Why should #ifdef be avoided in .c files?

A programmer I respect said that in C code, #if and #ifdef should be avoided at all costs, except possibly in header files. Why would it be considered bad programming practice to use #ifdef in a .c file?

Hard to maintain. Better use interfaces to abstract platform specific code than abusing conditional compilation by scattering #ifdefs all over your implementation.

E.g.

void foo() {
#ifdef WIN32
   // do Windows stuff
#else
   // do Posix stuff
#endif
   // do general stuff
}

Is not nice. Instead have files foo_w32.c and foo_psx.c with

foo_w32.c:

void foo() {
    // windows implementation
}

foo_psx.c:

void foo() {
    // posix implementation
}

foo.h:

void foo();  // common interface

Then have 2 makefiles¹: Makefile.win, Makefile.psx, with each compiling the appropriate .c file and linking against the right object.

Minor amendment:

If foo()'s implementation depends on some code that appears in all platforms, E.g. common_stuff()², simply call that in your foo() implementations.

E.g.

common.h:

void common_stuff();  // May be implemented in common.c, or maybe has multiple
                      // implementations in common_{A, B, ...} for platforms
                      // { A, B, ... }. Irrelevant.

foo_{w32, psx}.c:

void foo()  {  // Win32/Posix implementation
   // Stuff
   ...
   if (bar) {
     common_stuff();
   }
}

While you may be repeating a function call to common_stuff(), you can't parameterize your definition of foo() per platform unless it follows a very specific pattern. Generally, platform differences require completely different implementations and don't follow such patterns.

---

1. Makefiles are used here illustratively. Your build system may not use make at all, such as if you use Visual Studio, CMake, Scons, etc.
2. Even if common_stuff() actually has multiple implementations, varying per platform.