This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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)tofreethespacepointedtobyp 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.

       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(3POSIX)

       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.

       The realloc() function shall fail if:

       ENOMEM Insufficient memory is available.

       Thefollowingsectionsareinformative.

       None.

       This  standard defers to the ISO C standard. While that standard currently has language that might permit
       realloc(p,0),wherep 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.

       realloc — memory reallocator

       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.

       None.

       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.

calloc(), free(), malloc()

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

       #include <stdlib.h>

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