### 10.9 Caveats of using `protoize`

The conversion programs `protoize` and `unprotoize` can sometimes change a source file in a way that won't work unless you rearrange it.

- `protoize` can insert references to a type name or type tag before the definition, or in a file where they are not defined.

  If this happens, compiler error messages should show you where the new references are, so fixing the file by hand is straightforward.

- There are some C constructs which `protoize` cannot figure out. For example, it can't determine argument types for declaring a pointer-to-function variable; this you must do by hand. `protoize` inserts a comment containing ‘`???`’ each time it finds such a variable; so you can find all such variables by searching for this string. ISO C does not require declaring the argument types of pointer-to-function types.
- Using `unprotoize` can easily introduce bugs. If the program relied on prototypes to bring about conversion of arguments, these conversions will not take place in the program without prototypes. One case in which you can be sure `unprotoize` is safe is when you are removing prototypes that were made with `protoize`; if the program worked before without any prototypes, it will work again without them.

  You can find all the places where this problem might occur by compiling the program with the `-Wconversion` option. It prints a warning whenever an argument is converted.

- Both conversion programs can be confused if there are macro calls in and around the text to be converted. In other words, the standard syntax for a declaration or definition must not result from expanding a macro. This problem is inherent in the design of C and cannot be fixed. If only a few functions have confusing macro calls, you can easily convert them manually.
- `protoize` cannot get the argument types for a function whose definition was not actually compiled due to preprocessing conditionals. When this happens, `protoize` changes nothing in regard to such a function. `protoize` tries to detect such instances and warn about them.

  You can generally work around this problem by using `protoize` step by step, each time specifying a different set of `-D` options for compilation, until all of the functions have been converted. There is no automatic way to verify that you have got them all, however.

- Confusion may result if there is an occasion to convert a function declaration or definition in a region of source code where there is more than one formal parameter list present. Thus, attempts to convert code containing multiple (conditionally compiled) versions of a single function header (in the same vicinity) may not produce the desired (or expected) results.

  If you plan on converting source files which contain such code, it is recommended that you first make sure that each conditionally compiled region of source code which contains an alternative function header also contains at least one additional follower token (past the final right parenthesis of the function header). This should circumvent the problem.

- `unprotoize` can become confused when trying to convert a function definition or declaration which contains a declaration for a pointer-to-function formal argument which has the same name as the function being defined or declared. We recommend you avoid such choices of formal parameter names.
- You might also want to correct some of the indentation by hand and break long lines. (The conversion programs don't write lines longer than eighty characters in any case.)