What happens when you type gcc main.c

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.