OpenWrt: Adding Patches to a Shared Library

This document explains how to:

• Create a shared library (libbar.so)

• Apply patches to modify its behavior

• Export the library + headers to OpenWrt

• Build a test application that dynamically links against the patched library

• Install and run these components on OpenWrt

This demonstrates how OpenWrt’s patching system, shared library workflow, and dynamic linking infrastructure operate.

In this section, you are going to learn:

How to patch and build a shared library in OpenWrt?

How to export headers and shared objects into staging & rootfs?

How to build dynamic applications that link against patched libraries?

Topics in this section:

• Step 1: Directory Layout

• Step 1.1: Technical Notes

• Step 2: Source Files

• Step 2.1: Technical Notes

• Step 3: Patch File

• Step 3.1: Technical Notes

• Step 4: Shared Library Makefile

• Step 4.1: Technical Notes

• Step 5: Build Shared Library

• Step 5.1: Technical Notes

• Step 6: Install Shared Library on OpenWrt VM

• Step 6.1: Technical Notes

• Step 7: Test Application Directory Structure

• Step 7.1: Technical Notes

• Step 8: Test Application Patch

• Step 8.1: Technical Notes

• Step 9: Test Application Makefile

• Step 9.1: Technical Notes

• Step 10: Build & Install Test Application

• Step 10.1: Technical Notes

Step 1: Directory Layout

package/my/libbar/
├── Makefile
├── patches/
│   └── 0001-modify-behavior.patch
└── src/
    ├── bar.c
    └── bar.h

Step 1.1: Technical Notes

• patches/ contains quilt-formatted patches.

• All patches applied automatically via Build/Patch.

• Shared library objects must be built with -fPIC.

Step 2: Source Files

bar.h

int bar_mul(int a, int b);

bar.c

#include "bar.h"
int bar_mul(int a, int b) { return a * b; }

Step 2.1: Technical Notes

• bar.h exported to /usr/include.

• bar.c patched before compile.

Step 3: Patch File

0001-modify-behavior.patch

--- a/bar.c
+++ b/bar.c
@@ -1,5 +1,5 @@
 int bar_mul(int a, int b) {
-    return a * b;
+    return (a * b) + 2;
 }

Step 3.1: Technical Notes

• Patch modifies computation.

• Applied before Build/Compile.

Step 4: Shared Library Makefile

include $(TOPDIR)/rules.mk
PKG_NAME:=libbar
PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)
include $(INCLUDE_DIR)/package.mk

define Package/libbar
  SECTION:=libs
  CATEGORY:=Libraries
  TITLE:=Simple shared bar library
endef

define Build/Prepare
    mkdir -p $(PKG_BUILD_DIR)
    $(CP) ./src/* $(PKG_BUILD_DIR)/
    $(call Build/Patch,$(PKG_BUILD_DIR))
endef

define Build/Compile
    $(TARGET_CC) $(TARGET_CFLAGS) -fPIC -c \
        $(PKG_BUILD_DIR)/bar.c \
        -o $(PKG_BUILD_DIR)/bar.o

    $(TARGET_CC) -shared -o $(PKG_BUILD_DIR)/libbar.so \
        $(PKG_BUILD_DIR)/bar.o
endef

define Build/InstallDev
    $(INSTALL_DIR) $(1)/usr/include
    $(INSTALL_DATA) $(PKG_BUILD_DIR)/bar.h $(1)/usr/include/

    $(INSTALL_DIR) $(1)/usr/lib
    $(INSTALL_DATA) $(PKG_BUILD_DIR)/libbar.so $(1)/usr.lib/
endef

define Package/libbar/install
    $(INSTALL_DIR) $(1)/usr/include
    $(INSTALL_DATA) $(PKG_BUILD_DIR)/bar.h $(1)/usr/include/

    $(INSTALL_DIR) $(1)/usr/lib
    $(INSTALL_DATA) $(PKG_BUILD_DIR)/libbar.so $(1)/usr/lib/
endef

$(eval $(call BuildPackage,libbar))

Step 4.1: Technical Notes

• Build/Patch transforms code before compile.

• -fPIC ensures relocatable shared object.

• Headers + library exported to staging + rootfs.

Step 5: Build Shared Library

make package/my/libbar/compile V=s

Step 5.1: Technical Notes

• Result contains patched libbar.so.

• Output placed in bin/packages/....

Step 6: Install Shared Library on OpenWrt VM

scp -P <PORT> libbar*.apk root@127.0.0.1:/tmp/
apk add --allow-untrusted /tmp/libbar*.apk

Step 6.1: Technical Notes

• libbar.so lands in /usr/lib.

• Patched math logic immediately active.

Step 7: Test Application Directory Structure

package/my/bar-test/
├── Makefile
├── patches/
│   └── 0001-change-message.patch
└── src/main.c

printf("bar_mul(3,5) returned %d\n", bar_mul(3,5));

Step 7.1: Technical Notes

• Test program verifies shared library patching.

Step 8: Test Application Patch

--- a/main.c
+++ b/main.c
@@ -5,6 +5,6 @@
 int main() {
     int result = bar_mul(3,5);
-    printf("bar_mul(3,5) returned %d\n", result);
+    printf("Patched: bar_mul(3,5) = %d\n", result);
     return 0;
 }

Step 8.1: Technical Notes

• Demonstrates userland patching.

• Logic unchanged; output modified.

Step 9: Test Application Makefile

DEPENDS:=+libbar

define Build/Prepare
    mkdir -p $(PKG_BUILD_DIR)
    $(CP) ./src/* $(PKG_BUILD_DIR)/
    $(call Build/Patch,$(PKG_BUILD_DIR))
endef

define Build/Compile
    $(TARGET_CC) -I$(STAGING_DIR)/usr/include \
        -L$(STAGING_DIR)/usr/lib \
        $(PKG_BUILD_DIR)/main.c \
        -lbar \
        -o $(PKG_BUILD_DIR)/bar-test
endef

Step 9.1: Technical Notes

• DEPENDS ensures libbar installed first.

• Dynamically links against patched library.

Step 10: Build & Install Test Application

make package/my/bar-test/compile V=s
scp -P <PORT> bar-test*.apk root@127.0.0.1:/tmp/
apk add --allow-untrusted /tmp/bar-test*.apk
bar-test

Expected:

Patched: bar_mul(3,5) = 17

Step 10.1: Technical Notes

• bar_mul normally returns 15 → patched to 17.

• Dynamic linking picks up patched library without recompiling.