4.2. Makefile-Based Projects

Simple Makefile-based projects use and interact with the cross-toolchain environment variables established when you run the cross-toolchain environment setup script. The environment variables are subject to general make rules.

This section presents a simple Makefile development flow and provides an example that lets you see how you can use cross-toolchain environment variables and Makefile variables during development.

The main point of this section is to explain the following three cases regarding variable behavior:

Note

Regardless of how you set your variables, if you use the "-e" option with make, the variables from the SDK setup script take precedence:
     $ make -e target
                

The remainder of this section presents a simple Makefile example that demonstrates these variable behaviors.

In a new shell environment variables are not established for the SDK until you run the setup script. For example, the following commands show a null value for the compiler variable (i.e. CC).

     $ echo ${CC}

     $
            

Running the SDK setup script for a 64-bit build host and an i586-tuned target architecture for a core-image-sato image using the current 2.6 Yocto Project release and then echoing that variable shows the value established through the script:

     $ source /opt/poky/2.6/environment-setup-i586-poky-linux
     $ echo ${CC}
     i586-poky-linux-gcc -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux
            

To illustrate variable use, work through this simple "Hello World!" example:

  1. Create a Working Directory and Populate It: Create a clean directory for your project and then make that directory your working location.

         $ mkdir $HOME/helloworld
         $ cd $HOME/helloworld
                        

    After setting up the directory, populate it with files needed for the flow. You need a main.c file from which you call your function, a module.h file to contain headers, and a module.c that defines your function.

    Create the three files as follows:

    • main.c:

           #include "module.h"
           void sample_func();
           int main()
           {
           	sample_func();
           	return 0;
           }
                                      

    • module.h:

           #include <stdio.h>
           void sample_func();
                                      

    • module.c:

           #include "module.h"
           void sample_func()
           {
      	     printf("Hello World!");
      	     printf("\n");
           }
                                      

  2. Source the Cross-Toolchain Environment Setup File: As described earlier in the manual, installing the cross-toolchain creates a cross-toolchain environment setup script in the directory that the SDK was installed. Before you can use the tools to develop your project, you must source this setup script. The script begins with the string "environment-setup" and contains the machine architecture, which is followed by the string "poky-linux". For this example, the command sources a script from the default SDK installation directory that uses the 32-bit Intel x86 Architecture and the Thud Yocto Project release:

         $ source /opt/poky/2.6/environment-setup-i586-poky-linux
                        

  3. Create the Makefile: For this example, the Makefile contains two lines that can be used to set the CC variable. One line is identical to the value that is set when you run the SDK environment setup script, and the other line sets CC to "gcc", the default GNU compiler on the build host:

         # CC=i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux
         # CC="gcc"
         all: main.o module.o
         	 ${CC} main.o module.o -o target_bin
         main.o: main.c module.h
    	     ${CC} -I . -c main.c
         module.o: module.c module.h
    	     ${CC} -I . -c module.c
         clean:
    	     rm -rf *.o
    	     rm target_bin
                        

  4. Make the Project: Use the make command to create the binary output file. Because variables are commented out in the Makefile, the value used for CC is the value set when the SDK environment setup file was run:

         $ make
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c main.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c module.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux main.o module.o -o target_bin
                        

    From the results of the previous command, you can see that the compiler used was the compiler established through the CC variable defined in the setup script.

    You can override the CC environment variable with the same variable as set from the Makefile by uncommenting the line in the Makefile and running make again.

         $ make clean
         rm -rf *.o
         rm target_bin
         #
         # Edit the Makefile by uncommenting the line that sets CC to "gcc"
         #
         $ make
         gcc -I . -c main.c
         gcc -I . -c module.c
         gcc main.o module.o -o target_bin
                        

    As shown in the previous example, the cross-toolchain compiler is not used. Rather, the default compiler is used.

    This next case shows how to override a variable by providing the variable as part of the command line. Go into the Makefile and re-insert the comment character so that running make uses the established SDK compiler. However, when you run make, use a command-line argument to set CC to "gcc":

         $ make clean
         rm -rf *.o
         rm target_bin
         #
         # Edit the Makefile to comment out the line setting CC to "gcc"
         #
         $ make
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c main.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c module.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux main.o module.o -o target_bin
         $ make clean
         rm -rf *.o
         rm target_bin
         $ make CC="gcc"
         gcc -I . -c main.c
         gcc -I . -c module.c
         gcc main.o module.o -o target_bin
                        

    In the previous case, the command-line argument overrides the SDK environment variable.

    In this last case, edit Makefile again to use the "gcc" compiler but then use the "-e" option on the make command line:

         $ make clean
         rm -rf *.o
         rm target_bin
         #
         # Edit the Makefile to use "gcc"
         #
         $ make
         gcc -I . -c main.c
         gcc -I . -c module.c
         gcc main.o module.o -o target_bin
         $ make clean
         rm -rf *.o
         rm target_bin
         $ make -e
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c main.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux -I . -c module.c
         i586-poky-linux-gcc  -m32 -march=i586 --sysroot=/opt/poky/2.5/sysroots/i586-poky-linux main.o module.o -o target_bin
                        

    In the previous case, the "-e" option forces make to use the SDK environment variables regardless of the values in the Makefile.

  5. Execute Your Project: To execute the project (i.e. target_bin), use the following command:

         $ ./target_bin
         Hello World!
                        

    Note

    If you used the cross-toolchain compiler to build target_bin and your build host differs in architecture from that of the target machine, you need to run your project on the target device.

    As expected, the project displays the "Hello World!" message.