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Build and package your own software for OpenWRT

Today I am going to discuss how to build and package your own software for OpenWRT.

When I say “your own software” in this case I am referring to a C program which you want to cross-compile for the target SoC and install using the opkg package manager included in OpenWRT.

The program I wrote is a little more complicated than your standard “Hello World” application. Here’s what I wanted to do:
1) use libconfig to read a configuration file in /etc/config/ and then perform actions based on the configuration described in this file
2) use sqlite3 to create a database
3) write some meaningful data to the database

Here’s the program flow:
1) Open /etc/config/example-sqlite and read the values into variables
2) Open (or create) a new SQLite3 database file at the location defined in the above configuration file
3) Determine if the SQLite file is initialized with the target table we want to write to, and if not, create the table
4) Write the system load average to the database
5) Quit

To recap, this program is different from “Hello World” in the following ways:
1) It must read and understand a configuration file in libconfig syntax; this requires linking against the libconfig library, which we must tell opkg is a dependency
2) It must create or open an SQLite 3 database; this requires linking against the sqlite3 library, which we must tell opkg is a depenedency
3) It must perform some useful operations on this SQLite file

Let’s start with compiling the C file on your native architecture. Sure, you can just use cc/gcc from bash, but this isn’t any good to OpenWRT SDK, which expects that each package will have a makefile which can be used to compile the software.

load2sqlite.c

#include <sys/types.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <sqlite3.h>
#include <libconfig.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

int main(int argc, char *argv[]) {
// ...

Most importantly above, we are including sqlite3.h for SQLite support, libconfig.h, and sys/stat.h, fcntl.h,errno.h to check if the SQLite3 database file exists or not.

You can compile this by hand quite easily, just by doing:
cc load2sqlite.c -lsqlite3 -lconfig -o load2sqlite

Okay, but how do we make this ready for OpenWRT SDK? By writing a makefile!

makefile

PROFILE = -O2 -s
CFLAGS = $(PROFILE)
LDFLAGS = -lsqlite3 -lconfig

all: main

# build it
main:
	$(CC) $(CFLAGS) load2sqlite.c $(LDFLAGS) -o load2sqlite

# clean it
clean:
	rm load2sqlite

Okay, so now if you type make in the directory, magically you will end up with an executable called load2sqlite!

But, this is a native binary, and it’s somewhat unlikely that your OpenWRT device is on the same architecture.

load2sqlite: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=9661b88e92b553d0556cbeeafccf04d2526c770f, stripped

If you run it, you’ll see that it looks for the sqlite database file, can’t find it, and so initalizes a new one with the “readings” table.

[[email protected] src]$ ./load2sqlite 
Database file /tmp/sqlite3.db does not exist
Initialized database with readings table
[[email protected] src]$ echo "select * from readings;" | sqlite3 /tmp/sqlite3.db 
2015-10-28 22:48:42|0.57|0.56|0.57

And if you run it again, without removing the SQLite3 file that was created, you’ll see this output:

[[email protected] src]$ ./load2sqlite
SQLite database opened
Found readings table
[[email protected] src]$ echo "select * from readings;" | sqlite3 /tmp/sqlite3.db 
2015-10-28 22:48:42|0.57|0.56|0.57
2015-10-28 22:49:00|0.47|0.54|0.57

Before we proceed further, I want to show you the directory structure so you have an idea of where we just were when we did this compilation. We are currently in the the src directory.

load2sqlite/
|-- Makefile
|-- README
`-- src
    |-- load2sqlite.c
    |-- load2sqlite.conf
    `-- makefile

Now let’s move up to the load2sqlite directory and work on the OpenWRT Makefile (seen above).

Here is the complete file, and then we will discuss it section by section:
Makefile

#
# Copyright (C) 2006-2015 OpenWrt.org
#
# This is free software, licensed under the GNU General Public License v2.
# See /LICENSE for more information.
#

include $(TOPDIR)/rules.mk

PKG_NAME:=load2sqlite
PKG_VERSION:=1.0.1
PKG_RELEASE:=5
PKG_MAINTAINER:=Hal Martin 
PKG_LICENSE:=GPL-2
PKG_CONFIG_DEPENDS:=libsqlite3 libconfig

include $(INCLUDE_DIR)/package.mk

PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)-$(PKG_VERSION)

TARGET_LDFLAGS+= \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib/libconfig/lib \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib/sqlite/lib

define Package/load2sqlite
  SECTION:=utils
  CATEGORY:=Utilities
  DEPENDS:=+libsqlite3 +libconfig
  TITLE:=SQLite example program, creates or opens a user defined SQLite database
  URL:=https://github.com/halmartin/load2sqlite
  MENU:=1
endef

define Package/load2sqlite/description
 Example SQLite is a sample program built using libsqlite3 and libconfig
 which creates or opens a user-defined SQLite3 database and performs some
 simple verification checks on the file to ensure that the target table (readings)
 exists, and if not creates the table, then inserts a row with the current system
 time, and the load (1 minute, 5 minute, 15 minute).
endef

