Calloc is defined to return memory that's all-bits-zero, which in turn defines the value for many datatypes. The overwhelming majority of C implementations additionally define the values of all-bits-zero floats and pointers. IMO it's reasonable to write programs targeting only such implementations, provided such a dependency is clearly documented (just as e.g. it's reasonable to write C programs targeting only implementations on which floating-point arithmetic is IEEE 754, despite this not being required by the C standard).