REALLOC(P) POSIX Programmer's Manual REALLOC(P)
realloc - memory reallocator
void *realloc(void *ptr, size_t size);
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
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
The order and contiguity of storage allocated by successive calls to realloc() is unspeci-
fied. 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 and set errno to
The realloc() function shall fail if:
ENOMEM Insufficient memory is available.
The following sections are informative.
calloc() , free() , malloc() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 REALLOC(P)