This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
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
POSIX.1‐2008 defers to the ISO C standard.
The realloc() function shall deallocate the old object pointed to by
ptr and return a pointer to a new object that has the size specified
by size. The contents of the new object shall be the same as that of
the old object prior to deallocation, up to the lesser of the new and
old sizes. Any bytes in the new object beyond the size of the old
object have indeterminate values. If the size of the space requested
is zero, the behavior shall be implementation-defined: either a null
pointer is returned, or the behavior shall be as if the size were
some non-zero value, except that the returned pointer shall not be
used to access an object. 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
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, realloc() shall return a pointer to the
(possibly moved) allocated space. If size is 0, either:
* A null pointer shall be returned and errno set to an
* A unique pointer that can be successfully passed to free() shall
be returned, and the memory object pointed to by ptr shall be
freed. The application shall ensure that the pointer is not used
to access an object.
If there is not enough available memory, realloc() shall return a
null pointer and set errno to [ENOMEM]. If realloc() returns a null
pointer and errno has been set to [ENOMEM], the memory referenced by
ptr shall not be changed.
The description of realloc() has been modified from previous versions
of this standard to align with the ISO/IEC 9899:1999 standard.
Previous versions explicitly permitted a call to realloc(p, 0) tofree the space pointed to by p and return a null pointer. While this
behavior could be interpreted as permitted by this version of the
standard, the C language committee have indicated that this
interpretation is incorrect. Applications should assume that if
realloc() returns a null pointer, the space pointed to by p has not
been freed. Since this could lead to double-frees, implementations
should also set errno if a null pointer actually indicates a failure,
and applications should only free the space if errno was changed.
This standard defers to the ISO C standard. While that standard
currently has language that might permit realloc(p, 0), where p is
not a null pointer, to free p while still returning a null pointer,
the committee responsible for that standard is considering clarifying
the language to explicitly prohibit that alternative.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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 original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 REALLOC(3P)