You can insert code for debugging into a driver through conditional compiles by using a preprocessor symbol such as DEBUG or by using a global variable. With conditional compilation, unnecessary code can be removed in the production driver. Use a variable to set the amount of debugging output at runtime. The output can be specified by setting a debugging level at runtime with an ioctl or through a debugger. Commonly, these two methods are combined.
The following example relies on the compiler to remove unreachable code, in this case, the code following the always-false test of zero. The example also provides a local variable that can be set in /etc/system or patched by a debugger.
#ifdef DEBUG /* comments on values of xxdebug and what they do */ static int xxdebug; #define dcmn_err if (xxdebug) cmn_err #else #define dcmn_err if (0) cmn_err #endif /* ... */ dcmn_err(CE_NOTE, "Error!\n");
This method handles the fact that cmn_err(9F) has a variable number of arguments. Another method relies on the fact that the macro has one argument, a parenthesized argument list for cmn_err(9F). The macro removes this argument. This macro also removes the reliance on the optimizer by expanding the macro to nothing if DEBUG is not defined.
#ifdef DEBUG /* comments on values of xxdebug and what they do */ static int xxdebug; #define dcmn_err(X) if (xxdebug) cmn_err X #else #define dcmn_err(X) /* nothing */ #endif /* ... */ /* Note:double parentheses are required when using dcmn_err. */ dcmn_err((CE_NOTE, "Error!"));
You can extend this technique in many ways. One way is to specify different messages from cmn_err(9F), depending on the value of xxdebug. However, in such a case, you must be careful not to obscure the code with too much debugging information.