If you're being sensible and using -Wall with gcc, it will pick up the %zu thing anyway: sizeof-test.c: In function ‘main’: sizeof-test.c:5:3: warning: format

If you're being sensible and using -Wall with gcc, it will pick up the %zu thing anyway: sizeof-test.c: In function ‘main’: sizeof-test.c:5:3: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘long unsigned int’ Compilers have warnings so that you don't have to remember archaic bits of C. :)