Calloc clears the memory to all bits zero. All bits zero doesn't necessarily mean zero values. Not many people will make this distinction and happily write broken code thinking that calloc always clears the values to zero accordingly.

calloc's zero fill is all-bits-zero, and is therefore guaranteed to yield the value 0 for all integral types, and '\0' for character types -- but does not guarantee useful null pointer values or floating-point zero values.

Some C implementations represent null pointers by special, nonzero bit patterns when it can be arranged so that inadvertently using those values triggers automatic hardware traps.