NAME
mmap, munmap - map or unmap files or devices into memory SYNOPSIS
#include
void *mmap(void *addr, sizet length, int prot, int flags, int fd, offt offset); int munmap(void *addr, sizet length); See NOTES for information on feature test macro requirements. 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 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 MAPANONYMOUS 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(SCPAGESIZE). 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 PROTNONE or the bitwise OR of one or more of the following flags: PROTEXEC Pages may be executed. PROTREAD Pages may be read. PROTWRITE Pages may be written. PROTNONE 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: MAPSHARED 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. MAPPRIVATE 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: MAP32BIT (since Linux 2.4.20, 2.6) Put the mapping into the first 2 Gigabytes of the process
address space. This flag is supported only 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 prob‐ lem, so use of this flag is not required on those systems. The MAP32BIT flag is ignored when MAPFIXED is set. MAPANON Synonym for MAPANONYMOUS. Deprecated. MAPANONYMOUS 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 MAPANONYMOUS (or MAPANON) is specified, and portable applications should ensure this. The use of MAPANONYMOUS in conjunction with MAPSHARED is supported on Linux only since kernel 2.4. MAPDENYWRITE 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.) MAPEXECUTABLE This flag is ignored. MAPFILE Compatibility flag. Ignored. MAPFIXED 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 dis‐ couraged. MAPGROWSDOWN Used for stacks. Indicates to the kernel virtual memory system that the mapping should extend downward in memory. MAPHUGETLB (since Linux 2.6.32) Allocate the mapping using "huge pages." See the Linux kernel source file Documentation/vm/hugetlbpage.txt for further infor‐ mation. MAPLOCKED (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. MAPNONBLOCK (since Linux 2.5.46) Only meaningful in conjunction with MAPPOPULATE. Don't perform
read-ahead: create page tables entries only for pages that are already present in RAM. Since Linux 2.6.23, this flag causes MAPPOPULATE to do nothing. One day the combination of MAPPOP‐ ULATE and MAPNONBLOCK may be reimplemented. MAPNORESERVE 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/overcommitmemory in proc(5). In kernels before 2.6, this flag had effect only for private writable mappings. MAPPOPULATE (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. MAPPOPULATE is supported for private mappings only since Linux 2.6.23. MAPSTACK (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. MAPUNINITIALIZED (since Linux 2.6.33) Don't clear anonymous pages. This flag is intended to improve performance on embedded devices. This flag is honored only if the kernel was configured with the CONFIGMMAPALLOWUNINITIAL‐ IZED option. Because of the security implications, that option is normally enabled only on embedded devices (i.e., devices where one has complete control of the contents of user memory).
Of the above flags, only MAPFIXED is specified in POSIX.1-2001. How‐ ever, most systems also support MAPANONYMOUS (or its synonym MAPANON). Some systems document the additional flags MAPAUTOGROW, MAPAUTORESRV, MAPCOPY, and MAPLOCAL. 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 statime 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 stctime and stmtime field for a file mapped with PROTWRITE and MAPSHARED will be updated after a write to the mapped region, and before a subsequent msync(2) with the MSSYNC or MSASYNC flag, if one occurs. RETURN VALUE On success, mmap() returns a pointer to the mapped area. On error, the
value MAPFAILED (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 MAPPRIVATE was requested, but fd is not open for reading. Or MAPSHARED was requested and PROTWRITE is set, but fd is not open in read/write (ORDWR) mode. Or PROTWRITE 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 MAPANONYMOUS 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 MAPPRIVATE or MAPSHARED, 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 process's maximum number of map‐ pings would have been exceeded. EPERM The prot argument asks for PROTEXEC but the mapped area belongs
