Lecture 1, Thu 09/27

Introduction, Compilation Review

Course Syllabus

Be sure to read the course syllabus for information on the logistics of this course.

https://ucsb-cs32-f18.github.io/info/syllabus/

Makefiles (a simple example)

// main.cpp
#include <iostream>

using namespace std;

int main() {
	cout << "Hello CS 32!" << endl;
	return 0;
}

Can compile this with g++ or clang++:

$ g++ main.cpp
$ ls
a.out		main.cpp
$ ./a.out
Hello CS 32!

• Note that the executable is called a.out. This is the default name of the executable.

Using make command

$ make main
c++     main.cpp   -o main

• This default behavior of make tries to compile the .cpp with that name and output the executable to that name.
• Works well for very simple programs that exist in one file.

Change the output of the executable with the -o flag

$ g++ -o main main.cpp
$ ls
functions.cpp	functions.h	main		main.cpp
$ ./main
Hello CS 32!

• Changes the executable name from a.out to main

C++ Build Process

1. Preprocessor: Text-based program that runs before the compilation step. Looks for statements such as #include and modifies the source which is the input for compilation.

2. Compiler: A program that translates source code into “object code,” which is a lower-level representation optimized for executing instructions on the specific platform. Lower level representations are usually stored in a .o (object) file.

3. Linker: Resolves dependencies and maps appropriate functions located in various object files. The output of the linker is an executable file for the specific platform.

Compiling multiple files example

--------
// functions.h

int doubleInt(int x);
--------
// functions.cpp

int doubleInt(int x) {
	return 2 * x;
}
--------
// main.cpp
#include <iostream>
#include “functions.h”

using namespace std;

int main() {
	cout << doubleInt(12) << endl;
return 0;
}
--------

• We can’t just compile main.cpp anymore…

  $ g++ main.cpp
  Undefined symbols for architecture x86_64:
  "doubleInt(int)", referenced from:
      _main in main-7c4a04.o
  ld: symbol(s) not found for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  

• we see a “linker” error.
• The linker doesn’t know where to find the doubleInt() function definition.
• We need to compile functions.cpp so main.cpp knows where to find the doubleInt() function.

  $ g++ -o main main.cpp functions.cpp
  $ ls
  functions.cpp	functions.h	main		main.cpp
  $ ./main
  24
  

• Makefiles are useful since writing commands with many files is tedious and error-prone.
• Makefiles are used to define compilation rules and dependencies so we can simply type make [some_target] and not have to type everything out all the time.

General format of a Makefile

[target]: [dependencies]
	[commands]

Example:
--------
# Makefile

main: functions.cpp main.cpp
	g++ -o main main.cpp functions.cpp
--------

$ make main
g++ -o main main.cpp functions.cpp
$ make main
make: `main' is up to date.

• In this case, make uses timestamps to determine if something needs to be recompiled.
• If a recent change occurred, then it will recompile. If nothing changes, then nothing is done since everything is up to date

Note:

• The object (.o) files are not explicitly generated.
• You will need to use the -c (compile only) flag.
• Having make use .o files as a dependency is useful for maintenance reasons.
  • Only recompiles source files that have changed.

# Makefile

main: functions.o main.o
	g++ -o main main.o functions.o

$ make main
c++    -c -o functions.o functions.cpp
c++    -c -o main.o main.cpp
g++ -o main main.o functions.o
$ ls
Makefile	functions.h	main		main.o
functions.cpp	functions.o	main.cpp

Variables in Makefiles

# Makefile
#CXX=clang++
CXX=g++
DEPENDENCIES=functions.o main.o

main: ${DEPENDENCIES}
	${CXX} -o main ${DEPENDENCIES}

make clean

• Useful when trying to remove the executable and all .o files so everything can be recompiled from scratch.

--------
# Makefile
main: functions.o main.o
	g++ -o main main.o functions.o

clean:
	rm -f *.o main
--------

• If we want to “start fresh” and recompile everything, it’s nice to have a “clean” target to remove all object files and the executable

Compiling a specific version of C++

• We can specify the specific C++ version to use when compiling our programs with a special std flag.

main: ${DEPENDENCIES}
	${CXX} -std=c++11 -o main ${DEPENDENCIES}
clean:
	rm -f *.o main

Using $@ and $^

• Obtaining the target and dependencies is a common pattern when writing Makefile rules.
• We can use $@ and $^ to obtain the target and dependencies respectively.
  • $@ - target
  • $^ - dependencies
• Example:

main: ${DEPENDENCIES}
	${CXX} -std=c++11 -o $@ $^

# Expands out to
# main: functions.o main.o
	g++ -std=c++11 -o main functions.o main.o