Adding Patch to a Shared Library

In this section, you are going to learn

How to create patches ?

How to add patches to Shared Library ?

How to add patches using diff and patch commands ?

How to add or import patches using quilt ?

Topics in this section,

• Adding Patch to Shared Library : FAQs

• Adding Patch to Shared Library : Creating Patches using diff and patch commands

• Adding Patch to Shared Library : Adding Patches to Static Library using diff and patch commands

• Adding Patch to Shared Library : Creating Patches using quilt commands

• Adding Patch to Shared Library : Adding Patches to Static Library using quilt commands

Adding Patches to Application : FAQs

Let us answer few basic questions

What is a patch ?

See Answer

• It is a file or set of changes made to a source code of program to modify or update it.

• Used to fix bugs or add new features to existing ones.

What are the ways to generate and apply patch ?

See Answer

• using diff and patch commands.

• using quilt commands.

What is use of diff command ?

See Answer

• command use to compare two files line by line.

What is use of patch command ?

See Answer

• applies changes to the source code by reading the diff file.

What is the command to generate patch file ?

See Answer

diff -u helloworld_org.c helloworld.c > add_patch_to_helloworld.diff

What is the command to apply the patches to source code ?

See Answer

patch helloworld_org.c < add_patch_to_helloworld.diff

What is the command to remove the patches from source code ?

See Answer

patch -R helloworld_org.c < add_patch_to_helloworld.diff

What is quilt ?

See Answer

• Tools used for managing a series of patches applied to the source code repository.

What quilt init does ?

See Answer

• quilt init command is used to create a new quilt series.

$ quilt init
The quilt meta-data is now initialized.

What quilt new does ?

See Answer

• quilt new is used to create a patch file.

* quilt new patch_file_name.patch

What quilt add does ?

See Answer

• quilt add is used to add the source code files which needs to be modified.

quilt add <file1> <file2> ..... <fileN>

What quilt refresh does ?

See Answer

• quilt refresh is used to update the series with latest modifications.

What quilt series does ?

See Answer

• quilt series is used to display the name of the patches under that series.

What quilt push does ?

See Answer

• quilt push is used to apply the patches.

What is quilt pop does ?

See Answer

• quilt pop is used to unapply the top most patch in the series.

What quilt top does ?

See Answer

• quilt top is used to display the currently applied patch.

• It displays the top most patch present in the series.

Let us now explore it in depth !

Creating Patches using diff and patch

• Step 1: Edit the file which needs to be modified.

• Step 2: Generate a patch file using the diff command.

diff -u <original_file_name> <modified_file_name> > <patch_file_name>

• Step 3: Now the patches to the file can be applied using patch command.

patch <orginal_file_name> < <patch_file_name>

• Now a patch is created and the changes is applied to the file.

Adding patches to shared library using diff and patch commands

libmathsquare.h

#ifndef LIBMATHSQUARE_H
#define LIBMATHSQUARE_H

int square(int x);

#endif

libmathsquare_original.c

#include "libmathsquare.h"

int square(int x) {
	return x * x;
}

libmathsquare_modified.c

#include "libmathsquare.h"
#include <stdio.h>

int square(int x) {
	printf("patch applied to shared library\n");
	return x * x;
}

Generating patch file using diff command

$ diff -u libmathsquare_original.c libmathsquare_modified.c > add_patch_to_shared_lib.diff

Adding Patches using patch command

$ patch libmathsquare_original.c < add_patch_to_shared_lib.diff
patching file libmathsquare_original.c

add_patch_to_shared_lib.diff

--- libmathsquare_original.c	2024-01-02 15:40:09.467460416 +0530
+++ libmathsquare_modified.c	2024-01-02 15:39:18.636036723 +0530
 -1,6 +1,8
 #include "libmathsquare.h"
 #include <stdio.h>
 
 int square(int x) {
+	printf("patch applied to shared library\n");
 	return x * x;
 }
 

Generating library file

$ gcc -o libmathsquare.so -shared libmathsquare_original.c

Compiling and executing the application

$ gcc -o main main.c -L./ -lmathsquare
$ export LD_LIBRARY_PATH=$PWD:$LD_LIBRARY_PATH
$ ./main
patch applied to shared library
Square of 5 is: 25

Creating Patches

• Step 1: create a new quilt series using quilt init command.

