hello.c

#include <stdio.h>
#define pi 3.14
#ifdef __unix__ /* __unix__ is usually defined by compilers targeting Unix systems */
printf("using linux");
#elif defined _WIN32 /* _WIN32 is usually defined by compilers targeting 32 or 64 bit Windows systems */
printf("using windows");
#endif

int main(int argc, char const *argv[])
{
  printf("Hello World!\nThis is pi number: %2f", pi); //This is a comment
  return 0;
}