2.1 Compiling a simple C program
The classic example program for the C language is Hello World.
Here is the source code for our version of the program:
printf ("Hello, world!\n");
We will assume that the source code is stored in a file called 'hello.c'.
To compile the file 'hello.c' with
gcc, use the following
$ gcc -Wall hello.c -o hello
This compiles the source code in 'hello.c' to machine code and
stores it in an executable file 'hello'. The output file for the
machine code is specified using the
-o option. This option is
usually given as the last argument on the command line. If it is
omitted, the output is written to a default file called 'a.out'.
Note that if a file with the same name as the executable file already
exists in the current directory it will be overwritten.
-Wall turns on all the most commonly-used compiler
warnings---it is recommended that you always use this option!
There are many other warning options which will be discussed in later
-Wall is the most important. GCC will not
produce any warnings unless they are enabled. Compiler warnings are an
essential aid in detecting problems when programming in C and C++.
In this case, the compiler does not produce any warnings with the
-Wall option, since the program is completely valid. Source code
which does not produce any warnings is said to compile cleanly.
To run the program, type the path name of the executable like this:
This loads the executable file into memory and causes the CPU to begin
executing the instructions contained within it. The path
refers to the current directory, so
./hello loads and runs the
executable file 'hello' located in the current directory.