Removing conditional macros when tracing C/C++ code with Vim

Working with source code buried in conditional macros(#if, #ifdef, #ifndef, ...) is sometimes really annoying, especially when we are trying to get the whole picture of what it does instead of every details in it.

Simply expanding the conditionals with gcc -E usually doesn't help in this situation. Expansion of macros as well as the header files even makes the picture muddier. The unifdef(1) utility come in handy for this.

$ man unifdef(1)

UNIFDEF(1)                    Programmer's Manual                   UNIFDEF(1)

NAME
     unifdef, unifdefall — remove preprocessor conditionals from code

SYNOPSIS
     unifdef [-bBcdeKknsStV] [-Ipath] [-Dsym[=val]] [-Usym] [-iDsym[=val]] [-iUsym] ...
             [-o outfile] [infile]
     unifdefall [-Ipath] ... file

Apply it as vim filter command repeatedly and incrementally until you get your picture clear enough.

 vim /tmp/dummy.c

#if (LOG_LEVEL > 3)
  #define log printk
#else
  #define log(...) 
#endif

#ifdef ENABLE_FOO
void foo_xxx()
{
    ...
}

#endif

#ifdef ENABLE_BAR
void bar_yyy()
{
    ...
}
#endif

:%! unifdef -k -UENABLE_FOO

 vim /tmp/dummy.c

#if (LOG_LEVEL > 3)
  #define log printk
#else
  #define log(...) 
#endif

#ifdef ENABLE_BAR
void bar_yyy()
{
    ...
}
#endif

:%! unifdef -k -DENABLE_BAR -DLOG_LEVEL=4

 vim /tmp/dummy.c


  #define log printk

void bar_yyy()
{
    ...
}

Note that:

By default, unifdef ignores #if and #elif lines with constant expressions; it can be told to process them by specifying the -k flag on the command line.