/in/foo.c:1:9: error: expected '=', ',', ';', 'asm' or '__attribute__' before '<' token
 include <stdio.h>
         ^
/in/foo.c: In function 'solve_quadratic':
/in/foo.c:9:3: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   printf("%.2f\n%.2f\n", x1, x2);
   ^~~~~~
/in/foo.c:9:3: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:9:3: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:12:3: warning: incompatible implicit declaration of built-in function 'printf'
   printf("%.2f\n", root);
   ^~~~~~
/in/foo.c:12:3: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:16:3: warning: incompatible implicit declaration of built-in function 'printf'
   printf("%.2f + %.2fi\n%.2f - %.2fi\n", real_part, imaginary_part, real_part, -imaginary_part);
   ^~~~~~
/in/foo.c:16:3: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c: In function 'main':
/in/foo.c:22:2: warning: incompatible implicit declaration of built-in function 'printf'
  printf("请输入a, b, c（逗号分隔）: ");
  ^~~~~~
/in/foo.c:22:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/in/foo.c:23:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%lf,%lf,%lf", &a, &b, &c);
  ^~~~~
/in/foo.c:23:2: warning: incompatible implicit declaration of built-in function 'scanf'
/in/foo.c:23:2: note: include '<stdio.h>' or provide a declaration of 'scanf'