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04-life-of-code.md

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Understanding a Linux based OS: S1E4 - Life of Code

A computer program is a collection of instructions that can be executed by a computer to perform a specific task.

Here we shall focus on C language, which is a simple, yet powerful system programming language.

C is a compiled language. The process of compilation of C program looks as follows.

         source.c
            |
            V
     -----------------
     | Preprocessing |
     -----------------
            |
            V
    --------------------
    | Lexical Analysis |
    --------------------
            |
            V
    -------------------
    | Syntax Analysis |
    -------------------
            |
            V
   ---------------------
   | Semantic Analysis |
   ---------------------
            |
            V
    --------------------
    |   Intermediate    |
    |      Code         |
    |   Generation      |
    |       and         |
    |    Optimization   |
    ---------------------
            |
            V
    --------------------
    |     Target        |
    |      Code         |
    |   Generation      |
    |       and         |
    |    Optimization   |
    ---------------------
             |
             V
      -----------------
      |    Linking    |
      -----------------
             |
             V
           a.out

Viewing these steps in clang

  • Preprocessing
    clang -E source.c
  • Lexical Analysis
    clang -fsyntax-only -Xclang -dump-tokens  source.c
  • Syntax Analysis
    clang -Xclang -ast-dump source.c
  • Intermediate Code Generation
    clang -S -emit-llvm source.c 
    # source.ll is intermediate code
  • Target Code Generation
    clang -c source.c
    # generates source.o
    # To view its disassembly
    # objdump -d -s source.o
  • Linking
    clang -o source.o
  • Execution
    ./a.out

While executing, some errors like "Segmentation fault" may occur. Although it is possible to monitor the values using printf(), some better way is needed.

gdb - The debugger

  • Allows to step into the code line by line, print the values of expression, evaluate arbitrary expressions, ...
  • We will see its basic functionalities
  • Before gdb is invoked, we have to add debug symbols to the executable code for the ease of understanding
  • gcc -g source.c
  • And then, gdb a.out
  • Following table lists some of gdb commands along their shorthands and uses
Command Shorthand Purpose
run r Runs the executable
backtrace bt Shows the stack trace in case of error
break b Add breakpoint at particular function or line
list l Show the code around particular line
step s Execute a line of code
print p Prints the value of an expression
quit q Exit from the debugger

valgrind - The memory inspector

  • Programs can dynamically allocate heap memory using malloc()
  • This memory has to be deallocated after the particular task is done using free()
  • Otherwise, it will lead to a memory leak, where no other program would be able to use a particular portion of memory, even after the program is killed
  • valgrind executes the programs, and reports memory leaks, if any.
    valgrind program