/in/foo.c: In function 'main':
/in/foo.c:6:2: warning: implicit declaration of function 'scan9f' [-Wimplicit-function-declaration]
  scan9f("%lf%lf%lf",&a,&b,&c);
  ^~~~~~
/in/foo.c:5:28: warning: variable 'q' set but not used [-Wunused-but-set-variable]
 {double a,b,c,disc,x1,x2,p,q;
                            ^
/in/foo.c:5:26: warning: variable 'p' set but not used [-Wunused-but-set-variable]
 {double a,b,c,disc,x1,x2,p,q;
                          ^
/in/foo.c:10:2: warning: 'x1' is used uninitialized in this function [-Wuninitialized]
  printf("x1=%lf\nx2=%lf\n",x1,x2);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/in/foo.c:10:2: warning: 'x2' is used uninitialized in this function [-Wuninitialized]
/tmp/ccDAz2JT.o: In function `main':
foo.c:(.text.startup+0x1d): undefined reference to `scan9f'
collect2: error: ld returned 1 exit status