Main.cc: In function 'int f(int, int)': Main.cc:9:1: warning: control reaches end of non-void function [-Wreturn-type] } ^ /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/crt1.o: In function `_start': (.text+0x20): undefined reference to `main' collect2: error: ld returned 1 exit status