Lesson 1.3. Compilation

Introduction

The programming languages do not all run in the same way, we distinguish two major classes: compiled languages and interpreted languages.

Interpreted languages

An interpreted language requires the use of an interpreter that will run the source code. The interpreter is an intermediate program that parses and executes the lines (usually one by one). The most popular interpreted languages are : MATLAB, PHP, Python, Java ... Each of these languages requires the installation of the associated
interpreter to run a program.

Compiled languages

Running a compiled program is in two steps:

The source code is first transformed into machine language.
Then the transformed code is directly executed in the processor

The first step, the transformation of the source code into machine language is called compilation. The output of the compilation is an executable file, typically an .exe file under Windows. Here are some examples of compiled languages: C, C++, C#, swift, Pascal...

In general, compiled languages are more efficient because they are executed directly in the processor. To get an idea of the performance, this page presents a comparison between Python and C++. The same program takes 15 minutes in Python, against 30 seconds in C++. The graph below gives an order of magnitude of the execution ratios of the different languages. We can see that C is 10 times faster than PHP 7 and 40 times faster than the same program in Python.

The counterpart is the compilation time: if it is generally instantaneous, it can sometimes be much longer on large projects. For example the compilation of of a Linux kernel can take about ten hours. Obviously, if the source code is not modified it is useless to restart the compilation before each execution.

Compiler

Before executing a C code, it is imperative to compile it with a compiler. In the example Hello world seen before, if you look carefully at the console output, you will notice that this command is executed before the execution of the program :

clang-7 -pthread -lm -o main main.c

clang-7 is the compiler command
-pthread et -lm are compiler options
-o main (-o = output) specifies the output file (the executable file)
main.c is the source code to compile

The compiler creates a main file which is executable. This file is run right after with the command ./main. If you type this command again in the command again in the console, the program will run a second time.

Compiling step

During compilation, the source code is analyzed before being converted. Of course, the source code must respect rules to be compilable. The first rule to know is that each instruction ends with a semicolon. Here are the lines of the main function of the program Hello world :

printf("Hello World\n");
return 0;

The semicolon indicates to the compiler that the instruction ends and the next one starts. This allows it to parse the code automatically. If you omit a semicolon, the compiler will not know the difference between the two
instructions, for him you will have written something like :

printf("Hello World\n")return 0;

printf and return become a single instruction. Some editors like Qt Creator display a small arrow to indicate the error line. It is not uncommon for the compiler to place the arrow on the second line and not on the line where the semicolon is omitted, because from the compiler's point of view the two lines are the same instruction.

For readability reasons, we write the programs with one instruction per line. But the compiler does not look at the line breaks. The only separations that matter to him are semicolons. The program Hello world could be written on a single line as long as the instructions are separated by semicolons:

#include <stdio.h> 
int main(void) { printf("Hello World\n"); return 0; }

This program, even if it works, is unreadable. This type of writing is to be banned.

Note: the first line does not have a semicolon, because it is not a statement, but a preprocessor directive. Indeed, this line is not intended to be executed. It is used to transmit information to the compiler. Here, it tells the compiler that he should use the stdio.h library.

Tester l'exemple en ligne

Compiling errors

When the code is transformed into machine language, the compiler analyzes the code and detects errors if there are any:

Errors: this is a problem in the source code that prevents the code from being compiled. In case of errors, the compiler does not generate the executable file.
Warnings : this is a problem that the compiler detects, but which will not prevent the compilation of the program. The executable file is generated. But be careful, the executable may not have the expected behavior.

When deploying an application, the best is to have no errors, nor warnings. For errors, it is imperative. About the warnings, they are not always fixed. But if you keep the warning, you must understand the origin of the problem and do it knowingly.

Quiz

Which statements are true?

An interpreter parses the code line by line.

An interpreter converts code into machine language.

An interpreter runs the code line by line.

An interpreter executes all the code at once.

Check

About compilers...

A source code compiled under Windows will run under Android.

A compiler converts code into machine language.

A compiler runs the code.

A compiler produces an executable file.

Check

What is the problem with the code below?

int main(void){printf("Strange exercise !\n");return 0;}

Line returns are missing for it to run.

It is not easily readable.

It is missing spaces so that it can be compiled.

It is difficult to understand.

Check

What is the problem with the code below?

int main(void) {
  printf("Drole d'exercice !\n")
  return 0
};

There is no problem.

A semi-colon is missing at the end of the printf.

There is a semi-colon missing at the end of the return

There should not be a semicolon after the braces.

Check

With the following compilation result:

main.c:3:26: error: expected ';' after expression
  printf("Hello\n")

The compilation was a real success.

There was an error during compilation.

The code is missing a semicolon.

The executable file is always generated, even if there is an error.

Check

At the time of compilation a warning appears:

main.c:5:8: warning: using the result of an assignment as a condition without parentheses
      [-Wparentheses]
  if (i=5) printf ("Hello");

It doesn't matter. The program works properly since it compiles.

I analyze the message to understand the source of the problem.

The executable file is not generated.

It is imperative to fix this problem.

Check

Lesson 1.2 Summary Lesson 1.4

Lesson 1.3. Compilation

Introduction

Interpreted languages

Compiled languages

Compiler

Compiling step

Compiling errors

Quiz

See also