Patching your application: Editing existing files

This is the seventh chapter in the “Hello, world!” for OpenWrt article series. At this point, you should've already accomplished the following tasks:

• Commissioned your development environment
• Prepared, configured and built the tools and the cross-compilation toolchain
• Configured the PATH environment variable
• Created a simple “Hello, world!” application using native compilation tools
• Created a local package feed for your application
• Created a package manifest file for your application
• Included your new package feed into the OpenWrt build system
• Updated the package index, and installed your package from the feed
• Built, deployed and tested the application on your target device
• Migrated your application to use GNU make
• Tested building your application using GNU make
• Updated the package manifest to use GNU make as a build tool
• Prepared the source code, and created a patch to add new files to the package

If you missed one or more of these steps, review the previous chapters.

Our first patch that we created in our previous chapter added two new files, and implemented a new function called add. However, this function is not being called yet, so the functionality it provides is going to waste.

In order to utilize the functionality, we create an additional patch which modifies the helloworld.c file. First, we prepare the source codes for patching, and create a new patch context:

cd /home/buildbot/source
make package/helloworld/{clean,prepare} QUILT=1
cd build_dir/target-.../helloworld-1.0/
quilt push -a
quilt new 101-use_module.patch

Our task is to modify the helloworld.c file, so we issue the following command to add the file into the patch context, and open it for editing:

quilt edit helloworld.c

Note that since the file already exists, it is not necessary to add the file into the patch context using quilt add. In the previous chapter, we intentionally used this command to highlight the difference between adding a file and editing an existing file.

Let's modify the source file to include our header file, and call the new function:

helloworld.c

#include <stdio.h>
#include "functions.h"
 
int main(void)
{
    int result = add(2, 3);
 
    printf("\nHello, world!\nThe sum is '%d'", result);
    return 0;     
}

Save your changes, review them using quilt diff and move them to the patch context using quilt refresh.

Finally navigate back to the back to the root folder of the OpenWrt build system and update the package with our new patch. To do so, issue:

cd /home/buildbot/source
make package/helloworld/update

In this chapter, we created a second patch that edits existing files in order to take our newly added add function into use. We then finalized the patch and migrated it into the package proper.

This concludes the “Hello, world!” for OpenWrt article series.