Notes

Getting started with C language on a Mac

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Programming
4 minutes

machine: MacBook Pro (2020, M1 chip)

why: recently, i have developed a curious interest in computer science history and the genius that came out of Bell Labs. This includes UNIX, and the language C. So i just wanted to go through some programming fundamentals in this OG language of the OG operating system.

Notes

  • C comes pre-installed on a Mac. Check with clang --version
  • Files with source code end in .c
  • Files with compiled code end in .out (if you haven’t passed a custom filename, see next point)
  • You compile your file with the cc command. For example cc hello.c. By default, it’ll output to a file called a.out. You can pass an output file name with the -o flag. For example, cc hello.c -o hello and it’ll save your compiled code to a file called hello (notice the lack of .out at the end)
  • The output file it gives you is an executable (marked by an asterisk in the Terminal listing). You can run any executable file in your current directory by prepending the filename with ./. For example: ./a.out
  • I tried executing the file by typing just a.out but got a zsh: command not found: a.out instead. ./a.out works.

Hello World

1# include <stdio.h>
2
3int main () {
4  printf("hello, world\n");
5}

The book doesn’t mention the int, but if you run the code without it, you’ll get a warning like the following:

hello.c:3:1: warning: type specifier missing, defaults to 'int'
      [-Wimplicit-int]
main () {
^
1 warning generated.

It’ll still compile though, if you ls now, you’ll see a file called a.out (that’s the default file name) in the same directory.

Machine code

The compiled code is a bunch of gibberish (machine code). You can’t view it in VS Code, it’ll refuse to display it saying it is either binary or uses an unsupported format, and if you try to see file contents in the Terminal with cat a.out, you’ll see bats!

����X� H__PAGEZERO�__TEXT@@__text__TEXT`?#`?�__stubs__TEXT�?��__stub_helper__TEXT�?�?�__cstring__TEXT�?�?__unwind_info__TEXT�?H�?�__DATA_CONST@@@@__got__DATA_CONST@�__DATA�@�@__la_symbol_ptr__DATA�__data__DA�H__LINKEDIT�@�"�� �0�0h���H
                 P��
                     /usr/lib/dyld�.Ly(�;໽x݀eg)2

                                               a*(�`?
                                                     8<
                                                       /usr/lib/libSystem.B.dylib&`)h�UH��H��H�=7��1ɉE��H��]Ð�%v@L�u@AS�%e�h�����hello, world
`?44�?4
       �?#Q@dyld_stub_binderQr�s@_printf�__mh_execute_header!main%�~��`?$ __mh_execute_header_main_printfdyld_stub_binder__dyld_private%

What it compiles down to is machine code,and since i am not a machine, i don’t really care about what it looks like. I care about what i can do with its powers.

The main() function

Unlike the programming languages that i am already familiar with (JavaScript, Python, Bash, PHP), C programs must have a function called main. main is your entry point. You can define other function with other names, but if you wanted to execute something as part of your main program, you’ll have it in a function called main. Your program begins executing at the the beginning of main and goes line by line from there. Every program must have a function called main somewhere.

Languages that are derived from C (e.g. C++, C#, Objective-C) also by default must have a main function. Same is the case with Java, every Java application begins with a class definition, and every class must have the main method. Pascal (another language that pre-dates C) is similar to C in this regard, only Pascal programs start with the program keyword.

1// Language: Java
2class HelloWorld {
3    public static void main(String[] args) {
4        System.out.println("Hello, World!");
5    }
6}

1// Language: C
2# include <stdio.h>
3
4int main () {
5  printf("hello, world\n");
6}

 1// Languages: C#
 2namespace HelloWorld
 3{
 4  class Hello {
 5    static void Main(string[] args)
 6    {
 7      System.Console.WriteLine("Hello World!");
 8    }
 9  }
10}

// Language: Pascal
program HelloWorld(output);
begin
  writeln('Hello, world!');
end.

After learning about this hardcore main requirement, i now understand why some frontend devs call their JS and CSS files main.js and main.css, they probably have a CS background ¯_(ツ)_/¯ and this is probably where the file names come from

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