====== Hello World ======

  $ sudo apt install build-essential gdb valgrind

  $ gcc --version
  gcc (Debian 12.2.0-14) 12.2.0
  Copyright (C) 2022 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

  $ make --version
  GNU Make 4.3
  Built for x86_64-pc-linux-gnu
  Copyright (C) 1988-2020 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.

  $ gdb --version
  GNU gdb (Debian 13.1-3) 13.1
  Copyright (C) 2023 Free Software Foundation, Inc.
  License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
  This is free software: you are free to change and redistribute it.
  There is NO WARRANTY, to the extent permitted by law.

  $ valgrind --version
  valgrind-3.19.0

<code C>
/* first-example.c */
#include <stdio.h>
int main(void)
{
  printf("Hello, world!\n");
  return 0;
}
</code>

  $ gcc first-example.c
  $ ./a.out
  Hello, world!
  $ rm a.out

  
  * a.out : assembler output

  $ gcc first-example.c -o first-example
  $ ./first-example
  Hello, world!
  $ rm first-example

  $ make first-example
  cc     first-example.c   -o first-example
  $ ./first-example
  Hello, world!
  $ rm first-example