realloc(3p) — Linux manual page


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

PROLOG         top

       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         top

       realloc — memory reallocator

SYNOPSIS         top

       #include <stdlib.h>

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

DESCRIPTION         top

       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 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

       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         top

       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 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.

ERRORS         top

       The realloc() function shall fail if:

       ENOMEM Insufficient memory is available.

       The following sections are informative.

EXAMPLES         top



       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         top



       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         top

       calloc(3p), free(3p), malloc(3p)

       The Base Definitions volume of POSIX.1‐2008, stdlib.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                         REALLOC(3P)

Pages that refer to this page: stdlib.h(0p)calloc(3p)free(3p)malloc(3p)