Invoking the preprocessor from a shell or shell-like program - shell

Question

As mentioned in the title I want to invoke the preprocessor from a shell.
Let me clarify:
Suppose I have to invoke Preprocessor for command patch in Linux say:
patch -p1 -D `"{what and how should i write here }"` < patch.patch
Patch command has an option -D where I can define a preprocessor (to my understanding). I tried searching on Google I got only one link I couldn't understand it properly.
Please guide me with an example (or proper reference).
What all can be written in -D option of any command?
Are constructs same for all command are different for different commands?
What can be the input variables?

Answer 1

The -D option to patch means that the changes made to the patched file will be surrounded by #ifndef X / #else / #endif if X is what you specify as the argument to the -D option.
For example:
$ cat file-1.c
#include <stdio.h>
int main(void)
{
printf("Hello world\n");
return 0;
}
$ cat file-2.c
#include <stdio.h>
int main(void)
{
puts("Hello world");
return 0;
}
$ diff -u file-1.c file-2.c > patch
$ patch -DPRINTF_TO_PUTS -i patch --verbose
Hmm... Looks like a unified diff to me...
The text leading up to this was:
--------------------------
|--- file-1.c 2013-02-01 00:33:01.000000000 -0800
|+++ file-2.c 2013-02-01 00:33:17.000000000 -0800
--------------------------
Patching file file-1.c using Plan A...
Hunk #1 succeeded at 2.
done
$ file-1.c
#include <stdio.h>
int main(void)
{
#ifndef PRINTF_TO_PUTS
printf("Hello world\n");
#else
puts("Hello world");
#endif
return 0;
}
$
So, to answer your questions:
You should put a valid C identifier after -D on the patch command line.
Different commands apply different meanings to any given option letter. You can find information about common interpretations for options at The Art of Unix Programming, but there are only 52 alphabetic (single-letter) options and there are many more different meanings for arguments to different commands than that.
I'm not sure what you mean by this.
None of this invokes a C preprocessor from the command line. If you need to do that, look for a program cpp on your system. If you can't find cpp anywhere, you may end up invoking gcc -E instead.