thibault cholez - [email protected]/teaching/oci/dcr_lesson_1.pdf ·...

OCI - session 1: From C to C++ (and some C++ features)

Thibault CHOLEZ - [email protected]

TELECOM Nancy - Universite de Lorraine

LORIA - INRIA Nancy Grand-Est

From Nicolas Rougier’s C++ crash course

16/02/2017

Introduction Reminder on C compiler Reminder on memory managment Some features Exercices

Plan

1 Introduction

2 Reminder on C compiler

3 Reminder on memory managment

4 Some features

5 Exercices

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Plan

1 Introduction



4 Some features

5 Exercices

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First words

Goal of the class

Hands-on C++ !

Learning similarities and differencies regarding C and JAVA

Learning basic concepts and features of C++

Learning more advanced features used in embedded systems

Structure of the class

Three sessions of two hours

For each session : 30 min talk on C++ concepts, then shortpractical exercices on computer

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First words

Program

From C to C++ : reminder

C++ features and object programming

Tools for reliable programming

Optimizations and performance

Environment

Basic Linux OS and tools : g++, console, text editor

No Integrated Development Environment (hides complexity)

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Why a class on C++ ?

C and C++ are still the most widely used programming languages

For operating systems (SLOC in debian : 49% in C, 21% in C++)

For general purpose software (SourceForge statistics : 18% ofsoftware in C++, 16% in C)

... and also for embedded software (with an emphasis on real timeor low ressource constraints) !

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Why not JAVA for embedded software ?

Pros

Programmer friendly : features for reliability, garbage collector, etc.

Portable ”write once, run anywhere” : same byte-code workseverywhere (not tied to hardware but to JVM, great advantage todeal with smartphones heterogeneity)

Cons

JRE simulates hardware : slower execution (not as bad as it used tobe thanks to ”just-in-time” compilation but still)

Waste of ressources (memory, CPU)

Odd coupling with hardware : JRE as a stack machine while Dalvikis register-based

Unpredictability of JVM is an issue for real time system : you don’twant the garbage collector to work during during a critical time

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Stack VS Registers

Add register a to register b and store the result in c

Stack based (Java)push apush baddpop c

Register based (Dalvik)add a,b,c

Which wins ?

a program written for a stack based machine takes much more inmemory (not fitted to embedded systems)

a program written for a register based machine tends to be slowerthan its equivalent for a stack based machine. But fast enough forembedded devices

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C / C++

Pros

C is the closest high-level language to the machine, C++ is ”just”an improved version with object-oriented features

Close to the hardware, efficient execution and low ressource usage(with proper code)

No interference of VM to disturb execution

Possibility to mix C++ with asm code

Cons

Hard to write efficient and reliable code without strong knowledgeand experience (that’s why you are here)

Specific compilation for each hardware architecture needed

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Plan

1 Introduction



4 Some features

5 Exercices

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Pre-processor

Just like in C, very dumb ”text editing” tool, still useful

Features

#define MACRO NAME value

find and replace, (in C) good to define constant in UPPER CASE

#include <header file>

line replaced by whole content of the header file

#ifdef macro_name

/* Code to use if macro defined */

#else

/* Code to use otherwise */

#endif

#ifndef SOME_UNIQUE_NAME

#define SOME_UNIQUE_NAME

#endif

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Compilation and Link edition

Simplest way

gcc -o binary source1.c source2.c source3.c

Better way

We want to approach modular programming for many reasons(separation of concerns, team work, maintenance)

Source code organized in different meaningful files and folders, asautonomous as possible

Do not recompile everything for a small change : recompile only themodified file(s) and redo the link edition

The linker creates the exec file from multiple object files / librairies

gcc -c source1.c

gcc -o binary source1.o source2.o source3.o

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Separate compilation and Makefile

Makefile

Simple automated tool dealing with file dependencies duringseparate compilation :

target: dependencies

commands

additionnal rules are commonly used : ”all” to generate the mainexecutable, ”clean” to remove object files, etc.

also, additionnal variables are commonly used :

CC for the compiler, CFLAGS for the compilation optionsLDFLAGS for the linker options, EXEC for the name of theexecutable file

