Programming

Hi,

I am seeing varying results about, when realloc() fails in reallocation.

Which one is correct out of the below?

a) realloc() maintains the original pointer (i.e) the original pointer is left unaltered/untouched but relloc() returns the NULL value.
b) original buffer pointer is lost (i.e) the original pointer may point to some location may be zero (NULL) as well, and realloc() does not free() the original memory, so a memory leak

POSIX (IEEE Std 1003.1-2008) states that:
See realloc

It is silent on what happens to the original memory, but my experience is that the original memory is preserved.

The leak may be from code like this:

Code:

    char *ptr;
       .
       .
       .
    ptr = ( char * ) realloc( ptr, newsize );

If the realloc() call in that code returns NULL, the original pointer value is overwritten and lost, thus resulting in a memory leak. This code is better:

Code:

    char *ptr;
    char *tmp_ptr;
       .
       .
       .
    tmp_ptr = ( char * ) realloc( ptr, newsize );
    if ( NULL != tmp_ptr )
    {
        ptr = tmp_ptr;
    }
    else
    {
        /* error-handling code */
        .
        .
        .
    }

realloc(3) on my system states:

So it seems that if realloc() fails, your original memory isn't free()'d and you can use it as if you'd never called realloc() in the first place.