2.6. Patching software

Procédure

1. Preserve the original source code:

   $ cp -p cello.c cello.c.orig

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   The -p option is used to preserve mode, ownership, and timestamps.

2. Modify cello.c as needed:

   #include <stdio.h>
   
   int main(void) {
       printf("Hello World from my very first patch!\n");
       return 0;
   }

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3. Generate a patch using the diff utility:

   $ diff -Naur cello.c.orig cello.c
   --- cello.c.orig        2016-05-26 17:21:30.478523360 -0500
   + cello.c     2016-05-27 14:53:20.668588245 -0500
   @@ -1,6 +1,6 @@
    #include<stdio.h>
   
    int main(void){
   -    printf("Hello World!\n");
   +    printf("Hello World from my very first patch!\n");
        return 0;
    }
   \ No newline at end of file

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   Lines starting with a - are removed from the original source code and replaced with the lines that start with +.

   Using the Naur options with the diff command is recommended because it fits the majority of usual use cases. However, in this particular case, only the -u option is necessary. Particular options ensure the following:

   • -N (or --new-file) - Handles absent files as if they were empty files.
   • -a (or --text) - Treats all files as text. As a result, the files that diff classifies as binaries are not ignored.
   • -u (or -U NUM or --unified[=NUM]) - Returns output in the form of output NUM (default 3) lines of unified context. This is an easily readable format that allows fuzzy matching when applying the patch to a changed source tree.

   • -r (or --recursive) - Recursively compares any subdirectories that are found.

     For more information on common arguments for the diff utility, see the diff manual page.

4. Save the patch to a file:

   $ diff -Naur cello.c.orig cello.c > cello-output-first-patch.patch

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5. Restore the original cello.c:

   $ cp cello.c.orig cello.c

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   The original cello.c must be retained, because when an RPM is built, the original file is used, not the modified one. For more information, see Working with SPEC files.

Procédure

1. Redirect the patch file to the patch command:

   $ patch < cello-output-first-patch.patch
   patching file cello.c

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2. Check that the contents of cello.c now reflect the patch:

   $ cat cello.c
   #include<stdio.h>
   
   int main(void){
       printf("Hello World from my very first patch!\n");
       return 1;
   }

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3. Build and run the patched cello.c:

   $ make clean
   rm cello
   
   $ make
   gcc -g -o cello cello.c
   
   $ ./cello
   Hello World from my very first patch!

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