MMAP(2) Linux Programmers Manual MMAP(2) NAME mmap, munmap - map or unmap files or devices into memory SYNOPSIS #include 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 call ing 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 map ping. 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 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 map ping (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 visi ble 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 map ping 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 Put the mapping into the first 2GB of the process address space. Ignored when MAP_FIXED is set. This flag is currently only sup ported on x86-64, for 64-bit programs. MAP_ANON Synonym for MAP_ANONYMOUS. Deprecated. MAP_ANONYMOUS The mapping is not backed by any file; its contents are initial ized 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 Dont 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 dis couraged. 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. Dont 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_POP ULATE 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 map ping, 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. How ever, 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. 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 con taining a part of the indicated range are unmapped, and subsequent ref erences 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 dont 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 sup port memory mapping. ENOMEM No memory is available, or the processs maximum number of map pings 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 corre spond 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 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 adddress 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 #include #include #include #include #include #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, OReilly, pp. 128-129 and 389-391. COLOPHON This page is part of release 3.05 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/. Linux 2008-06-05 MMAP(2)