1MREMAP(2) Linux Programmer's Manual MREMAP(2)
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6 mremap - remap a virtual memory address
7
9 #define _GNU_SOURCE /* See feature_test_macros(7) */
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 If the value of old_size is zero, and old_address refers to a shareable
28 mapping (see mmap(2) MAP_SHARED), then mremap() will create a new map‐
29 ping of the same pages. new_size will be the size of the new mapping
30 and the location of the new mapping may be specified with new_address;
31 see the description of MREMAP_FIXED below. If a new mapping is
32 requested via this method, then the MREMAP_MAYMOVE flag must also be
33 specified.
34 In Linux the memory is divided into pages. A user process has (one or)
36 several linear virtual memory segments. Each virtual memory segment
37 has one or more mappings to real memory pages (in the page table).
38 Each virtual memory segment has its own protection (access rights),
39 which may cause a segmentation violation if the memory is accessed
40 incorrectly (e.g., writing to a read-only segment). Accessing virtual
41 memory outside of the segments will also cause a segmentation viola‐
42 tion.
43 mremap() uses the Linux page table scheme. mremap() changes the map‐
45 ping between virtual addresses and memory pages. This can be used to
46 implement a very efficient realloc(3).
47 The flags bit-mask argument may be 0, or include the following flag:
49 MREMAP_MAYMOVE
51 By default, if there is not sufficient space to expand a mapping
52 at its current location, then mremap() fails. If this flag is
53 specified, then the kernel is permitted to relocate the mapping
54 to a new virtual address, if necessary. If the mapping is relo‐
55 cated, then absolute pointers into the old mapping location
56 become invalid (offsets relative to the starting address of the
57 mapping should be employed).
58 MREMAP_FIXED (since Linux 2.3.31)
60 This flag serves a similar purpose to the MAP_FIXED flag of
61 mmap(2). If this flag is specified, then mremap() accepts a
62 fifth argument, void *new_address, which specifies a page-
63 aligned address to which the mapping must be moved. Any previ‐
64 ous mapping at the address range specified by new_address and
65 new_size is unmapped. If MREMAP_FIXED is specified, then
66 MREMAP_MAYMOVE must also be specified.
67 If the memory segment specified by old_address and old_size is locked
69 (using mlock(2) or similar), then this lock is maintained when the seg‐
70 ment is resized and/or relocated. As a consequence, the amount of mem‐
71 ory locked by the process may change.
72
74 On success mremap() returns a pointer to the new virtual memory area.
75 On error, the value MAP_FAILED (that is, (void *) -1) is returned, and
76 errno is set appropriately.
77
79 EAGAIN The caller tried to expand a memory segment that is locked, but
80 this was not possible without exceeding the RLIMIT_MEMLOCK
81 resource limit.
82 EFAULT "Segmentation fault." Some address in the range old_address to
84 old_address+old_size is an invalid virtual memory address for
85 this process. You can also get EFAULT even if there exist map‐
86 pings that cover the whole address space requested, but those
87 mappings are of different types.
88 EINVAL An invalid argument was given. Possible causes are:
90 * old_address was not page aligned;
92 * a value other than MREMAP_MAYMOVE or MREMAP_FIXED was speci‐
94 fied in flags;
95 * new_size was zero;
97 * new_size or new_address was invalid;
99 * the new address range specified by new_address and new_size
101 overlapped the old address range specified by old_address and
102 old_size;
103 * MREMAP_FIXED was specified without also specifying
105 MREMAP_MAYMOVE;
106 * old_size was zero and old_address does not refer to a share‐
108 able mapping (but see BUGS);
109 * old_size was zero and the MREMAP_MAYMOVE flag was not speci‐
111 fied.
112 ENOMEM The memory area cannot be expanded at the current virtual
114 address, and the MREMAP_MAYMOVE flag is not set in flags. Or,
115 there is not enough (virtual) memory available.
116
118 This call is Linux-specific, and should not be used in programs
119 intended to be portable.
120
122 Prior to version 2.4, glibc did not expose the definition of
123 MREMAP_FIXED, and the prototype for mremap() did not allow for the
124 new_address argument.
125 If mremap() is used to move or expand an area locked with mlock(2) or
127 equivalent, the mremap() call will make a best effort to populate the
128 new area but will not fail with ENOMEM if the area cannot be populated.
129
131 Before Linux 4.14, if old_size was zero and the mapping referred to by
132 old_address was a private mapping (mmap(2) MAP_PRIVATE), mremap() cre‐
133 ated a new private mapping unrelated to the original mapping. This
134 behavior was unintended and probably unexpected in user-space applica‐
135 tions (since the intention of mremap() is to create a new mapping based
136 on the original mapping). Since Linux 4.14, mremap() fails with the
137 error EINVAL in this scenario.
138
140 brk(2), getpagesize(2), getrlimit(2), mlock(2), mmap(2), sbrk(2), mal‐
141 loc(3), realloc(3)
142 Your favorite text book on operating systems for more information on
144 paged memory (e.g., Modern Operating Systems by Andrew S. Tanenbaum,
145 Inside Linux by Randolf Bentson, The Design of the UNIX Operating Sys‐
146 tem by Maurice J. Bach)
147
149 This page is part of release 5.04 of the Linux man-pages project. A
150 description of the project, information about reporting bugs, and the
151 latest version of this page, can be found at
152 https://www.kernel.org/doc/man-pages/.
153Linux 2019-03-06 MREMAP(2)