Thursday, July 2, 2020

Avoiding bitrot in C macros

A common approach to debug messages that can be toggled at compile-time in C programs is:

#define DPRINTF(fmt, ...) do { fprintf(stderr, fmt, ## __VA_ARGS__); } while (0)
#define DPRINTF(fmt, ...)

Usually the ENABLE_DEBUG macro is not defined in normal builds, so the C preprocessor expands the debug printfs to nothing. No messages are printed at runtime and the program's binary size is smaller since no instructions are generated for the debug printfs.

This approach has the disadvantage that it suffers from bitrot, the tendency for source code to break over time when it is not actively built and used. Consider what happens when one of the variables used in the debug printf is not updated after being renamed:

- int r;
+ int radius;
  DPRINTF("radius %d\n", r);

The code continues to compile after r is renamed to radius because the DPRINTF() macro expands to nothing. The compiler does not syntax check the debug printf and misses that the outdated variable name r is still in use. When someone defines ENABLE_DEBUG months or years later, the compiler error becomes apparent and that person is confronted with fixing a new bug on top of whatever they were trying to debug when they enabled the debug printf!

It's actually easy to avoid this problem by writing the macro differently:

#define ENABLE_DEBUG 0
#define DPRINTF(fmt, ...) do { \
        if (ENABLE_DEBUG) { \
            fprintf(stderr, fmt, ## __VA_ARGS__); \
        } \
    } while (0)

When ENABLE_DEBUG is not defined the macro expands to:

do {
    if (0) {
        fprintf(stderr, fmt, ...);
} while (0)

What is the difference? This time the compiler parses and syntax checks the debug printf even when it is disabled. Luckily compilers are smart enough to eliminate deadcode, code that cannot be executed, so the binary size remains small.

This applies not just to debug printfs. More generally, all preprocessor conditionals suffer from bitrot. If an #if ... #else ... #endif can be replaced with equivalent unconditional code then it's often worth doing.