A co-worker had a mysterious C++ compile error: a symbol was undefined, but it was clearly present in the header file. We figured there was an ifdef affecting it, but how to find the culprit in the 11,000-line header file?

I tossed off this Python script:

# showifstruct.py
import re, sys

reDirective = re.compile(r'\s*#\s*([a-z]+)')

f = file(sys.argv[1])
targetLnum = int(sys.argv[2])

# A stack. Each element is a list of tuples.
# Each tuple is a linenumber/text pair of a directive that affects the current line.
ifstack = []

lnum = 0
for l in f.readlines():
    lnum += 1
    if lnum == targetLnum:
        break
    matchDir = reDirective.match(l)
    if matchDir:
        directive = matchDir.group(1)
        if directive in ['if', 'ifdef', 'ifndef']:
            # Start a group
            ifstack.append( [[lnum, l]] )
        elif directive == 'else':
            ifstack[-1].append( [lnum, l] )
        elif directive == 'endif':
            del ifstack[-1]

indent = ''
for ifs in ifstack:
    for lnum, line in ifs:
        print "%6d: %s%s" % (lnum, indent, line),
    indent += '  '

Give it a file name and a line number, and it will track what preprocessor directives affect the line in question. It prints the lines that matter, indented to show structure, with line numbers:

$ showifstruct WinUser.h 11100
    12: #ifndef _WINUSER_
 10384:   #if(WINVER >= 0x0500)
 10483:     #ifndef NOWINABLE