small gcc introduction

GCC Intro/hello.c

@@ -0,0 +1,6 @@
+#include <stdio.h>
+ 
+int main() {
+    printf("Hello, world!\n");
+    return 0;
+}

GCC Intro/hello.cpp

@@ -0,0 +1,8 @@
+#include <iostream>
+using namespace std;
+
+int main()
+{
+    cout << "Hello, world!" << endl;
+    return 0;
+}

GCC Intro/makefile

@@ -0,0 +1,10 @@
+all: hello.exe
+
+hello.exe: hello.o
+	 gcc -o hello.exe hello.o
+
+hello.o: hello.c
+	 gcc -c hello.c
+     
+clean:
+	 rm hello.o hello.exe

GCC Intro/readme.md

@@ -0,0 +1,144 @@
+# GCC Intro
+The GNU C and C++ compiler are called gcc and g++, respectively.
+
+The GNU Toolchain, including GCC, is included in all Unixes. It is the standard compiler for most Unix-like operating systems. Test its installed with 
+```bash
+gcc --version
+```
+
+## Hello World - C
+The first program is a simple "Hello World" in C. To compile the hello.c:
+```bash
+gcc hello.c
+```
+This compiles and links source file hello.c into executable a. The default output executable is called 'a.out'. To run the program:
+```bash
+# grant executable rights
+chmod a+x a.out
+# run
+./a.out
+```
+To specify the output filename, use -o option:
+```bash
+gcc -o hello hello.c
+chmod a+x hello
+./hello
+```
+
+## Hello World - C++
+You need to use g++ to compile C++ program, as follows. We use the -o option to specify the output file name. It's very similar to gcc.
+```bash
+g++ -o hello hello.cpp
+chmod a+x hello
+./hello
+```
+
+There are a few more GCC Compiler Options. A few commonly-used GCC compiler options are:
+- o: specifies the output executable filename.
+- Wall: prints "all" Warning messages.
+- g: generates additional symbolic debuggging information for use with gdb debugger.
+
+## Compile multiple source files
+Suppose that your program has two source files: file1.cpp, file2.cpp. You could compile all of them in a single command:
+```bash
+g++ -o myprog.exe file1.cpp file2.cpp 
+```
+Usually compile each of the source files separately into object file, and link them together in the later stage. In this case, changes in one file does not require re-compilation of the other files.
+```bash
+g++ -c file1.cpp
+g++ -c file2.cpp
+g++ -o myprog.exe file1.o file2.o
+```
+
+## GNU Make
+The `make` utility automates the mundane aspects of building executable from source code. `make` uses a so-called makefile, which contains rules on how to build the executables.
+
+Simply create a file named `makefile` in the same directory as the source file, this file contains rules to build the exectable.
+To run the the `make` utility use
+```bash
+make
+```
+Running make without argument starts the target "all" in the makefile. 
+
+### Rulesets
+A makefile consists of a set of rules. A rule consists of 3 parts: a target, a list of pre-requisites and a command, like
+```bash
+target: pre-req-1 pre-req-2 ...
+	command
+```
+You can also specify the target to be made in the make command by using its targets name of the makefile
+```bash
+make clean
+```
+
+### Comments
+Makefiles can be commented by prevailing a line with `#` like
+```makefile
+# this is a comment
+target: requisites ...
+	command
+```
+
+### Variables
+Next to that you can use Variables for a more programatically way of building your projects. Variables begin with a `$` and are enclosed within parantheses or braces
+```makefile
+$(CC), $(CC_FLAGS), $@, $^.
+```
+Automatic variables are set by make after a rule is matched. There include:
+- `$@`: the target filename.
+- `$*`: the target filename without the file extension.
+- `$<`: the first prerequisite filename.
+- `$^`: the filenames of all the prerequisites, separated by spaces, discard duplicates.
+- `$+`: similar to $^, but includes duplicates.
+- `$?`: the names of all prerequisites that are newer than the target, separated by spaces.
+
+E.g. an advanced makefile could look like
+```makefile
+all: hello.exe
+ 
+# $@ matches the target; $< matches the first dependent
+hello.exe: hello.o
+	gcc -o $@ $<
+
+hello.o: hello.c
+	gcc -c $<
+     
+clean:
+	rm hello.o hello.exe
+```
+
+### Virtual Paths
+Use VPATH (uppercase) to specify the directory to search for dependencies and target files, e.g.
+```makefile
+# Search for dependencies and targets from "src" and "include" directories
+# directories are separated by space
+VPATH = src include
+```
+
+Use vpath (lowercase) to be more precise about the file type and its search directory, e.g.
+```makefile
+# Search for .c files in "src" directory; .h files in "include" directory
+# The pattern matching character '%' matches filename without the extension
+vpath %.c src
+vpath %.h include
+```
+
+### Pattern Rules
+A pattern rule, which uses pattern matching character `%` as the filename, can be applied to create a target, if there is no explicit rule. For example,
+```makefile
+# Applicable for create .o object file.
+# '%' matches filename.
+# $< is the first pre-requisite
+# $(COMPILE.c) consists of compiler name and compiler options
+# $(OUTPUT_OPTIONS) could be -o $@
+%.o: %.c
+	$(COMPILE.c) $(OUTPUT_OPTION) $<
+ 
+# Applicable for create executable (without extension) from object .o object file
+# $^ matches all the pre-requisites (no duplicates)
+%: %.o
+$(LINK.o) $^ $(LOADLIBES) $(LDLIBS) -o $@
+```
+
+
+Find more informations at [GNU GCC](http://gcc.gnu.org/)