GCC tutorial

Quickie Intro to GCC

gcc is your friendly Gnu C compiler and g++ is the C++ compiler. This document serves as a very simple bunch of hints to start using gcc, and is not meant to be clear or concise : experiment yourself to figure it out (no fun (especially for me) if I tell you *everything*).

Compiling simple programs

Say you have a file hello.c as follows :

#include 

void main ()
{
    printf("hello");
}

You can compile and run it from the unix prompt as follows :

	% gcc hello.c

This creates an executable called "a.out". You can run it by typing

	% ./a.out

at the prompt. a.out is the default name for an executable. Use the "-o" option to change the name :

	% gcc -o hello hello.c

creates an executable called "hello".

Include Directories

Sometimes the header files that you write are not in the same directory as the .c file that #include's it. E.g. you have a structure in a file "foo.h" that resides in /homes/me/include. If I want to include that file in hello.c I can do the following :

add
```
 #include < foo.h >
```
to hello.c

compile with the -I option :

	% gcc -o hello hello.c -I/homes/me/include

This basically tells gcc to look for #include's in /homes/me/include in addition to other directories you specify with -I

Other options

Warnings : -Wall turns on most warnings. man gcc for more options.
Debugging : if you intend to use the debugger (gdb), also use the -g option
Optimizations : -O or -O2

Compiling multiple files

Basic idea : compile each .c file into a .o file, then compile the .o files (along with any libraries) into an executable. One of these .c files obviously has to define the main function, or else the linker will gag.

E.g. Suppose we have main.c, foo.c and bar.c and want to create an executable fubar, and suppose further that we need the math library:

	% gcc -c foo.c
	% gcc -c main.c
	% gcc -c bar.c
	% gcc -o fubar foo.o main.o bar.o -lm

The first three commands generate foo.o, main.o and bar.o respectively. The last line links them together along with the math library.

Libraries

Specify library paths with the -L option. e.g. -L/usr/lib tells gcc to look for libraries in /usr/lib. As with includes, you can add as many -L's as necessary. Specify which libraries to link with -l

Emacs Support

Tired of finding the line an error occurred on when compiling your C code? As with everything else in UNIX, there is good support for compiling and fixing errors in Emacs. To use this, simply type meta-x compile in Emacs. Then enter the gcc or make commandline you would normally use. Emacs will create a new window and execute the command in that window. When there are compilation errors, Emacs can help you go straight to them. In Emacs under X, middle-click on the error message. In Emacs under text mode, type ctrl-x ` (ctrl-x backquote) to go to successive erroneous lines.

cse451-TA@cs.washington.edu