1MREMAP(2) Linux Programmer's Manual MREMAP(2)
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6 mremap - remap a virtual memory address
7
9 #define _GNU_SOURCE
10 #include <sys/mman.h>
11 void *mremap(void *old_address, size_t old_size,
13 size_t new_size, int flags, ... /* void *new_address */);
14
16 mremap() expands (or shrinks) an existing memory mapping, potentially
17 moving it at the same time (controlled by the flags argument and the
18 available virtual address space).
19 old_address is the old address of the virtual memory block that you
21 want to expand (or shrink). Note that old_address has to be page
22 aligned. old_size is the old size of the virtual memory block.
23 new_size is the requested size of the virtual memory block after the
24 resize. An optional fifth argument, new_address, may be provided; see
25 the description of MREMAP_FIXED below.
26 In Linux the memory is divided into pages. A user process has (one or)
28 several linear virtual memory segments. Each virtual memory segment
29 has one or more mappings to real memory pages (in the page table).
30 Each virtual memory segment has its own protection (access rights),
31 which may cause a segmentation violation if the memory is accessed
32 incorrectly (e.g., writing to a read-only segment). Accessing virtual
33 memory outside of the segments will also cause a segmentation viola‐
34 tion.
35 mremap() uses the Linux page table scheme. mremap() changes the map‐
37 ping between virtual addresses and memory pages. This can be used to
38 implement a very efficient realloc(3).
39 The flags bit-mask argument may be 0, or include the following flag:
41 MREMAP_MAYMOVE
43 By default, if there is not sufficient space to expand a mapping
44 at its current location, then mremap() fails. If this flag is
45 specified, then the kernel is permitted to relocate the mapping
46 to a new virtual address, if necessary. If the mapping is relo‐
47 cated, then absolute pointers into the old mapping location
48 become invalid (offsets relative to the starting address of the
49 mapping should be employed).
50 MREMAP_FIXED (since Linux 2.3.31)
52 This flag serves a similar purpose to the MAP_FIXED flag of
53 mmap(2). If this flag is specified, then mremap() accepts a
54 fifth argument, void *new_address, which specifies a page-
55 aligned address to which the mapping must be moved. Any previ‐
56 ous mapping at the address range specified by new_address and
57 new_size is unmapped. If MREMAP_FIXED is specified, then
58 MREMAP_MAYMOVE must also be specified.
59 If the memory segment specified by old_address and old_size is locked
61 (using mlock(2) or similar), then this lock is maintained when the seg‐
62 ment is resized and/or relocated. As a consequence, the amount of mem‐
63 ory locked by the process may change.
64
66 On success mremap() returns a pointer to the new virtual memory area.
67 On error, the value MAP_FAILED (that is, (void *) -1) is returned, and
68 errno is set appropriately.
69
71 EAGAIN The caller tried to expand a memory segment that is locked, but
72 this was not possible without exceeding the RLIMIT_MEMLOCK
73 resource limit.
74 EFAULT "Segmentation fault." Some address in the range old_address to
76 old_address+old_size is an invalid virtual memory address for
77 this process. You can also get EFAULT even if there exist map‐
78 pings that cover the whole address space requested, but those
79 mappings are of different types.
80 EINVAL An invalid argument was given. Possible causes are: old_address
82 was not page aligned; a value other than MREMAP_MAYMOVE or
83 MREMAP_FIXED was specified in flags; new_size was zero; new_size
84 or new_address was invalid; or the new address range specified
85 by new_address and new_size overlapped the old address range
86 specified by old_address and old_size; or MREMAP_FIXED was spec‐
87 ified without also specifying MREMAP_MAYMOVE.
88 ENOMEM The memory area cannot be expanded at the current virtual
90 address, and the MREMAP_MAYMOVE flag is not set in flags. Or,
91 there is not enough (virtual) memory available.
92
94 This call is Linux-specific, and should not be used in programs
95 intended to be portable.
96
98 Prior to version 2.4, glibc did not expose the definition of
99 MREMAP_FIXED, and the prototype for mremap() did not allow for the
100 new_address argument.
101
103 brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), mal‐
104 loc(3), realloc(3), feature_test_macros(7)
105 Your favorite OS text book for more information on paged memory. (Mod‐
107 ern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Randolf
108 Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)
109
111 This page is part of release 3.25 of the Linux man-pages project. A
112 description of the project, and information about reporting bugs, can
113 be found at http://www.kernel.org/doc/man-pages/.
114Linux 2010-06-10 MREMAP(2)