I first tried to use some preprocessor directives, like they did in hxpCTF
```
#define _str(x) #x
#define str(x) _str(x)
#define batpwn str(
const char* flag =
#include './flag.txt'
)
```

This however didn't compile, with error about the open parenthesis.
So I changed directions and tried to find a different way to include a file while compilation, after looking at some other writeups for simillar challenges I saw this cool thing you can do with inline assembly `.incbin`

Solution:
```
int main(void) {
__asm__ (
".incbin \"./flag.txt\""
);
return 0;
}
```
Let the service compile this code, then use `strings` on it to find the flag included.
Flag: `batpwn{CTFs_the_only_way_towards_actual_hacking}`