什么情况？

In file included from hello.c:2:0:
test.c: In function 'say':
test.c:3:5: warning: implicit declaration of function 'printLine' [-Wimplicit-function-declaration]
     printLine();
     ^~~~~~~~~
hello.c: At top level:
hello.c:3:13: warning: conflicting types for 'printLine'
 extern void printLine ()     //这里定义的方法对吗？
             ^~~~~~~~~
In file included from hello.c:2:0:
test.c:3:5: note: previous implicit declaration of 'printLine' was here
     printLine();
     ^~~~~~~~~
test.c: In function 'say':
test.c:3:5: warning: implicit declaration of function 'printLine' [-Wimplicit-function-declaration]
     printLine();
     ^~~~~~~~~
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