<cstdio>

Unsigned integral type

size_t corresponds to the integral data type returned by the language operator sizeof and is defined in the <cstdio> header file (among others) as an unsigned integral type.

In <cstdio>, it is used as the type of some parameters in the functions fread, fwrite and setvbuf, and in the case of fread and fwrite also as its returning type.
In all cases it is used as a type to represent the length or count in bytes of a specific buffer or string.