oracular (3) realloc.3posix.gz

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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>


       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 .

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