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.TH REALLOC "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
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.

.SH NAME
realloc
\(em memory reallocator
.SH SYNOPSIS
.LP
.nf
#include <stdlib.h>
.P
void *realloc(void *\fIptr\fP, size_t \fIsize\fP);
.fi
.SH DESCRIPTION
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\(hy2008 defers to the ISO\ C standard.
.P
The
\fIrealloc\fR()
function shall deallocate the old object pointed to by
.IR ptr
and return a pointer to a new object that has the size specified by
.IR 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.
.P
If
.IR ptr
is a null pointer,
\fIrealloc\fR()
shall be equivalent to
\fImalloc\fR()
for the specified size.
.P
If
.IR ptr
does not match a pointer returned earlier by
\fIcalloc\fR(),
\fImalloc\fR(),
or
\fIrealloc\fR()
or if the space has previously been deallocated by a call to
\fIfree\fR()
or
\fIrealloc\fR(),
the behavior is undefined.
.P
The order and contiguity of storage allocated by successive calls to
\fIrealloc\fR()
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.
.SH "RETURN VALUE"
Upon successful completion,
\fIrealloc\fR()
shall return a pointer to the (possibly moved) allocated space. If
.IR size
is 0, either:
.IP " *" 4
A null pointer shall be returned
and
.IR errno
set to an implementation-defined value.
.IP " *" 4
A unique pointer that can be successfully passed to
\fIfree\fR()
shall be returned, and the memory object pointed to by
.IR ptr
shall be freed. The application shall ensure that the pointer is not
used to access an object.
.P
If there is not enough available memory,
\fIrealloc\fR()
shall return a null pointer
and set
.IR errno
to
.BR [ENOMEM] .
If
\fIrealloc\fR()
returns a null pointer
and
.IR errno
has been set to
.BR [ENOMEM] ,
the memory referenced by
.IR ptr
shall not be changed.
.SH ERRORS
The
\fIrealloc\fR()
function shall fail if:
.TP
.BR ENOMEM
Insufficient memory is available.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The description of
\fIrealloc\fR()
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
.IR realloc \c
(\fIp\fI, 0) to free the space pointed to by
.IR 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
\fIrealloc\fR()
returns a null pointer, the space pointed to by
.IR p
has not been freed. Since this could lead to double-frees, implementations
should also set
.IR errno
if a null pointer actually indicates a failure, and applications should
only free the space if
.IR errno
was changed.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
This standard defers to the ISO\ C standard. While that standard currently has
language that might permit
.IR realloc \c
(\fIp\fI, 0), where
.IR p
is not a null pointer, to free
.IR p
while still returning a null pointer, the committee responsible for that
standard is considering clarifying the language to explicitly prohibit
that alternative.
.SH "SEE ALSO"
.IR "\fIcalloc\fR\^(\|)",
.IR "\fIfree\fR\^(\|)",
.IR "\fImalloc\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "\fB<stdlib.h>\fP"
.SH COPYRIGHT
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 .

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