `PROT_MPROTECT` macro can be used to request additional permissions for later use with `mprotect`(2). For example `PROT_MPROTECT(PROT_READ)` requests that future `PROT_READ` mappings are allowed and can be enabled using mprotect(2), but does not currently grant read mappings to the returned memory segment. This is necessary for switching pages between writable and executable when PaX MPROTECT restrictions are in place. See mremap(2) for a sample use case.

Note that, due to hardware limitations, on some platforms `PROT_WRITE` may imply `PROT_READ`, and `PROT_READ` may imply `PROT_EXEC`. Portable programs should not rely on these flags being separately enforceable.

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. Note that either `MAP_SHARED` or `MAP_PRIVATE` must be specified. Sharing, mapping type and options are specified in the flags argument by OR'ing the following values:

`MAP_ALIGNED(n)`
  Request that the allocation be aligned to the given boundary. The parameter n should be the base 2 logarithm of the desired alignment (e.g., to request alignment to 16K, use 14 as the value for n). The alignment must be equal to or greater than the platform's page size as returned by sysconf(3) with the `_SC_PAGESIZE` request. The following constants are defined for convenience:
  - `MAP_ALIGNMENT_64KB`
  - `MAP_ALIGNMENT_16MB`
  - `MAP_ALIGNMENT_4GB`
  - `MAP_ALIGNMENT_1TB`
  - `MAP_ALIGNMENT_256TB`
  - `MAP_ALIGNMENT_64PB`
`MAP_ANON`
  Map anonymous memory not associated with any specific file. The file descriptor is not used for creating `MAP_ANON` regions, and must be specified as -1. The mapped memory will be zero filled.
`MAP_FILE`
  Mapped from a regular file or character-special device memory. Read accesses beyond the end of of the file or device but less than the current page size will be zero-filled. Write accesses beyond the end of the file or device but less than the current page size will not affect the file or device. References beyond the end of file that are beyond the current page size will result in the delivery of `SIGBUS` signal.
`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 pagesize. Use of this option is discouraged.
`MAP_HASSEMAPHORE`
  Notify the kernel that the region may contain semaphores and that special handling may be necessary.
`MAP_INHERIT`
  Permit regions to be inherited across execve(2) system calls.
`MAP_NORESERVE`
  Only reserve address space, but do not reserve swap space or any other resources for this mapping. Access to the address space is not guaranteed and may result in a segmentation violation. Unimplemented.
`MAP_REMAPDUP`
  Only valid for mremap(2).
`MAP_RENAME`
  Assign the referenced private pages to the file descriptor provided. Unimplemented.
`MAP_STACK`
  Allocate a memory segment that can be used either for a process or thread stack. This currently has no effect, but its use is reserved for architectures that might require special treatment of that address space. Unimplemented.
`MAP_TRYFIXED`
  Attempt to use the address addr even if it falls within the normally protected process data or text segment memory regions. If the requested region of memory is actually present in the memory map, a different address will be selected as if `MAP_TRYFIXED` had not been specified. If addr is `NULL`, this flag is ignored and the system will select a mapping address.
`MAP_WIRED`
  Lock the mapped region into memory as with mlock(2).
`MAP_PRIVATE`
  Modifications made by this process are private, however modifications made by other processes using `MAP_SHARED` will be seen.
`MAP_SHARED`
  Modifications are shared.

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

The current design does not allow a process to specify the location of swap space. In the future we may define an additional mapping type, `MAP_SWAP`, in which the file descriptor argument specifies a file or device to which swapping should be done.

If `MAP_FIXED` is not specified, the system will attempt to place the mapping in an unused portion of the address space chosen to minimize possible collision between mapped regions and the heap.

# 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. The symbol `MAP_FAILED` is defined in the header `<sys/mman.h>`. No successful return from `mmap`() will 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.

  PaX mprotect restrictions prohibit the requested protection.

[`EBADF`]
  fd is not a valid open file descriptor.
[`EINVAL`]
  `MAP_FIXED` was specified and the addr parameter was not page aligned or was outside of the valid address range for a process.

  `MAP_ANON` was specified and fd was not -1.

[`ENODEV`]
  fd did not reference a regular or character special file.
[`ENOMEM`]
  `MAP_FIXED` was specified and the addr parameter wasn't available.

  `MAP_ANON` was specified and insufficient memory was available.

[`EOVERFLOW`]
  fd references a regular file and the value of offset plus len would exceed the offset maximum established in its open file description.

# SEE ALSO

madvise(2), mincore(2), mlock(2), mprotect(2), mremap(2), msync(2), munmap(2), getpagesize(3), sysconf(3)

# STANDARDS

The `mmap`() function conforms to IEEE Std 1003.1b-1993 (“POSIX.1b”).

# HISTORY

The `mmap`() interface was first designed in 4.2BSD.

May 2, 2018 | NetBSD 9.4