1REALLOC(3P)                POSIX Programmer's Manual               REALLOC(3P)
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PROLOG

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

NAME

12       realloc — memory reallocator
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SYNOPSIS

15       #include <stdlib.h>
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17       void *realloc(void *ptr, size_t size);
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DESCRIPTION

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.
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25       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.
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37       If ptr is a null pointer, realloc() shall be equivalent to malloc() for
38       the specified size.
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40       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.
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44       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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RETURN VALUE

55       Upon successful completion, realloc() shall return  a  pointer  to  the
56       (possibly moved) allocated space. If size is 0, either:
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58        *  A null pointer shall be returned and, if ptr is not a null pointer,
59           errno shall be set to an implementation-defined value.
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61        *  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.
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65       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.
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ERRORS

71       The realloc() function shall fail if:
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73       ENOMEM Insufficient memory is available.
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75       The following sections are informative.
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EXAMPLES

78       None.
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APPLICATION USAGE

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.
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RATIONALE

94       None.
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FUTURE DIRECTIONS

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.
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SEE ALSO

104       calloc(), free(), malloc()
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106       The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
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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 .
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119       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 .
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126IEEE/The Open Group                  2017                          REALLOC(3P)
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