$ quilt init
The quilt meta-data is now initialized.

• Step 2: create a patch file using quilt new command.

$ quilt new add_hello_world_patch.patch
Patch patches/add_hello_world_patch.patch is now on top

• Step 3: add the file which needs to be modified using quilt add command.

$ quilt add helloworld.c
File helloworld.c added to patch patches/add_hello_world_patch.patch

• Step 4: edit the file and make the modifications needed.

$ vi helloworld.c

• Step 5: once the changes is added run quilt refresh to update the series with latest modifications.

$ quilt refresh
Refreshed patch patches/add_hello_world_patch.patch

• Now a patch is created and changes has been added to the series.

Adding Patches to an Application

libmathsquare.h

#ifndef LIBMATHSQUARE_H
#define LIBMATHSQUARE_H

int square(int x);

#endif

libmathsquare.c

#include "libmathsquare.h"
#include <stdio.h>

int square(int x) {
	printf("patch applied to shared library\n");
	return x * x;
}

static library makefile

CFLAGS = -Wall -O2 -fPIC
LIBRARY_NAME = libmathsquare
SHARED_LIBRARY = $(LIBRARY_NAME).so
SOURCE = $(LIBRARY_NAME).c
HEADERS = $(LIBRARY_NAME).h

PATCHES_DIR = patches

all: apply_patches $(SHARED_LIBRARY)

$(SHARED_LIBRARY): $(SOURCE)
	gcc $(CFLAGS) -shared -o $(SHARED_LIBRARY) $(SOURCE)

apply_patches:
	quilt push || true

.PHONY: clean
clean:
	quilt pop -a || true
	rm -f $(SHARED_LIBRARY)

main.c

#include <stdio.h>
#include "libmathsquare.h"

int main() {
	int num = 5;
	printf("Square of %d is: %d\n", num, square(num));
	return 0;
}

application makefile

CFLAGS = -Wall -O2
EXECUTABLE = main
SOURCE = main.c
LIBRARY_NAME = libmathsquare
SHARED_LIBRARY = $(LIBRARY_NAME).so

LIBRARY_DIR = ../lib
PATCHES_DIR = patches

all: apply_patches $(EXECUTABLE)

$(EXECUTABLE): $(SOURCE) $(LIBRARY_DIR)/$(SHARED_LIBRARY)
	gcc $(CFLAGS) -o $(EXECUTABLE) $(SOURCE) -L$(LIBRARY_DIR) -lmathsquare -I$(LIBRARY_DIR)

apply_patches:
	quilt push || true

.PHONY: clean
clean:
	quilt pop -a || true
	rm -f $(EXECUTABLE)

Adding patches to helloworld.c

$ quilt init
The quilt meta-data is now initialized.
$ quilt new add_static_lib_patch.patch
Patch patches/add_static_lib_patch.patch is now on top
$ quilt add libmathsquare.c
File libmathsquare.c added to patch patches/add_static_lib_patch.patch
$ vi libmathsquare.c
$ quilt refresh
Refreshed patch patches/add_static_lib_patch.patch

add_shared_lib_patch.patch

Index: lib/libmathsquare.c
===================================================================
--- lib.orig/libmathsquare.c
+++ lib/libmathsquare.c
 #include "libmathsquare.h"
 #include <stdio.h>
 
 int square(int x) {
+	printf("patch applied to shared library\n");
 	return x * x;
 }
 

Compiling the Library

$ make
Applying patch patches/add_shared_lib_patch.patch
patching file libmathsquare.c

Now at patch patches/add_shared_lib_patch.patch
gcc -Wall -O2 -fPIC -shared -o libmathsquare.so libmathsquare.c

Compiling the application

$ make
No series file found
gcc -Wall -O2 -o main main.c -L../lib -lmathsquare -I../lib

Executing the application

$ export LD_LIBRARY_PATH=../lib/:$LD_LIBRARY_PATH
$ ./main
Patch added to static library
Square of 5 is: 25

See Also

• Other topics of linux x86

  • Adding Hello World Application

  • Adding Static Library

  • Adding Shared Library

  • Adding Hello World Kernel Module

  • Adding Patch to an Application

  • Adding Patch to a Static Library

  • Adding Patch to Kernel Module

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