to a file on a file system that was mounted no-exec. ETXTBSY MAPDENYWRITE was set but the object specified by fd is open for writing. EOVERFLOW
On 32-bit architecture together with the large file extension
(i.e., using 64-bit offt): the number of pages used for length plus number of pages used for offset would overflow unsigned long (32 bits). 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, POSIXMAPPEDFILES is defined in
to a value greater than 0. (See also sysconf(3).) NOTES This page describes the interface provided by the glibc mmap() wrapper function. Originally, this function invoked a system call of the same name. Since kernel 2.4, that system call has been superseded by mmap2(2), and nowadays the glibc mmap() wrapper function invokes mmap2(2) with a suitably adjusted value for offset. On some hardware architectures (e.g., i386), PROTWRITE implies PROTREAD. It is architecture dependent whether PROTREAD implies PROTEXEC or not. Portable programs should always set PROTEXEC 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 MAPFIXED 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 MAPFIXED flag is specified, and addr is 0 (NULL), then the mapped address will be 0 (NULL). Certain flags constants are defined only if either BSDSOURCE or SVIDSOURCE is defined. (Requiring GNUSOURCE also suffices, and requiring that macro specifically would have been more logical, since these flags are all Linux specific.) The relevant flags are: MAP32BIT, MAPANONYMOUS (and the synonym MAPANON), MAPDENYWRITE, MAPEXECUTABLE, MAPFILE, MAPGROWSDOWN, MAPHUGETLB, MAPLOCKED, MAPNONBLOCK, MAPNORESERVE, MAPPOPULATE, and MAPSTACK. BUGS On Linux there are no guarantees like those suggested above under MAPNORESERVE. By default, any process can be killed at any moment when the system runs out of memory. In kernels before 2.6.7, the MAPPOPULATE flag has effect only if prot is specified as PROTNONE. 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. POSIX specifies that the system shall always zero fill any partial page at the end of the object and that system will never write any modifica‐ tion of the object beyond its end. On Linux, when you write data to such partial page after the end of the object, the data stays in the page cache even after the file is closed and unmapped and even though the data is never written to the file itself, subsequent mappings may see the modified content. In some cases, this could be fixed by call‐ ing msync(2) before the unmap takes place; however, this doesn't work on tmpfs (for example, when using POSIX shared memory interface docu‐ mented in shmoverview(7)). 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
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#define handleerror(msg) \ do { perror(msg); exit(EXITFAILURE); } while (0) int main(int argc, char *argv[]) { char *addr; int fd; struct stat sb; offt offset, paoffset; sizet length; ssizet s; if (argc < 3 || argc > 4) {
fprintf(stderr, "%s file offset [length]\n", argv[0]); exit(EXITFAILURE); } fd = open(argv[1], ORDONLY);
if (fd == -1) handleerror("open");
if (fstat(fd, &sb) == -1) /* To obtain file size */ handleerror("fstat"); offset = atoi(argv[2]);
paoffset = offset & ~(sysconf(SCPAGESIZE) - 1); /* offset for mmap() must be page aligned */ if (offset >= sb.stsize) { fprintf(stderr, "offset is past end of file\n"); exit(EXITFAILURE); } if (argc == 4) { length = atoi(argv[3]); if (offset + length > sb.stsize)
length = sb.stsize - offset; /* Can't display bytes past end of file */ } else { /* No length arg ==> display to end of file */
length = sb.stsize - offset; }
addr = mmap(NULL, length + offset - paoffset, PROTREAD, MAPPRIVATE, fd, paoffset); if (addr == MAPFAILED) handleerror("mmap");
s = write(STDOUTFILENO, addr + offset - paoffset, length); if (s != length) {
if (s == -1) handleerror("write"); fprintf(stderr, "partial write"); exit(EXITFAILURE); } exit(EXITSUCCESS); } SEE ALSO getpagesize(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2), remapfilepages(2), setrlimit(2), shmat(2), shmopen(3), shmoverview(7) The descriptions of the following files in proc(5): /proc/[pid]/maps, /proc/[pid]/mapfiles, and /proc/[pid]/smaps.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. COLOPHON
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Linux 2013-04-17 MMAP(2)