REALLOC(3POSIX) POSIX Programmer's Manual REALLOC(3POSIX)
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.
realloc — memory reallocator
void *realloc(void *ptr, size_t size);
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
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, realloc() shall return a pointer to the (possibly moved) allo‐
cated space. If size is 0, either:
* A null pointer shall be returned and errno set to an implementation-defined value.
* 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 realloc() function shall fail if:
ENOMEM Insufficient memory is available.
The following sections are informative.
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) to free 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
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 clar‐
ifying the language to explicitly prohibit that alternative.
calloc(), free(), malloc()
The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
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 discrep‐
ancy 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 https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 REALLOC(3POSIX)