C string realloc and concat

json_tests.c

char * strautocat(char **buffer, const char *str1)  //char **buffer, const char *format, vargs? Ive done this before
{
    assert(str1 != NULL); assert(buffer != NULL);
    if(*buffer == NULL){*buffer = (char *)calloc(sizeof(char)*DEFAULT_STRING_LENGTH,sizeof(char));}

    size_t allocated_size, required_size;
        allocated_size = malloc_usable_size(*buffer);
        required_size  = strlen(str1)+strlen(*buffer)+1;

    while(required_size > allocated_size)
    {
        printf("Growing buffer from %d to accomidate %d\n", allocated_size, required_size);
        *buffer = (char *)realloc(*buffer, malloc_usable_size(*buffer)*2);
        allocated_size = malloc_usable_size(*buffer);
    }
    strcat(*buffer, str1);
    return *buffer;
}

Should malloc_usable_size() be really used in production code?

Notes from the man pages:

The value returned by malloc_usable_size() may be greater than the requested size of the allocation because of alignment and minimum size constraints. Although the excess bytes can be overwritten by the application without ill effects, this is not good programming practice: the number of excess bytes in an allocation depends on the underlying implementation.

The main use of this function is for debugging and introspection.