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

define Build/Configure
endef

define Build/Compile
	$(MAKE) -C $(PKG_BUILD_DIR) $(TARGET_CONFIGURE_OPTS)
endef

define Package/load2sqlite/install
	$(INSTALL_DIR) $(1)/bin
	$(INSTALL_BIN) $(PKG_BUILD_DIR)/load2sqlite $(1)/bin/
	$(INSTALL_DIR) $(1)/etc/config
	$(INSTALL_CONF) $(PKG_BUILD_DIR)/load2sqlite.conf $(1)/etc/config/load2sqlite
endef

$(eval $(call BuildPackage,load2sqlite))

If you clone the OpenWRT source and take a look at basically any package, you’ll see a Makefile that looks similar to the one above.

Let’s look at the package information:

PKG_NAME:=load2sqlite
PKG_VERSION:=1.0.1
PKG_RELEASE:=5
PKG_MAINTAINER:=Hal Martin 
PKG_LICENSE:=GPL-2

Here is where we define core details of our package, such as the name (e.g. what opkg will know it as), the version (useful for upgrading later), maintainer, and license.

TARGET_LDFLAGS+= \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib/libconfig/lib \
  -Wl,-rpath-link=$(STAGING_DIR)/usr/lib/sqlite/lib

Since we want to build a program which links against external libraries, we must also tell the compiler where to find the header files for these libraries, so that the linking process does not fail during compilation. Above you can see that we are linking to libconfig and sqlite libraries.

define Package/load2sqlite
  SECTION:=utils
  CATEGORY:=Utilities
  DEPENDS:=+libsqlite3 +libconfig
  TITLE:=SQLite example program, creates or opens a user defined SQLite database
  URL:=https://github.com/halmartin/load2sqlite
  MENU:=1
endef

This is where you define the package for the OpenWRT build system and declare things like dependencies, and the description that will be present when you run menuconfig (which is how you will select your package to be built as part of an image).

Without declaring dependencies, you may find that you can build, package, and install your software, but it won’t run! So, by declaring the dependencies (packages which provide the libraries we link against) we ensure that when we type opkg install load2sqlite and libconfig and libsqlite3 are not installed, opkg knows to go and install them before installing our program. Now we can safely run the program because all the required libraries are installed on the device!

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

define Build/Configure
endef

Since our utility is quite simple, as *NIX software goes, the preparation steps are to create the build directory and copy the source from the source directory to the build directory. Since there is nothing to configure in our sample program, the configure step is empty (otherwise the OpenWRT build system will attempt to configure the package and fail because we haven’t bothered to implement this).

define Build/Compile
	$(MAKE) -C $(PKG_BUILD_DIR) $(TARGET_CONFIGURE_OPTS)
endef

define Package/load2sqlite/install
	$(INSTALL_DIR) $(1)/bin
	$(INSTALL_BIN) $(PKG_BUILD_DIR)/load2sqlite $(1)/bin/
	$(INSTALL_DIR) $(1)/etc/config
	$(INSTALL_CONF) $(PKG_BUILD_DIR)/load2sqlite.conf $(1)/etc/config/load2sqlite
endef

Finally, compile and install the software. As you can see above, I didn’t include an install directive in the makefile of the application, it is instead done manually within the OpenWRT Makefile. This is your choice, since I was designing this program specifically to run on OpenWRT, I saw no need to incorporate the installation steps in the makefile of the program.

And, finally:

$(eval $(call BuildPackage,load2sqlite))

This line is required for OpenWRT to build the package. Forget this line, and you will sit there wondering why your package is not being built!


Okay, now we have prepared our software to be built for OpenWRT. It would be stupid of me to get this far and not tell you how to compile it using the OpenWRT toolchain!

Following the excellent OpenWRT documentation, we need to set up a buildroot.

Install the dependencies (instructions for Debian/Ubuntu):

sudo apt-get install git-core build-essential libssl-dev libncurses5-dev unzip subversion mercurial

Clone the OpenWRT Chaos Calmer release:

git clone git://git.openwrt.org/15.05/openwrt.git

I find that the stock OpenWRT repository is a bit light on some of the software I like to have on my routers, so I take step 3 and install the additional feeds:

cd openwrt
./scripts/feeds update -a
./scripts/feeds install -a

Follow step 4 to ensure you have all the required dependencies installed on your host system!

make defconfig
make prereq
# don't forget to copy load2sqlite to package/utils/ before running this step, or the package won't appear in the menu!
make menuconfig

If everything has gone well thus far (e.g. no errors in the OpenWRT Makefile, and you put load2sqlite in package/utils/ then you should see the following in your menuconfig:

menuconfig_load2sqlite

menuconfig_load2sqlite_desc

Now I already have an official OpenWRT build installed on my router, so I don’t need to build an entire image, just the package I want to install. To do this, we must first build the cross compilation toolchain required to compile for a different CPU architecture.

