main.c
extern void func(); int main(int argc, const char *argv[]) { func(); return 0; }
lib.c
#include <stdio.h>
void func(int i) { printf("param: %d\n", i); }
$ gcc main.c lib.c
$ ./a.out
param: 134513712
$ g++ main.c lib.c
/tmp/ccxkNQ9h.o: In function `main':
main.c:(.text+0x7): undefined reference to `func()'
collect2: ld returned 1 exit status