realloc

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



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.

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(3P)