.\" Copyright (C) 1996 Andries Brouwer .\" and Copyright (C) 2006, 2007 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified 1997-01-31 by Eric S. Raymond .\" Modified 2000-03-25 by Jim Van Zandt .\" Modified 2001-10-04 by John Levon .\" Modified 2003-02-02 by Andi Kleen .\" Modified 2003-05-21 by Michael Kerrisk .\" MAP_LOCKED works from 2.5.37 .\" Modified 2004-06-17 by Michael Kerrisk .\" Modified 2004-09-11 by aeb .\" Modified 2004-12-08, from Eric Estievenart .\" Modified 2004-12-08, mtk, formatting tidy-ups .\" Modified 2006-12-04, mtk, various parts rewritten .\" 2007-07-10, mtk, Added an example program. .\" 2008-11-18, mtk, document MAP_STACK .\" .TH MMAP 2 2014-08-19 "Linux" "Linux Programmer's Manual" .SH NAME mmap, munmap \- map or unmap files or devices into memory .SH SYNOPSIS .nf .B #include .sp .BI "void *mmap(void *" addr ", size_t " length \ ", int " prot ", int " flags , .BI " int " fd ", off_t " offset ); .BI "int munmap(void *" addr ", size_t " length ); .fi See NOTES for information on feature test macro requirements. .SH DESCRIPTION .BR mmap () creates a new mapping in the virtual address space of the calling process. The starting address for the new mapping is specified in .IR addr . The .I length argument specifies the length of the mapping. If .I 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 .I 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. .\" Before Linux 2.6.24, the address was rounded up to the next page .\" boundary; since 2.6.24, it is rounded down! 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 .B MAP_ANONYMOUS below), are initialized using .I length bytes starting at offset .I offset in the file (or other object) referred to by the file descriptor .IR fd . .I offset must be a multiple of the page size as returned by .IR sysconf(_SC_PAGE_SIZE) . .LP The .I prot argument describes the desired memory protection of the mapping (and must not conflict with the open mode of the file). It is either .B PROT_NONE or the bitwise OR of one or more of the following flags: .TP 1.1i .B PROT_EXEC Pages may be executed. .TP .B PROT_READ Pages may be read. .TP .B PROT_WRITE Pages may be written. .TP .B PROT_NONE Pages may not be accessed. .LP The .I 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 .IR flags : .TP 1.1i .B 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 .BR msync (2) or .BR munmap () is called. .TP .B 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 .BR mmap () call are visible in the mapped region. .LP Both of these flags are described in POSIX.1-2001. In addition, zero or more of the following values can be ORed in .IR flags : .TP .BR 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 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. .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 Modern x86-64 processors no longer have this performance problem, so use of this flag is not required on those systems. The .B MAP_32BIT flag is ignored when .B MAP_FIXED is set. .TP .B MAP_ANON Synonym for .BR MAP_ANONYMOUS . Deprecated. .TP .B MAP_ANONYMOUS The mapping is not backed by any file; its contents are initialized to zero. The .I fd and .I offset arguments are ignored; however, some implementations require .I fd to be \-1 if .B MAP_ANONYMOUS (or .BR MAP_ANON ) is specified, and portable applications should ensure this. The use of .B MAP_ANONYMOUS in conjunction with .B MAP_SHARED is supported on Linux only since kernel 2.4. .TP .B MAP_DENYWRITE This flag is ignored. .\" Introduced in 1.1.36, removed in 1.3.24. (Long ago, it signaled that attempts to write to the underlying file should fail with .BR ETXTBUSY . But this was a source of denial-of-service attacks.) .TP .B MAP_EXECUTABLE This flag is ignored. .\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link. .\" (Long ago, it signaled that the underlying file is an executable. .\" However, that information was not really used anywhere.) .\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of .\" MAP_DENYWRITE? .TP .B MAP_FILE Compatibility flag. Ignored. .\" On some systems, this was required as the opposite of .