The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


realloc - memory reallocator


#include <stdlib.h>

void *realloc(void *
ptr, size_t size);


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

The realloc() function shall change the size of the memory object pointed to by ptr to the size specified by size. The contents of the object shall remain unchanged up to the lesser of the new and old sizes. If the new size of the memory object would require movement of the object, the space for the previous instantiation of the object is freed. If the new size is larger, the contents of the newly allocated portion of the object are unspecified. If size is 0 and ptr is not a null pointer, the object pointed to is freed. If the space cannot be allocated, the object shall remain unchanged.

If ptr is a null pointer, realloc() shall be equivalent to malloc() for the specified size.

If ptr does not match a pointer returned earlier by calloc(), malloc(), or realloc() or if the space has previously been deallocated by a call to free() or realloc(), the behavior is undefined.

The order and contiguity of storage allocated by successive calls to realloc() is unspecified. The pointer returned if the allocation succeeds shall be suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object in the space allocated (until the space is explicitly freed or reallocated). Each such allocation shall yield a pointer to an object disjoint from any other object. The pointer returned shall point to the start (lowest byte address) of the allocated space. If the space cannot be allocated, a null pointer shall be returned.


Upon successful completion with a size not equal to 0, realloc() shall return a pointer to the (possibly moved) allocated space. If size is 0, either a null pointer or a unique pointer that can be successfully passed to free() shall be returned. If there is not enough available memory, realloc() shall return a null pointer [CX] [Option Start]  and set errno to [ENOMEM]. [Option End]


The realloc() function shall fail if:

[CX] [Option Start] Insufficient memory is available. [Option End]

The following sections are informative.










calloc(), free(), malloc(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

End of informative text.

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POSIX ® is a registered Trademark of The IEEE.
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