Added description and examples for gcc documentation

_commands/basics/gcc.md

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+
+**gcc**
+---
+
+
+`gcc` is a C and C++ compiler for UNIX systems developed by GNU project.
+
+```
+$ gcc -o testfile testfile.c
+# complies the code in testfile.c into an exectuable called "testfile"
+$ chmod a+x testfile
+# change the permission to executable file mode
+$ ./testfile
+# executes the program "testfile", the resulting ouput of the program
+# if any, will show right under this command.
+```
+
+
+**Useful Options / Examples**
+
+  ### `gcc -Wall`
+
+enables all the complier's warning messages.
+
+```
+# imagine you have the following code named myfile.c
+// myfile.c
+#include <stdio.h>
+
+int main()
+{
+    printf("Program run!\n");
+    int i=10;
+}
+```
+If this code is complied as shown in the first example above, then we 
+perform: `$ gcc -o myfile myfile.c` and the resulting output is `$` (nothing). However with the `-Wall` flag, we obtain the informative error messages:
+```
+$ gcc -Wall -o myfile myfile.c
+myfile.c In function 'main':
+myfile.c:6:6: warning: unused variable 'i'
+myfile.c:7:1: warning: control reaches end of non-void function
+$
+```
+
+
+### `gcc -I`
+
+allows user to include directory of header files.
+
+Imagine you have the following code called "myfile.c" and note that it includes a header file named "myheader.h" located in the directory /project/source :
+
+```
+// myfile.c
+#include <stdio.h>
+#include "myheader.h"
+
+void main()
+{
+    int num = NUM1;
+    printf("num=%d\n", num);
+}
+```
+If complied normally without the `-I` flag, the result is:
+```
+$ gcc -o myfile myfile.c
+myfile.c:2:22: fatal error: myheader.h: No such file or directory
+compilation terminated.
+$
+```
+Now, using the `-I` flag and including the directory that "myheader.h" is located in, we obtain successful compilation:
+```
+$ gcc -Iproject/source -o myfile myfile.c
+$ ./myfile
+num=5
+$
+```
+
+
+### `gcc -O`
+
+sets the complier's optimization level. The resulting flags and optimizations are listed below:
+
+- `-O0` (default option) performs optimization for compile time.
+
+- `-O1` optimization for code size and execution time.
+
+- `-O2` further optimization for code size and execution time.
+
+- `-O3` highest optimization for code size and execution time.
+
+- `-Os` optimization for code size.
+
+usage example: `$ gcc -O0 myfile.c -o myfile`
+
+
+### `gcc -g`
+
+generate debug information to be used GDB debugger. The resulting flags and debug info are listed below:
+
+- `-g0` no debug information
+
+- `-g1` minimal debug information
+
+- `-g` default debug information
+
+- `-g3` maximum debug information
+
+imagine we have a file named "myfile.c" with the following code:
+```
+#include <stdio.h>
+
+void main()
+{
+    printf("Program run!!\n");
+}
+```
+
+ If we perform the command `$ gcc -o -g myfile myfile.c` we can then perform the GDB debugger on the file "myfile" as:
+
+ ```
+ $ gdb myfile
+(gdb) run
+Starting program: /home/ubuntu/myfile
+Program run!!
+Program exited with code 012.
+(gdb) quit
+$
+```
+
+
+### `gcc -D`
+
+Define a macro to be used by the preprocessor.
+
+Imagine you have the file "myfile.c" containing the following code:
+
+```
+#include <stdio.h>
+
+void main()
+{
+    #ifdef DEBUG    
+       printf("Debug run\n");
+    #else
+       printf("Release run\n");
+    #endif
+}
+```
+
+if the code is compiled without the `-D` flag, the output is:
+
+```
+$ gcc -o myfile myfile.c
+$ ./myfile
+Release run
+$
+```
+
+However, if we include the `-D` flag, we obtain the desired output:
+
+```
+$ gcc -D DEBUG -o myfile myfile.c
+$ ./myfile
+Debug run
+$
+```
+
+### `gcc -o`
+
+writes the build output to an output file.
+
+For example, if you have the file named "myfile.c" containing the following code:
+
+```
+#include <stdio.h>
+
+void main()
+{
+    printf("Program run\n");
+}
+```
+
+if you want to write the build output to a file named "myfile" and run it, we can perform this as shown below:
+
+```
+$ gcc -o myfile myfile.c
+$ ./myfile
+Program run
+$
+```
+
+
+