gcc - How to find what preprocessor does to a C program

There are gcc options that allow us to do only preprocessing or preprocessing and compiling or all preprocessing, compiling and linking to make an executable. If we need to see what does the compiler does during preprocessing we can ask it to do only preprocessing and see the result to find out what it does in preprocessing. This can be useful in many situations where we need to identify preprocessor errors, in academics etc.
Lets understand this with an example:
I am using Fedora8 with gcc - gcc (GCC) 4.1.2 20070925 (Red Hat 4.1.2-33)

Example contains a C file named "first.c" and a header file named "first.h" as following:

first.c
-------
#include "first.h"
#define MAX 10
int main()
{
int i;
int j;

printf("hello world \n");
i=MAX;
j=MIN;
printf("i - %d and MAX is %d\n",i,MAX);
printf("j - %d and MIN is %d\n",j,MIN);
return 0;
}
----

first.h
----
#ifndef __FIRST
#define __FIRST

#define MIN 5
#include <stdio.h>
#endif
----

if you compile the program and run it you will get
#gcc first.c
#./a.out
hello world
i - 10 and MAX is 10
j - 5 and MIN is 5

What we need to find out here is what preprocessor does to this program, as we all know, preprocessor does include the header file first.h in the program, which internally includes stdio.h (in my case it is /usr/include/stdio.h file), replaces MACROS (MIN and MAX here) with their values. In my case the gcc gives "-E" option for preprocessing only, find out your option by seeing manual page. So I run

#gcc -E first.c > first_only_preprocessor.out

and when you see the output file "first_only_preprocessor.out", you will find out what preprocessor does to our example C file.

Lets see, for example, what it does to our printfs, where we are printing MAX and MIN, I searched MAX and MIN in the output as follows:

# grep 'MIN\|MAX' first_only_preprocessor.out
OR
# gcc -E first.c | grep 'MIN\|MAX'

output is
printf("i - %d and MAX is %d\n",i,10);
printf("j - %d and MIN is %d\n",j,5);

As you see, preprocessor correctly replaces MAX with 10 and MIN with 5 :)

Think on the ideas where you can use this ...

Networking FAQs

1) Why ARP packets don't have fixed-format header? - To allow it to work on different networks, as length of fields that contain address depends on networks.
2) If ICMP messages are encapsulated and sent using IP, Why ICMP is considered Internet Layer protocol and not a higher level protocol? - Since it is a required part of IP, it is delivered via IP since it may need to travel across several physical networks to reach final destination and thus cant be delivered via physical transport/ link layer alone.
3) Why hashing works well as routing algorithm in classful addressing and not in classless addressing - Since in classful, addresses are self-identifying and we can extract network portion and use that as key to hashing, while in classless, addresses are not self-identifying.

C - Questions one should ask...

Last week, I was explaining someone the difference between switch case and if-else if, I thought of writing this blog to ask some questions. Try finding the answers, if you cant, ping me, I will try to answer:

Plz note that, this is meant for beginners only and I will update the list as I come across the questions. If you have questions like this, do ping me :)

Q1: When to use "switch" and "if-else if"?
Q2: What should be the ideal/efficient way of ordering the cases in switch cases?
Q3: When you would rather not have a break in a switch case?
Q4: When you would want the default case to be the first case in a switch case?
Q5: When to use different types of loops?
Q6: How to use different number systems in programming?
Q7: When to use ternary operator?
Q8: When to use union then structure?
Q9: Whats the difference between heap and stack memory?
Q10: How to find out what preprocessor does to a C program file?
Q11: What is preprocessing, compiling and linking?
Q12: When to use pointers and when not to use them?
Q13: Whats the use of return value from main function, How you can use it?
Q14: When to use iteration(loops) and when to use recursion?