Saturday, June 4, 2011

Evil C++ #2: Using GCC's -ftrapv flag to debug integer overflows

In C++, overflowing an integer type won't cause an exception and can result in weird numbers propagating through your program. GCC's ftrapv flag has your back.

Thursday, June 2, 2011

Evil C++ #1: Brackets and "at" for accessing STL vector elements

This is the first in a series of code snippets that demonstrate C/C++ pitfalls.

(For an thorough explanation of the many ways C++ is out to get you, see Yossi Kreinin's excellent C++ FQA).

Ignoring GCC warnings on a per-file basis

In most cases, ignoring GCC warnings is a Bad Idea. Treating warnings as errors results in better code.

However, sometimes we are forced to deal with other people's code. For instance, a project I work on relies on JsonCpp. We include this in our source tree so that every user doesn't to have to go get JsonCpp source code in order to compile this thing.

Such dependencies can be a problem if you want really strict compiler options, since libraries will often be slightly incompatible with your particular standard (ANSI, C++0x, ...) or not be up to your lofty expectations. In my case, JsonCpp gives me a couple of warnings with GCC options -W, -Wall, -ansi, -pedantic. This means I can't compile my code with -Werror, which makes me sad. I certainly don't want to modify these external libraries.

Fortunately, in recent GCC versions ways of selectively disabling warnings have been added. If your problems are confined to headers, you can replace -I/path/to/headers with -isystem/path/to/headers and GCC will treat them as system headers, ignoring warnings.

Another less-desirable solution is to use pragmas. Headers can be marked as system headers by putting at the top:

#pragma GCC system_header

If the problems lie in the source files themselves, neither of these tricks work. We can, however, add to the top of the files causing the warnings things like this:

#pragma GCC diagnostic ignored "-Wunused-parameter"
#pragma GCC diagnostic ignored "-Woverflow"

to disable specific warnings generated by that file.

To figure out the names of the warnings causing the problems, recompile with the -fdiagnostics-show-option option on the g++ line. This is especially useful in the case of default warnings (i.e. those which aren't optional) like -Woverflow since they are harder to find in the documentation.

This isn't a great solution, since it does require some modification of the libraries. However, you can easily generate a patch from your changes and apply it to any new library versions should you decide later to upgrade them. Hopefully someday GCC will include an "ignore warnings from this file or subdirectory" option, but until then... it works.