\" MAP_ANONYMOUS -- mtk, 1 May 2007 .TP .B MAP_FIXED Don't interpret .I addr as a hint: place the mapping at exactly that address. .I addr must be a multiple of the page size. If the memory region specified by .I addr and .I 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, .BR mmap () will fail. Because requiring a fixed address for a mapping is less portable, the use of this option is discouraged. .TP .B MAP_GROWSDOWN Used for stacks. Indicates to the kernel virtual memory system that the mapping should extend downward in memory. .TP .BR MAP_HUGETLB " (since Linux 2.6.32)" Allocate the mapping using "huge pages." See the Linux kernel source file .I Documentation/vm/hugetlbpage.txt for further information. .TP .BR MAP_LOCKED " (since Linux 2.5.37)" Lock the pages of the mapped region into memory in the manner of .BR mlock (2). This flag is ignored in older kernels. .\" If set, the mapped pages will not be swapped out. .TP .BR MAP_NONBLOCK " (since Linux 2.5.46)" Only meaningful in conjunction with .BR MAP_POPULATE . 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 .BR MAP_POPULATE to do nothing. One day the combination of .BR MAP_POPULATE and .BR MAP_NONBLOCK may be reimplemented. .TP .B 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 .B SIGSEGV upon a write if no physical memory is available. See also the discussion of the file .I /proc/sys/vm/overcommit_memory in .BR proc (5). In kernels before 2.6, this flag had effect only for private writable mappings. .TP .BR 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. .BR MAP_POPULATE is supported for private mappings only since Linux 2.6.23. .TP .BR 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. .\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08 .\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7 .\" http://thread.gmane.org/gmane.linux.kernel/720412 .\" "pthread_create() slow for many threads; also time to revisit 64b .\" context switch optimization?" .TP .BR MAP_UNINITIALIZED " (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 .B CONFIG_MMAP_ALLOW_UNINITIALIZED 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). .LP Of the above flags, only .B MAP_FIXED is specified in POSIX.1-2001. However, most systems also support .B MAP_ANONYMOUS (or its synonym .BR MAP_ANON ). .LP Some systems document the additional flags .BR MAP_AUTOGROW , .BR MAP_AUTORESRV , .BR MAP_COPY , and .BR MAP_LOCAL . .LP Memory mapped by .BR mmap () is preserved across .BR fork (2), with the same attributes. .LP 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. .SS munmap() The .BR 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. .LP The address .I 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 .BR SIGSEGV . It is not an error if the indicated range does not contain any mapped pages. .SS Timestamps changes for file-backed mappings For file-backed mappings, the .I st_atime field for the mapped file may be updated at any time between the .BR mmap () and the corresponding unmapping; the first reference to a mapped page will update the field if it has not been already. .LP The .I st_ctime and .I st_mtime field for a file mapped with .B PROT_WRITE and .B MAP_SHARED will be updated after a write to the mapped region, and before a subsequent .BR msync (2) with the .B MS_SYNC or .B MS_ASYNC flag, if one occurs. .SH RETURN VALUE On success, .BR mmap () returns a pointer to the mapped area. On error, the value .B MAP_FAILED (that is, .IR "(void\ *)\ \-1" ) is returned, and .I errno is set appropriately. On success, .BR munmap () returns 0, on failure \-1, and .I errno is set (probably to .BR EINVAL ). .SH ERRORS .TP .B EACCES A file descriptor refers to a non-regular file. Or a file mapping was requested, but .I fd is not open for reading. Or .B MAP_SHARED was requested and .B PROT_WRITE is set, but .I fd is not open in read/write .RB ( O_RDWR ) mode. Or .B PROT_WRITE is set, but the file is append-only. .TP .B EAGAIN The file has been locked, or too much memory has been locked (see .BR setrlimit (2)). .TP .B EBADF .I fd is not a valid file descriptor (and .B MAP_ANONYMOUS was not set). .TP .B EINVAL We don't like .IR addr , .IR length , or .I offset (e.g., they are too large, or not aligned on a page boundary). .TP .B EINVAL (since Linux 2.6.12) .I length was 0. .TP .B EINVAL .I flags contained neither .B MAP_PRIVATE or .BR MAP_SHARED , or contained both of these values. .TP .B ENFILE .\" This is for shared anonymous segments .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() The system limit on the total number of open files has been reached. .\" .TP .\" .B ENOEXEC .\" A file could not be mapped for reading. .TP .B ENODEV The underlying filesystem of the specified file does not support memory mapping. .TP .B ENOMEM No memory is available, or the process's maximum number of mappings would have been exceeded. .TP .B EPERM The .I prot argument asks for .B PROT_EXEC but the mapped area belongs to a file on a filesystem that was mounted no-exec. .