Steps to develop patch and apply it to original source file

1. Create test.c

 Above file contains :

--------
[kamalma@test-1 C_Programming]$ cat test.c
#include
#include
int main()
 {
 printf("\n I'm kamal \n");
return 0;
}
--------

2. Compile it and check its output.

OP :

-------
[kamalma@test-1 C_Programming]$ ./test

 I'm kamal
[kamalma@test-1 C_Programming]$
-------

3. Modify test.c and add new lines or codes in it. Lets say it test_modifed.c. This file contains following codes :

------
[kamalma@test-1 C_Programming]$ cat test_modified.c
#include
#include
int main()
 {
 printf("\n I'm kamal \n");
 printf("\nI have added one more line. This comes from modifed code\n" );
return 0;
}
[kamalma@test-1 C_Programming]$
[kamalma@test-1 C_Programming]$ gcc -o test_modified test_modified.c
[kamalma@test-1 C_Programming]$ ./test_modified

 I'm kamal

I have added one more line. This comes from modifed code
[kamalma@test-1 C_Programming]$
------

3. Then execute following command to create a patch in the same directory.  :

 diff -u test.c test_modified.c > test.patch.1

Here test.path.1 will contain following :

-----
[kamalma@test-1 C_Programming]$ cat test.patch.1
--- test.c      2012-07-17 07:52:39.000000000 +0530
+++ test_modified.c     2012-07-17 07:54:41.000000000 +0530
@@ -3,5 +3,6 @@
 int main()
  {
  printf("\n I'm kamal \n");
+ printf("\nI have added one more line. This comes from modifed code\n" );
 return 0;
 }
[kamalma@test-1 C_Programming]$
-----

5. In order to apply patch to test.c file, you need to execute following command :

patch test.c < test.patch.1

Example :

---------
[kamalma@test-1 C_Programming]$ patch -u test.c < test.patch.1
patching file test.c
[kamalma@test-1 C_Programming]$ cat test.c
#include
#include
int main()
 {
 printf("\n I'm kamal \n");
 printf("\nI have added one more line. This comes from modifed code\n" );
return 0;
}
[kamalma@test-1 C_Programming]$
-----------

Reverting back :

----------
[kamalma@test-1 C_Programming]$ patch -R test.c < test.patch.1
patching file test.c
[kamalma@test-1 C_Programming]$ cat test.c
#include
#include
int main()
 {
 printf("\n I'm kamal \n");
return 0;
}
[kamalma@test-1 C_Programming]$
----------

You can do dry-run (test prior to be originally chaning codes in test.c)

patch -p0 --dry-run test.c < test.patch.1

6. Now recompiling the test.c program :

---------
[kamalma@test-1 C_Programming]$ gcc -o testnew test.c
[kamalma@test-1 C_Programming]$ ./testnew

 I'm kamal

I have added one more line. This comes from modifed code
[kamalma@test-1 C_Programming]$
---------