#include <stdlib.h>

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


Partially implemented


IEEE Std 1003.1-2017


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.


The realloc() function shall fail if:

  • ENOMEM - Insufficient memory is available.



Known bugs


See Also

  1. Standard library functions
  2. Table of Contents