1REALLOC(3P) POSIX Programmer's Manual REALLOC(3P)
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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.
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13 realloc — memory reallocator
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16 #include <stdlib.h>
17 void *realloc(void *ptr, size_t size);
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21 The functionality described on this reference page is aligned with the
22 ISO C standard. Any conflict between the requirements described here
23 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
24 defers to the ISO C standard.
25 The realloc() function shall deallocate the old object pointed to by
27 ptr and return a pointer to a new object that has the size specified by
28 size. The contents of the new object shall be the same as that of the
29 old object prior to deallocation, up to the lesser of the new and old
30 sizes. Any bytes in the new object beyond the size of the old object
31 have indeterminate values. If the size of the space requested is zero,
32 the behavior shall be implementation-defined: either a null pointer is
33 returned, or the behavior shall be as if the size were some non-zero
34 value, except that the returned pointer shall not be used to access an
35 object. If the space cannot be allocated, the object shall remain
36 unchanged.
37 If ptr is a null pointer, realloc() shall be equivalent to malloc() for
39 the specified size.
40 If ptr does not match a pointer returned earlier by calloc(), malloc(),
42 or realloc() or if the space has previously been deallocated by a call
43 to free() or realloc(), the behavior is undefined.
44 The order and contiguity of storage allocated by successive calls to
46 realloc() is unspecified. The pointer returned if the allocation suc‐
47 ceeds shall be suitably aligned so that it may be assigned to a pointer
48 to any type of object and then used to access such an object in the
49 space allocated (until the space is explicitly freed or reallocated).
50 Each such allocation shall yield a pointer to an object disjoint from
51 any other object. The pointer returned shall point to the start (lowest
52 byte address) of the allocated space. If the space cannot be allocated,
53 a null pointer shall be returned.
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56 Upon successful completion, realloc() shall return a pointer to the
57 (possibly moved) allocated space. If size is 0, either:
58 * A null pointer shall be returned and errno set to an implementa‐
60 tion-defined value.
61 * A unique pointer that can be successfully passed to free() shall be
63 returned, and the memory object pointed to by ptr shall be freed.
64 The application shall ensure that the pointer is not used to access
65 an object.
66 If there is not enough available memory, realloc() shall return a null
68 pointer and set errno to [ENOMEM]. If realloc() returns a null pointer
69 and errno has been set to [ENOMEM], the memory referenced by ptr shall
70 not be changed.
71
73 The realloc() function shall fail if:
74 ENOMEM Insufficient memory is available.
76 The following sections are informative.
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80 None.
81
83 The description of realloc() has been modified from previous versions
84 of this standard to align with the ISO/IEC 9899:1999 standard. Previous
85 versions explicitly permitted a call to realloc(p, 0) to free the space
86 pointed to by p and return a null pointer. While this behavior could be
87 interpreted as permitted by this version of the standard, the C lan‐
88 guage committee have indicated that this interpretation is incorrect.
89 Applications should assume that if realloc() returns a null pointer,
90 the space pointed to by p has not been freed. Since this could lead to
91 double-frees, implementations should also set errno if a null pointer
92 actually indicates a failure, and applications should only free the
93 space if errno was changed.
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96 None.
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99 This standard defers to the ISO C standard. While that standard cur‐
100 rently has language that might permit realloc(p, 0), where p is not a
101 null pointer, to free p while still returning a null pointer, the com‐
102 mittee responsible for that standard is considering clarifying the lan‐
103 guage to explicitly prohibit that alternative.
104
106 calloc(), free(), malloc()
107 The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
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111 Portions of this text are reprinted and reproduced in electronic form
112 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
113 -- Portable Operating System Interface (POSIX), The Open Group Base
114 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
115 cal and Electronics Engineers, Inc and The Open Group. (This is
116 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
117 event of any discrepancy between this version and the original IEEE and
118 The Open Group Standard, the original IEEE and The Open Group Standard
119 is the referee document. The original Standard can be obtained online
120 at http://www.unix.org/online.html .
121 Any typographical or formatting errors that appear in this page are
123 most likely to have been introduced during the conversion of the source
124 files to man page format. To report such errors, see https://www.ker‐
125 nel.org/doc/man-pages/reporting_bugs.html .
126IEEE/The Open Group 2013 REALLOC(3P)