/in/foo.c: In function 'a':
/in/foo.c:4:7: warning: implicit declaration of function 'b' [-Wimplicit-function-declaration]
    4 | int l=b(s);
      |       ^
/usr/bin/ld: /tmp/ccJJ7BQg.o: in function `a':
foo.c:(.text+0xc): undefined reference to `b'
collect2: error: ld returned 1 exit status