hello_world.c: In function `main': hello_world.c:3: warning: unknown escape sequence `\z'
This error message is reasonable self-explanatory. As usual, the first line indicates the program (hello_world.c) and the function (main) which contains the error. The second line, still referring to the program hello_world.c, encounters a problem at line 3. It gives a "warning", meaning that you will still be able to run the program. The warning, reasonably enough, is that an unknown escape sequence `\z' has been encountered. Notice that had the printf statement been printf("Hello, world\ze"), the compiler would still have only complained about `\z', because escape sequences are composed of a `\' followed by a single character.