REALLOC(3)                 POSIX Programmer's Manual                REALLOC(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       realloc -- memory reallocator

SYNOPSIS
       #include <stdlib.h>

       void *realloc(void *ptr, size_t size);

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.1-2017
       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 behavior is undefined if the returned pointer is
       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 undefined.

       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.

RETURN VALUE
       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, if ptr is not a null pointer,
           errno shall be set to an implementation-defined value.

        *  A pointer to the allocated space 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.

ERRORS
       The realloc() function shall fail if:

       ENOMEM Insufficient memory is available.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       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 was changed.

RATIONALE
       None.

FUTURE DIRECTIONS
       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.

SEE ALSO
       calloc(), free(), malloc()

       The Base Definitions volume of POSIX.1-2017, <stdlib.h>

COPYRIGHT
       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 http://www.opengroup.org/unix/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                  2017                           REALLOC(3)
