1MREMAP(2) Linux Programmer's Manual MREMAP(2)
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6 mremap - re-map 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);
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.
25 In Linux the memory is divided into pages. A user process has (one or)
27 several linear virtual memory segments. Each virtual memory segment
28 has one or more mappings to real memory pages (in the page table).
29 Each virtual memory segment has its own protection (access rights),
30 which may cause a segmentation violation if the memory is accessed
31 incorrectly (e.g., writing to a read-only segment). Accessing virtual
32 memory outside of the segments will also cause a segmentation viola‐
33 tion.
34 mremap() uses the Linux page table scheme. mremap() changes the map‐
36 ping between virtual addresses and memory pages. This can be used to
37 implement a very efficient realloc(3).
38 The flags bit-mask argument may be 0, or include the following flag:
40 MREMAP_MAYMOVE
42 By default, if there is not sufficient space to expand a mapping
43 at its current location, then mremap() fails. If this flag is
44 specified, then the kernel is permitted to relocate the mapping
45 to a new virtual address, if necessary. If the mapping is relo‐
46 cated, then absolute pointers into the old mapping location
47 become invalid (offsets relative to the starting address of the
48 mapping should be employed).
49 MREMAP_FIXED (since Linux 2.3.31)
51 This flag serves a similar purpose to the MAP_FIXED flag of
52 mmap(2). If this flag is specified, then mremap() accepts a
53 fifth argument, void *new_address, which specifies a page-
54 aligned address to which the mapping must be moved. Any previ‐
55 ous mapping at the address range specified by new_address and
56 new_size is unmapped. If MREMAP_FIXED is specified, then
57 MREMAP_MAYMOVE must also be specified.
58 If the memory segment specified by old_address and old_size is locked
60 (using mlock(2) or similar), then this lock is maintained when the seg‐
61 ment is resized and/or relocated. As a consequence, the amount of mem‐
62 ory locked by the process may change.
63
65 On success mremap() returns a pointer to the new virtual memory area.
66 On error, the value MAP_FAILED (that is, (void *) -1) is returned, and
67 errno is set appropriately.
68
70 EAGAIN The caller tried to expand a memory segment that is locked, but
71 this was not possible without exceeding the RLIMIT_MEMLOCK
72 resource limit.
73 EFAULT "Segmentation fault." Some address in the range old_address to
75 old_address+old_size is an invalid virtual memory address for
76 this process. You can also get EFAULT even if there exist map‐
77 pings that cover the whole address space requested, but those
78 mappings are of different types.
79 EINVAL An invalid argument was given. Possible causes are: old_address
81 was not page aligned; a value other than MREMAP_MAYMOVE or
82 MREMAP_FIXED was specified in flags; new_size was zero; new_size
83 or new_address was invalid; or the new address range specified
84 by new_address and new_size overlapped the old address range
85 specified by old_address and old_size; or MREMAP_FIXED was spec‐
86 ified without also specifying MREMAP_MAYMOVE.
87 ENOMEM The memory area cannot be expanded at the current virtual
89 address, and the MREMAP_MAYMOVE flag is not set in flags. Or,
90 there is not enough (virtual) memory available.
91
93 This call is Linux-specific, and should not be used in programs
94 intended to be portable.
95
97 Prior to version 2.4, glibc did not expose the definition of
98 MREMAP_FIXED, and the prototype for mremap() did not allow for the
99 new_address argument.
100
102 brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), mal‐
103 loc(3), realloc(3), feature_test_macros(7)
104 Your favorite OS text book for more information on paged memory. (Mod‐
106 ern Operating Systems by Andrew S. Tannenbaum, Inside Linux by Randolf
107 Bentson, The Design of the UNIX Operating System by Maurice J. Bach.)
108
110 This page is part of release 3.22 of the Linux man-pages project. A
111 description of the project, and information about reporting bugs, can
112 be found at http://www.kernel.org/doc/man-pages/.
113Linux 2005-09-13 MREMAP(2)