/in/foo.c: In function 'main':
/in/foo.c:24:5: warning: implicit declaration of function 'add' [-Wimplicit-function-declaration]
   24 |     add(a);
      |     ^~~
/usr/bin/ld: /tmp/cc1CIsFS.o: in function `main':
foo.c:(.text.startup+0x1f): undefined reference to `add'
collect2: error: ld returned 1 exit status