SPO600 Compiled C Lab

Purpose of this Lab
In this lab, you will investigate the relationship between basic C source code and the output of the C compiler.

Lab 2

1. Write a basic C program which prints a message on the screen, Hello World!-style -- something like this:

#include <stdio.h>

int main() {
    printf("Hello World!\n");
}

2. Compile the program using the GCC compiler. Include these compiler options (refer to the gcc manpage for details):

-g               # enable debugging information
-O0              # do not optimize (that's a capital letter and then the digit zero)
-fno-builtin     # do not use builtin function optimizations

3. The resulting binary is an ELF (Executable and Linkable Format) file, which contains multiple sections. These sections may contain object code, link tables, debugging symbols, program data (such as constants and the initial values of variables), metadata about the program and ELF sections, and comments.

Examine the binary produced by the previous step using the objdump program. These options may be useful -- see the manpage for objdump for other options:

-f          # display header information for the entire file
-s          # display per-section summary information
-d          # disassemble sections containing code
--source    # (implies -d) show source code, if available, along with disassembly

Using objdump, find the answers to these questions: (i) Which section contains the code you wrote? (ii) Which section contains the string to be printed?

objdump vs readelf
The output of strings using objdump is not formatted in the best way for some purposes. The readelf -p command may be a better tool for viewing some strings.

4. Try to gain a basic understanding of what the compiled code is doing. Focus on the section marked <main> but also take a look at the other sections.

5. Recompile the code with these changes:

(1) Add the compiler option -static. Note and explain the change in size, section headers, and the function call.

(2) Remove the compiler option -fno-builtin. Note and explain the change in the function call.

(3) Remove the compiler option -g. Note and explain the change in size, section headers, and disassembly output.

(4) Add additional arguments to the printf() function in your program. Note which register each argument is placed in. (Tip: Use sequential integer arguments after the first string argument. Go up to 10 arguments and note the pattern).

(5) Move the printf() call to a separate function named output(), and call that function from main(). Explain the changes in the object code.

(6) Remove -O0 and add -O3 to the gcc options. Note and explain the difference in the compiled code.

6. Blog about your results. Important! -- explain what you're doing so that a reader coming across your blog post understands the context (in other words, don't just jump into a discussion of optimization results -- give your post some context).

Make
Learn how to use make -- your life as a programmer will be much easier, and you'll get back years of your life!

External Resources

• For a general overview of ELF, see the Wikipedia article on Executable and Linkable Format