<cstdlib>

Unsigned integral type

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

In <cstlib>, it is used as the type of some parameters in the functions bsearch, qsort, calloc, malloc, realloc, mblen, mbtowc, mbstowcs and wcstombs, and in the case of mbstowcs and wcstombs 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.