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mmap.2

MMAP

Section: Linux Programmer's Manual (2)
Updated: 2008-12-01
Index Return to Main Contents
 

NAME

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

SYNOPSIS

#include <sys/mman.h>

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

DESCRIPTION

mmap() creates a new mapping in the virtual address space of the calling process. The starting address for the new mapping is specified in addr. The length argument specifies the length of the mapping.

If addr is NULL, then the kernel chooses the address at which to create the mapping; this is the most portable method of creating a new mapping. If addr is not NULL, then the kernel takes it as a hint about where to place the mapping; on Linux, the mapping will be created at a nearby page boundary. The address of the new mapping is returned as the result of the call.

The contents of a file mapping (as opposed to an anonymous mapping; see MAP_ANONYMOUS below), are initialized using length bytes starting at offset offset in the file (or other object) referred to by the file descriptor fd. offset must be a multiple of the page size as returned by sysconf(_SC_PAGE_SIZE).

The prot argument describes the desired memory protection of the mapping (and must not conflict with the open mode of the file). It is either PROT_NONE or the bitwise OR of one or more of the following flags:

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 argument determines whether updates to the mapping are visible to other processes mapping the same region, and whether updates are carried through to the underlying file. This behavior is determined by including exactly one of the following values in flags:

MAP_SHARED
Share this mapping. Updates to the mapping are visible to other processes that map this file, and are carried through to the underlying file. The file may not actually be updated until msync(2) or munmap() is called.
MAP_PRIVATE
Create a private copy-on-write mapping. Updates to the mapping are not visible to other processes mapping the same file, and are not carried through to the underlying file. It is unspecified whether changes made to the file after the mmap() call are visible in the mapped region.

Both of these flags are described in POSIX.1-2001.

In addition, zero or more of the following values can be ORed in flags:

MAP_32BIT (since Linux 2.4.20, 2.6)
Put the mapping into the first 2 Gigabytes of the process address space. This flag is only supported on x86-64, for 64-bit programs. It was added to allow thread stacks to be allocated somewhere in the first 2GB of memory, so as to improve context-switch performance on some early 64-bit processors. Modern x86-64 processors no longer have this performance problem, so use of this flag is not required on those systems. The MAP_32BIT flag is ignored when MAP_FIXED is set.
MAP_ANON
Synonym for MAP_ANONYMOUS. Deprecated.
MAP_ANONYMOUS
The mapping is not backed by any file; its contents are initialized to zero. The fd and offset arguments are ignored; however, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure this. The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is only supported on Linux since kernel 2.4.
MAP_DENYWRITE
This flag is ignored. (Long ago, it signaled 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_FILE
Compatibility flag. Ignored.
MAP_FIXED
Don't interpret addr as a hint: place the mapping at exactly that address. addr must be a multiple of the page size. If the memory region specified by addr 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. Because requiring a fixed address for a mapping is less portable, the use of this option is discouraged.
MAP_GROWSDOWN
Used for stacks. Indicates to the kernel virtual memory system that the mapping should extend downwards in memory.
MAP_LOCKED (since Linux 2.5.37)
Lock the pages of the mapped region into memory in the manner of mlock(2). This flag is ignored in older kernels.
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. Since Linux 2.6.23, this flag causes MAP_POPULATE to do nothing. One day the combination of MAP_POPULATE and MAP_NONBLOCK may be re-implemented.
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_POPULATE (since Linux 2.5.46)
Populate (prefault) page tables for a mapping. For a file mapping, this causes read-ahead on the file. Later accesses to the mapping will not be blocked by page faults. MAP_POPULATE is only supported for private mappings since Linux 2.6.23.

Of the above flags, only MAP_FIXED is specified in POSIX.1-2001. However, most systems also support MAP_ANONYMOUS (or its synonym MAP_ANON).

