# realloc

## 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.
    


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