/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/cc40q39m.o: in function `A':
foo.c:(.text+0xc): undefined reference to `B'
collect2: error: ld returned 1 exit status