1REALLOC(3P) POSIX Programmer's Manual REALLOC(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 realloc — memory reallocator
13
15 #include <stdlib.h>
16 void *realloc(void *ptr, size_t size);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The realloc() function shall deallocate the old object pointed to by
26 ptr and return a pointer to a new object that has the size specified by
27 size. The contents of the new object shall be the same as that of the
28 old object prior to deallocation, up to the lesser of the new and old
29 sizes. Any bytes in the new object beyond the size of the old object
30 have indeterminate values. If the size of the space requested is zero,
31 the behavior shall be implementation-defined: either a null pointer is
32 returned, or the behavior shall be as if the size were some non-zero
33 value, except that the behavior is undefined if the returned pointer is
34 used to access an object. If the space cannot be allocated, the object
35 shall remain unchanged.
36 If ptr is a null pointer, realloc() shall be equivalent to malloc() for
38 the specified size.
39 If ptr does not match a pointer returned earlier by calloc(), malloc(),
41 or realloc() or if the space has previously been deallocated by a call
42 to free() or realloc(), the behavior is undefined.
43 The order and contiguity of storage allocated by successive calls to
45 realloc() is unspecified. The pointer returned if the allocation suc‐
46 ceeds shall be suitably aligned so that it may be assigned to a pointer
47 to any type of object and then used to access such an object in the
48 space allocated (until the space is explicitly freed or reallocated).
49 Each such allocation shall yield a pointer to an object disjoint from
50 any other object. The pointer returned shall point to the start (lowest
51 byte address) of the allocated space. If the space cannot be allocated,
52 a null pointer shall be returned.
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55 Upon successful completion, realloc() shall return a pointer to the
56 (possibly moved) allocated space. If size is 0, either:
57 * A null pointer shall be returned and, if ptr is not a null pointer,
59 errno shall be set to an implementation-defined value.
60 * A pointer to the allocated space shall be returned, and the memory
62 object pointed to by ptr shall be freed. The application shall
63 ensure that the pointer is not used to access an object.
64 If there is not enough available memory, realloc() shall return a null
66 pointer and set errno to [ENOMEM]. If realloc() returns a null pointer
67 and errno has been set to [ENOMEM], the memory referenced by ptr shall
68 not be changed.
69
71 The realloc() function shall fail if:
72 ENOMEM Insufficient memory is available.
74 The following sections are informative.
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78 None.
79
81 The description of realloc() has been modified from previous versions
82 of this standard to align with the ISO/IEC 9899:1999 standard. Previous
83 versions explicitly permitted a call to realloc(p, 0) to free the space
84 pointed to by p and return a null pointer. While this behavior could be
85 interpreted as permitted by this version of the standard, the C lan‐
86 guage committee have indicated that this interpretation is incorrect.
87 Applications should assume that if realloc() returns a null pointer,
88 the space pointed to by p has not been freed. Since this could lead to
89 double-frees, implementations should also set errno if a null pointer
90 actually indicates a failure, and applications should only free the
91 space if errno was changed.
92
94 None.
95
97 This standard defers to the ISO C standard. While that standard cur‐
98 rently has language that might permit realloc(p, 0), where p is not a
99 null pointer, to free p while still returning a null pointer, the com‐
100 mittee responsible for that standard is considering clarifying the lan‐
101 guage to explicitly prohibit that alternative.
102
104 calloc(), free(), malloc()
105 The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
107
109 Portions of this text are reprinted and reproduced in electronic form
110 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
111 table Operating System Interface (POSIX), The Open Group Base Specifi‐
112 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
113 Electrical and Electronics Engineers, Inc and The Open Group. In the
114 event of any discrepancy between this version and the original IEEE and
115 The Open Group Standard, the original IEEE and The Open Group Standard
116 is the referee document. The original Standard can be obtained online
117 at http://www.opengroup.org/unix/online.html .
118 Any typographical or formatting errors that appear in this page are
120 most likely to have been introduced during the conversion of the source
121 files to man page format. To report such errors, see https://www.ker‐
122 nel.org/doc/man-pages/reporting_bugs.html .
123IEEE/The Open Group 2017 REALLOC(3P)