MAP_STACK (since Linux 2.6.27)
Allocate the mapping at an address suitable for a process or thread stack. This flag is currently a no-op, but is used in the glibc threading implementation so that if some architectures require special treatment for stack allocations, support can later be transparently implemented for glibc.

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

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.  

munmap()

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 addr 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.  

Timestamps changes for file-backed mappings

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(2) 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).  

ERRORS

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 addr, length, or offset (e.g., they are too large, or not aligned on a page boundary).
EINVAL
(since Linux 2.6.12) length was 0.
EINVAL
flags contained neither MAP_PRIVATE or MAP_SHARED, or contained both of these values.
ENFILE
The system limit on the total number of open files has been reached.
ENODEV
The underlying file system 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 file system 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).
 

CONFORMING TO

SVr4, 4.4BSD, POSIX.1-2001.  

AVAILABILITY

On POSIX systems on which mmap(), msync(2) and munmap() are available, _POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)  

NOTES

Since kernel 2.4, this system call has been superseded by mmap2(2). Nowadays, the glibc mmap() wrapper function invokes mmap2(2) with a suitably adjusted value for offset.

On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ. It is architecture dependent whether PROT_READ implies PROT_EXEC or not. Portable programs should always set PROT_EXEC if they intend to execute code in the new mapping.

The portable way to create a mapping is to specify addr as 0 (NULL), and omit MAP_FIXED from flags. In this case, the system chooses the address for the mapping; the address is chosen so as not to conflict with any existing mapping, and will not be 0. If the MAP_FIXED flag is specified, and addr is 0 (NULL), then the mapped address will be 0 (NULL).  

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.

SUSv3 specifies that mmap() should fail if length is 0. However, in kernels before 2.6.12, mmap() succeeded in this case: no mapping was created and the call returned addr. Since kernel 2.6.12, mmap() fails with the error EINVAL for this case.  

EXAMPLE

The following program prints part of the file specified in its first command-line argument to standard output. The range of bytes to be printed is specified via offset and length values in the second and third command-line arguments. The program creates a memory mapping of the required pages of the file and then uses write(2) to output the desired bytes.


#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define handle_error(msg) \
    do { perror(msg); exit(EXIT_FAILURE); } while (0)

int
main(int argc, char *argv[])
{
    char *addr;
    int fd;
    struct stat sb;
    off_t offset, pa_offset;
    size_t length;
    ssize_t s;

    if (argc < 3 || argc > 4) {
        fprintf(stderr, "%s file offset [length]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_RDONLY);
    if (fd == -1)
        handle_error("open");

    if (fstat(fd, &sb) == -1)           /* To obtain file size */
        handle_error("fstat");

    offset = atoi(argv[2]);
    pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);
        /* offset for mmap() must be page aligned */

    if (offset >= sb.st_size) {
        fprintf(stderr, "offset is past end of file\n");
        exit(EXIT_FAILURE);
    }

    if (argc == 4) {
        length = atoi(argv[3]);
        if (offset + length > sb.st_size)
            length = sb.st_size - offset;
                /* Can't display bytes past end of file */

    } else {    /* No length arg ==> display to end of file */
        length = sb.st_size - offset;
    }

    addr = mmap(NULL, length + offset - pa_offset, PROT_READ,
                MAP_PRIVATE, fd, pa_offset);
    if (addr == MAP_FAILED)
        handle_error("mmap");

    s = write(STDOUT_FILENO, addr + offset - pa_offset, length);
    if (s != length) {
        if (s == -1)
            handle_error("write");

        fprintf(stderr, "partial write");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
} /* main */
 

SEE ALSO

getpagesize(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2), setrlimit(2), shmat(2), shm_open(3), shm_overview(7)
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.  

COLOPHON

This page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


 

Index

NAME
SYNOPSIS
DESCRIPTION
munmap()
Timestamps changes for file-backed mappings
RETURN VALUE
ERRORS
CONFORMING TO
AVAILABILITY
NOTES
BUGS
EXAMPLE
SEE ALSO
COLOPHON

 

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