01-hello

Hello World!

Motivation

When learning to code, you have to start at some point. This usually is the "Hello World!" program.

Task

• You'll find the file hello.c in this folder. Read it carefully
• Try to explain every single line of the file.
• Let's assume your favourite compiler is gcc:
  • Run gcc -E hello.c. This will show you the output of the preprocessor
    • What happend to EXIT_SUCCESS?
    • Can you find the forward declaration of the function puts?
  • Run gcc -c -o hello.o -std=c99 -O0 -g -Wall -Wextra -Werror -pedantic hello.c
    • This will invoke the preprocessor and compile to generate the machine code for hello.c
    • -c tells GCC to compile, but not to link
    • -o hello.o tells GCC to store the comiled output as hello.o
    • -std=c99 informs GCC that the source code follows the C99 standard
    • -Wall -Wextra -pedantic tells GCC to inform you about any problems in your code. (Don't fool yourself by thinking a warning is only a minor problem! A warning indicates a serious flaw in your code!)
    • -Werror tells GCC to treat warnings as errors. This should always be enabled
    • -g is used to generate debug symbols. This is very usefull when using debug tools like GDB or Valgrind.
    • -O0 tells GCC to not optimize your code. This makes debugging much easier. Use -O3 or -Os in production code instead
  • Run gcc -o hello hello.o
    • This will invoke the linker. It will link in all required dependencies from the standard C library and create the final executable hello
    • Run ./hello to start the executable
    • Some programs depend on additional libraries. These have to be specified using the -l option. E.g. to link against libofat and libxml you have to specify -lowfat -lxml (the leading lib is droppend).