Warning: the OpenWRT buildroot is around 6GB on disk, so ensure you have the necessary space before starting!

make tools/install
# this will take a while the first time
make toolchain/install
# this will also take a while the first time

When we have the tools and toolchain compiled, we can compile our package:

make package/load2sqlite/compile

This will create an ipkg file in bin/ramips/packages/base/load2sqlite_1.0.1-5_ramips_24kec.ipk which we need to copy to our router to install:

scp bin/ramips/packages/base/load2sqlite_1.0.1-5_ramips_24kec.ipk [email protected]:/tmp/
# scp completes
ssh [email protected]
[email protected]'s password:

BusyBox v1.23.2 (2015-07-25 03:03:02 CEST) built-in shell (ash)

  _______                     ________        __
 |       |.-----.-----.-----.|  |  |  |.----.|  |_
 |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
 |_______||   __|_____|__|__||________||__|  |____|
          |__| W I R E L E S S   F R E E D O M
 -----------------------------------------------------
 CHAOS CALMER (15.05, r46767)
 -----------------------------------------------------
  * 1 1/2 oz Gin            Shake with a glassful
  * 1/4 oz Triple Sec       of broken ice and pour
  * 3/4 oz Lime Juice       unstrained into a goblet.
  * 1 1/2 oz Orange Juice
  * 1 tsp. Grenadine Syrup
 -----------------------------------------------------
[email protected]:~# opkg install /tmp/load2sqlite_1.0.1-5_ramips_24kec.ipk 
Installing load2sqlite (1.0.1-4) to root...
Installing libsqlite3 (3081101-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05/ramips/mt7620/packages/packages/libsqlite3_3081101-1_ramips_24kec.ipk.
Installing libpthread (0.9.33.2-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05/ramips/mt7620/packages/base/libpthread_0.9.33.2-1_ramips_24kec.ipk.
Installing libconfig (1.4.9-1) to root...
Downloading http://downloads.openwrt.org/chaos_calmer/15.05/ramips/mt7620/packages/base/libconfig_1.4.9-1_ramips_24kec.ipk.
Configuring libpthread.
Configuring libconfig.
Configuring libsqlite3.
Configuring load2sqlite.

Now that our package is installed, we can test it!

[email protected]:~# /bin/load2sqlite 
Database file /tmp/sqlite3.db does not exist
Initialized database with readings table

If you install sqlite3-cli we can inspect the row added to the file:

[email protected]:~# opkg install sqlite3-cli
[email protected]:~# echo "select * from readings;" | sqlite3 /tmp/sqlite3.db 
2015-10-31 20:47:33|0.76|0.4|0.25

Since this is just an example program, it is one-shot (e.g. not a daemon). If you really do want to track the load of our OpenWRT router, just add /bin/load2sqlite to crontab (e.g. every hour) and you’ll have this tracking info in the SQLite database.

If you run it multiple times, you get another row added to the file each time the program is run:

[email protected]:~# /bin/load2sqlite 
SQLite database opened
Found readings table
[email protected]:~# echo "select * from readings;" | sqlite3 /tmp/sqlite3.db 
2015-10-31 20:47:33|0.76|0.4|0.25
2015-10-31 20:53:43|0.02|0.2|0.22
2015-10-31 21:23:19|0.08|0.04|0.05

Note that by default the file is saved to /tmp/, which on OpenWRT is a ramdisk. This means that the file will be lost when you reboot, or if you leave it running unattended for too long, the file size will grow to the point where the ramdisk will consume all available memory and the router will crash. For this reason, I suggest you modify the configuration file /etc/config/load2sqlite to point to non-volatile storage (such as a USB stick).


Source code: https://github.com/halmartin/load2sqlite


Why write another OpenWRT software guide?

Well, while I was looking for resources on how to build and package software for OpenWRT, I ran into a lot of posts about people compiling simple “Hello World” programs for OpenWRT, but for my particular use case, I wanted to utilize multiple libraries in my program, and I couldn’t find any good instructions on how to compile a program with linked libraries for OpenWRT.

Disclaimer: I’m not a C expert, so maybe there are some headers there which are not strictly necessary, but it works for me and the executable size is quite small.

If you wish to further reduce the size of your executable, you can tell the compiler to strip it of the symbol table and relocation information. Do this by appending -s to the PROFILE line in the makefile. When I did this on my laptop, the output went from 9.9KB to 7.0KB, or a savings of 30%

I have tested this on Chaos Calmer (15.05), and I expect the instructions would also work on Barrier Breaker (14.07) however I didn’t try this, so I cannot say certainly that it will work.