gcc的-W选项

comparison between signed and unsigned integer expressions：
-Wno-sign-compare

defined but not used：
The GCC compiler flags that control unused warnings include:
-Wunused-function
-Wunused-label
-Wunused-parameter
-Wunused-value
-Wunused-variable
-Wunused (=all of the above)
Each of these has a corresponding negative form with "no-" inserted after the W which turns off the warning (in case it was turned on by -Wall, for example). Thus, in your case you should use
-Wno-unused-function