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.1‐2017 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

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

        *  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         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

SEE ALSO         top

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

       The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)

COPYRIGHT         top

       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 .

       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               2017                       REALLOC(3P)

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