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 start. The length argument specifies the length of the mapping.

If start 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 start 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 the next higher 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:

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

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

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

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).

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.

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(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

Use of a mapped region can result in these signals:

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.

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 start. 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), mremap(2), msync(2), remap_file_pages(2), setrlimit(2), shm_open(3)

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