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Example of MakefileCXX=g++

CXXFLAGS=-Wall

LDFLAGS=

EXEC=hello

all: $(EXEC)

hello: hello.o main.o

$(CXX) -o hello hello.o main.o $(LDFLAGS)

hello.o: hello.c

$(CXX) -o hello.o -c hello.c $(CXXFLAGS)

main.o: main.c hello.h

$(CXX) -o main.o -c main.c $(CXXFLAGS)

clean:

rm -rf *.o

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Plan

1 Introduction



4 Some features

5 Exercices

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Memory : stack vs heap

The memory of each process is split in 3 big segment : Data(Code+Globals), Stack, Heap. Their place / size is constrained bythe OS.

The stack

The stack stores temporary variables created by each function whichpushes a temporary stack frame for its execution.

The stack is a ”FILO” (first in, last out) data structure, that ismanaged and optimized by the CPU (fast read and write to stackvariables).

Another advantage of using the stack to store variables : no need toallocate memory by hand, memory is automatically freed

Limits : the stack has strong size limits, stack variables only existwhile the function that created them is running

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Memory : stack vs heap

The heap

Heap is the dynamic memory (manually managed by you)

Variables can be accessed globally and ”no” limit on memory size

Memory may become fragmented and wasted (memory leaks) if notproperly managed

Try to gather mallocs and frees in specific parts of your code(initialization and stop phases)

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Pointers

Pointers are variable storing a memory address : Pointer value =rank of a memory cell.

int *p declares a pointer variable p which is a pointer to an integervalue

*p is then the pointed value, interpreted according to the pointertype

the & operator retrieves the adress of something

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Memory management in C

malloc and free to use the heap memory

”High level” API to deal with chunks in heap

There is only 3 functions to know : request a new memory chunk,return a memory chunk, expend a memory chunk

#include <stdlib.h>

void*malloc(int size)

void free(void*p)

void*realloc(void*p,int size)

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Memory management in C++

New/Delete

The new and delete keywords are used to allocate and free memory.They are ”object-aware” so you’d better use them instead of mallocand free (no more sizeof(struct toto))

Never cross the streams (new/free or malloc/delete)

Delete does two things : it calls the destructor and it deallocates thememory.

int *a = new int;

delete a;

int *b = new int[5];

delete [] b;

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Plan

1 Introduction



4 Some features

5 Exercices

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References

A reference allows to declare an alias to another variable. As long asthe aliased variable lives, you can use indifferently the variable orthe alias.

References are extremely useful when used with function argumentssince it saves the cost of copying parameters into the stack whencalling the function.

int x;

int& foo = x;

foo = 42;

std::cout << x << std::endl;

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Namespaces

Namespace allows to group classes, functions and variable under acommon scope name that can be referenced elsewhere.

There exists some standard namespace in the standard templatelibrary such as std.

The example below should give values 3 and 5 :

namespace first { int var = 5; }

namespace second { int var = 3; }

cout << first::var << endl << second::var << endl;

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Default parameters

You can specify default values for function parameters. When thefunction is called with fewer parameters, default values are used.

Watch out, it can create ambiguity.

The code below should obtain values 4, 5 and 6.

float foo( float a=0, float b=1, float c=2 )

{return a+b+c;}

cout << foo(1) << endl

<< foo(1,2) << endl

<< foo(1,2,3) << endl;

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Overloading

Function overloading refers to the possibility of creating multiplefunctions with the same name as long as they have differentparameters (type and/or number).

It is not legal to overload a function based on the return type !

float add( float a, float b )

{return a+b;}

int add( int a, int b )

{return a+b;}

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Const and inline functions

Defines and macros are bad if not used properly as illustrated below

For constants, prefer the const notation (it adds type to theconstant)

#define SQUARE(x) x*x

int result = SQUARE(3+3);

const int two = 2;

int inline square(int x) {return x*x;}

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Plan

1 Introduction



4 Some features

5 Exercices

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Up to you

C++ Hello World

#include<iostream>

int main()

{

std::cout << "Hello, new world!\n";

}

Compilation and execution

g++ hello.cpp -o hello

./hello

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