Section: POSIX Programmer's Manual (3P)
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.
--- 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.1-2017 defers to the ISO C standard.
function shall deallocate the old object pointed to by
and return a pointer to a new object that has the size specified by
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 behavior is undefined if the returned pointer
is used to access an object. If the space cannot be allocated,
the object shall remain unchanged.
is a null pointer,
shall be equivalent to
for the specified size.
does not match a pointer returned earlier by
or if the space has previously been deallocated by a call to
the behavior is undefined.
The order and contiguity of storage allocated by successive calls to
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,
shall return a pointer to the (possibly moved) allocated space. If
is 0, either:
A null pointer shall be returned
is not a null pointer,
shall be set to an implementation-defined value.
A pointer to the allocated space shall be returned, and the
memory object pointed to by
shall be freed. The application shall ensure that the pointer is not
used to access an object.
If there is not enough available memory,
shall return a null pointer
returns a null pointer
has been set to
the memory referenced by
shall not be changed.
function shall fail if:
Insufficient memory is available.
The following sections are informative.
The description of
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
(p, 0) to free the space pointed to by
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
returns a null pointer, the space pointed to by
has not been freed. Since this could lead to double-frees, implementations
should also set
if a null pointer actually indicates a failure, and applications should
only free the space if
This standard defers to the ISO C standard. While that standard currently has
language that might permit
(p, 0), where
is not a null pointer, to free
while still returning a null pointer, the committee responsible for that
standard is considering clarifying the language to explicitly prohibit
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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 original Standard can be obtained online at
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