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Subjectmmap(2) man page
Here's a ham-fisted attempt to expand the rather sparse mmap(2) man page a little :

Can someone point out the errors and unclear bits, and missing stuff please ?

Shown in text here :


MMAP(2) Linux Programmer's Manual MMAP(2)

mmap, munmap - map or unmap files or devices into memory

#include <unistd.h>
#include <sys/mman.h>


void * mmap(void *start, size_t length, int prot , int flags, int fd, off_t offset);

int munmap(void *start, size_t length);


The mmap function asks to map length bytes starting at offset offset from the file (or
other object) specified by the file descriptor fd into memory, preferably at address
start. This latter address is a hint only, and is usually specified as 0. The actual
place where the object is mapped is returned by mmap. The prot argument describes the
desired memory protection (and must not conflict with the open mode of the file). It
has bits

PROT_EXEC Pages may be executed.

PROT_READ Pages may be read.

PROT_WRITE Pages may be written.

PROT_NONE Pages may not be accessed.

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. It has bits

MAP_FIXED Do not select a different address than the one specified. If the specified
address cannot be used, mmap will fail. If MAP_FIXED is specified, start
must be a multiple of the pagesize. Use of this option is discouraged, and
it may make the behaviour of future system calls affecting the process's
mappings unspecified.

MAP_SHARED Share this mapping with all other processes that map this object. Storing
to the region is equivalent to writing to the file. The file may not actu­
ally be updated until msync(2) or munmap(2) are called.

Create a private copy-on-write mapping. Stores to the region do not affect
the original file.

You must specify exactly one of MAP_SHARED and MAP_PRIVATE.

The above three flags are described in POSIX.1b (formerly POSIX.4). Linux also knows
about the following non-standard flags :

Consequent attempts to open the backing file for writing fail with the error
ETXTBSY. This flag is ignored and has no effect.

The mapping is tagged as a mapping of an executable. This flag is ignored
and has no effect.

Indicates to the kernel VM system that availability of free pages in memory
and swap should not be checked before accepting the mapping request. On some
systems this reserves the necessary number of swap pages.

MAP_LOCKED If set, the mapped pages will not be swapped out.

Indicates to the kernel VM system that the mapping should extend downwards
in memory.

The fd argument is ignored; the mapping is not backed by any file. Imple­
mented only in 2.4 kernels and above.

MAP_ANON Alias for MAP_ANONYMOUS. Deprecated.

MAP_FILE Compatibility flag. Ignored.

Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV, MAP_COPY, and

fd should be a valid file descriptor, unless MAP_ANONYMOUS is set, in which case the
argument is ignored.

offset should ordinarily be a multiple of the page size returned by getpagesize(2).

Memory mapped by mmap is preserved across fork(2), with the same attributes.

A file is mapped in multiples of the page size. For a file that is not a multiple of
the page size, the remaining memory is zeroed when mapped, and writes to that region
are ignored. The effect of changing the size of the underlying file of a mapping is

The munmap system call deletes the mappings for the specified address range, and causes
further references to addresses within the range to generate invalid memory references.
Consecutive mapped pages from more than one mmap call may be unmapped with one munmap
system call. The region is also automatically unmapped when the process is terminated.
On the other hand, closing the file descriptor does not unmap the region.

On success, mmap returns a pointer to the mapped area. On error, MAP_FAILED (-1) is
returned, and errno is set appropriately. On success, munmap returns 0, on failure -1,
and errno is set (probably to EINVAL).

EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not set).

EACCES A file descriptor refers to a non-regular file; MAP_PRIVATE was requested, but
fd is not open for reading; MAP_SHARED was requested and PROT_WRITE is set, but
fd is not open in read/write (O_RDWR) mode; PROT_WRITE is set, but the file is

EINVAL We don't like start or length or offset. (E.g., they are too large, or not
aligned on a PAGESIZE boundary.)

A file could not be mapped for reading.

MAP_DENYWRITE was set but the object specified by fd is open for writing.

EAGAIN The file has been locked, or too much memory has been locked.

ENOMEM No memory is available, or the process's maximum number of mappings would have
been exceeded.

ENODEV The underlying filesystem of the specified file does not support memory mapping.

Use of a mapped region can result in these signals:

Attempted write into a region specified to mmap as read-only.

SIGBUS Attempted access to a portion of the buffer that does not correspond to the file
(for example, beyond the end of the file, including the case where another pro­
cess has truncated the file).

SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD. Svr4 documents additional error codes ENXIO
and ENODEV. Some systems document additional error codes EMFILE and EOVERFLOW.

getpagesize(2), mremap(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4, O'Reilly,
pp. 128-129 and 389-391.
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