\" (Since 2.4.25 / 2.6.0.) .TP .B ETXTBSY .B MAP_DENYWRITE was set but the object specified by .I fd is open for writing. .TP .B EOVERFLOW On 32-bit architecture together with the large file extension (i.e., using 64-bit .IR off_t ): the number of pages used for .I length plus number of pages used for .I offset would overflow .I "unsigned long" (32 bits). .LP Use of a mapped region can result in these signals: .TP .B SIGSEGV Attempted write into a region mapped as read-only. .TP .B 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). .SH CONFORMING TO SVr4, 4.4BSD, POSIX.1-2001. .\" SVr4 documents additional error codes ENXIO and ENODEV. .\" SUSv2 documents additional error codes EMFILE and EOVERFLOW. .SH AVAILABILITY On POSIX systems on which .BR mmap (), .BR msync (2), and .BR munmap () are available, .B _POSIX_MAPPED_FILES is defined in \fI\fP to a value greater than 0. (See also .BR sysconf (3).) .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L. .\" -1: unavailable, 0: ask using sysconf(). .\" glibc defines it to 1. .SH NOTES On some hardware architectures (e.g., i386), .B PROT_WRITE implies .BR PROT_READ . It is architecture dependent whether .B PROT_READ implies .B PROT_EXEC or not. Portable programs should always set .B PROT_EXEC if they intend to execute code in the new mapping. The portable way to create a mapping is to specify .I addr as 0 (NULL), and omit .B MAP_FIXED from .IR 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 .B MAP_FIXED flag is specified, and .I addr is 0 (NULL), then the mapped address will be 0 (NULL). Certain .I flags constants are defined only if either .BR _BSD_SOURCE or .BR _SVID_SOURCE is defined. (Requiring .BR _GNU_SOURCE also suffices, and requiring that macro specifically would have been more logical, since these flags are all Linux-specific.) The relevant flags are: .BR MAP_32BIT , .BR MAP_ANONYMOUS (and the synonym .BR MAP_ANON ), .BR MAP_DENYWRITE , .BR MAP_EXECUTABLE , .BR MAP_FILE , .BR MAP_GROWSDOWN , .BR MAP_HUGETLB , .BR MAP_LOCKED , .BR MAP_NONBLOCK , .BR MAP_NORESERVE , .BR MAP_POPULATE , and .BR MAP_STACK . .\" .SS C library/kernel ABI differences This page describes the interface provided by the glibc .BR 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 .BR mmap2 (2), and nowadays .\" Since around glibc 2.1/2.2, depending on the platform. the glibc .BR mmap () wrapper function invokes .BR mmap2 (2) with a suitably adjusted value for .IR offset . .SH BUGS On Linux there are no guarantees like those suggested above under .BR 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 .B MAP_POPULATE flag has effect only if .I prot is specified as .BR PROT_NONE . SUSv3 specifies that .BR mmap () should fail if .I length is 0. However, in kernels before 2.6.12, .BR mmap () succeeded in this case: no mapping was created and the call returned .IR addr . Since kernel 2.6.12, .BR mmap () fails with the error .B 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 modification 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 calling .BR msync (2) before the unmap takes place; however, this doesn't work on tmpfs (for example, when using POSIX shared memory interface documented in .BR shm_overview (7)). .SH EXAMPLE .\" FIXME . Add an example here that uses an anonymous shared region for .\" IPC between parent and child. .PP 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 .BR write (2) to output the desired bytes. .SS Program source .nf #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\(aqt 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); } .fi .SH SEE ALSO .BR getpagesize (2), .BR mincore (2), .BR mlock (2), .BR mmap2 (2), .BR mprotect (2), .BR mremap (2), .BR msync (2), .BR remap_file_pages (2), .BR setrlimit (2), .BR shmat (2), .BR shm_open (3), .BR shm_overview (7) The descriptions of the following files in .BR proc (5): .IR /proc/[pid]/maps , .IR /proc/[pid]/map_files , and .IR /proc/[pid]/smaps . B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391. .\" .\" Repeat after me: private read-only mappings are 100% equivalent to .\" shared read-only mappings. No ifs, buts, or maybes. -- Linus .SH COLOPHON This page is part of release 3.74 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%http://www.kernel.org/doc/man\-pages/.