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mmap() - Unix, Linux System Call


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NAME

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

SYNOPSIS

#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);

DESCRIPTION

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 is either PROT_NONE or is the bitwise OR of one or more of the other PROT_* flags.

TagDescription
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
TagDescription
MAP_FIXED Do not select a different address than the one specified. If the memory region specified by start and len overlaps pages of any existing mapping(s), then the overlapped part of the existing mapping(s) will be discarded. If the specified address cannot be used, mmap() will fail. If MAP_FIXED is specified, start must be a multiple of the page size. Use of this option is discouraged.
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 actually be updated until msync(2) or munmap(2) are called.
MAP_PRIVATE Create a private copy-on-write mapping. Stores to the region do not affect the original file. It is unspecified whether changes made to the file after the mmap() call are visible in the mapped region.
You must specify exactly one of MAP_SHARED and MAP_PRIVATE.

The above three flags are described in POSIX.1-2001. Linux also knows about the following non-standard flags:

TagDescription
MAP_DENYWRITE
  This flag is ignored. (Long ago, it signalled that attempts to write to the underlying file should fail with ETXTBUSY. But this was a source of denial-of-service attacks.)
MAP_EXECUTABLE
  This flag is ignored.
MAP_NORESERVE
  Do not reserve swap space for this mapping. When swap space is reserved, one has the guarantee that it is possible to modify the mapping. When swap space is not reserved one might get SIGSEGV upon a write if no physical memory is available. See also the discussion of the file /proc/sys/vm/overcommit_memory in proc(5). In kernels before 2.6, this flag only had effect for private writable mappings.
MAP_LOCKED (since Linux 2.5.37)
  Lock the pages of the mapped region into memory in the manner of mlock(). This flag is ignored in older kernels.
MAP_GROWSDOWN
  Used for stacks. Indicates to the kernel VM system that the mapping should extend downwards in memory.
MAP_ANONYMOUS
  The mapping is not backed by any file; the fd and offset arguments are ignored. The use of this flag in conjunction with MAP_SHARED is only supported on Linux since kernel 2.4.
MAP_ANON
  Alias for MAP_ANONYMOUS. Deprecated.
MAP_FILE
  Compatibility flag. Ignored.
MAP_32BIT
  Put the mapping into the first 2GB of the process address space. Ignored when MAP_FIXED is set. This flag is currently only supported on x86-64 for 64bit programs.
MAP_POPULATE (since Linux 2.5.46)
  Populate (prefault) page tables for a file mapping, by performing read-ahead on the file. Later accesses to the mapping will not be bocked by page faults.
MAP_NONBLOCK (since Linux 2.5.46)
  Only meaningful in conjunction with MAP_POPULATE. Don’t perform read-ahead: only create page tables entries for pages that are already present in RAM.
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV, MAP_COPY, and MAP_LOCAL.

fd should be a valid file descriptor, unless MAP_ANONYMOUS is set. If MAP_ANONYMOUS is set, then fd is ignored on Linux. However, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this.

offset should be a multiple of the page size as 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 not written out to the file. The effect of changing the size of the underlying file of a mapping on the pages that correspond to added or removed regions of the file is unspecified.

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. The region is also automatically unmapped when the process is terminated. On the other hand, closing the file descriptor does not unmap the region.

The address start must be a multiple of the page size. All pages containing a part of the indicated range are unmapped, and subsequent references to these pages will generate SIGSEGV. It is not an error if the indicated range does not contain any mapped pages.

For file-backed mappings, the st_atime field for the mapped file may be updated at any time between the mmap() and the corresponding unmapping; the first reference to a mapped page will update the field if it has not been already.

The st_ctime and st_mtime field for a file mapped with PROT_WRITE and MAP_SHARED will be updated after a write to the mapped region, and before a subsequent msync() with the MS_SYNC or MS_ASYNC flag, if one occurs.

RETURN VALUE

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

NOTES

It is architecture dependent whether PROT_READ includes PROT_EXEC or not. Portable programs should always set PROT_EXEC if they intend to execute code in the new mapping.

ERRORS

TagDescription
EACCES A file descriptor refers to a non-regular file. Or MAP_PRIVATE was requested, but fd is not open for reading. Or MAP_SHARED was requested and PROT_WRITE is set, but fd is not open in read/write (O_RDWR) mode. Or PROT_WRITE is set, but the file is append-only.
EAGAIN The file has been locked, or too much memory has been locked (see setrlimit(2)).
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not set).
EINVAL We don’t like start or length or offset. (E.g., they are too large, or not aligned on a page boundary.)
ENFILE The system limit on the total number of open files has been reached.
ENODEV The underlying filesystem of the specified file does not support memory mapping.
ENOMEM No memory is available, or the process’s maximum number of mappings would have been exceeded.
EPERM The prot argument asks for PROT_EXEC but the mapped area belongs to a file on a filesystem that was mounted no-exec.
ETXTBSY
  MAP_DENYWRITE was set but the object specified by fd is open for writing.
Use of a mapped region can result in these signals:
SIGSEGV
  Attempted write into a region mapped 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 process has truncated the file).

AVAILABILITY

On POSIX systems on which mmap(), msync() and munmap() are available, _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)

CONFORMING TO

SVr4, 4.4BSD, POSIX.1-2001.

BUGS

On Linux there are no guarantees like those suggested above under MAP_NORESERVE. By default, any process can be killed at any moment when the system runs out of memory.

In kernels before 2.6.7, the MAP_POPULATE flag only has effect if prot is specified as PROT_NONE.

SEE ALSO

B.O. Gallmeister, POSIX.4, O’Reilly, pp. 128-129 and 389-391.

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