Hello, World

You write one file, compile it, and run it.

The program

A program needs a place to start. The compiler does not choose one; the C runtime calls the function main.

Printing needs a function that writes to the terminal, and puts from the C library does that. The {external} attribute tells the compiler that the implementation is somewhere else and that the linker will find it.

Write this into hello_world.st:

{external}
FUNCTION puts: DINT
    VAR_INPUT {ref}
        text: STRING;
    END_VAR
END_FUNCTION

FUNCTION main: DINT
    puts('hello, world!');
END_FUNCTION

Two details of the language show up already. A string literal stands between single quotation marks, and a $ inside it starts an escape, such as $N for a new line. This literal needs no $N, because puts writes a new line after the text.

Compile and run

plc hello_world.st -o hello_world --linker=cc
./hello_world

hello, world!

The compiler translated the file into an object file and then called cc to link that object with the C library into an executable. -o names the result. --linker=cc names the program that links; Install says which one your system has.

Leave out -o and the executable is named after the input file, here hello_world.st.out. Add -c and the compiler stops after the object file, hello_world.st.o.

When something is wrong

Now make a mistake on purpose. Write this into hello_world.st instead:

FUNCTION main: DINT
    VAR
        x: DINT;
    END_VAR

    x := 'text';
END_FUNCTION

error[E037]: Invalid assignment: cannot assign 'STRING' to 'DINT'
  ┌─ hello_world.st:6:5
  │
6 │     x := 'text';
  │     ^^^^^^^^^^^ Invalid assignment: cannot assign 'STRING' to 'DINT'

error: Compilation aborted due to critical errors.
Hint: You can use `plc explain <ErrorCode>` for more information

The first line carries the code of the message. plc explain E037 prints what the code means, with an example. The compiler also returns a non-zero exit code, so a script sees the failure.

To check the file without producing anything, use plc --check hello_world.st.

What’s next

One file on the command line is enough for one program. Real code lives in several files and needs the same options on every build. The next chapter puts both into a project.