Compiling and Linking
You may already be anxious to know how an executable file is made. Let's have a look at how a C program is compiled and translated into an executable file (binary code). As shown in Figure 2.1, there are at least three steps needed to create an executable file.
Figure 2.1. Make an executable file by the compiler and linker.
First, a program written in C, called source code, is made. Then the source code is compiled by a C compiler, which creates a new file. The new file is an object file. In the UNIX operating system, the name of an object file ends with the extension .o; in the DOS operating system, the extension is .obj.
You cannot execute the object file because there is some function code missing. You have to finish the next step: linking. Linking is done by invoking a special program called a linker, which normally comes with the compiler package.
A linker is used to link together the object file, the ANSI standard C library, and other user-generated libraries to produce an executable file—the binary code. In this stage, the binary code of the library functions that are called in the source code is combined with the object file; the result is saved into a new file—an executable file. As you learned in the first hour of this book, the name of an executable file usually ends with the extension .exe in DOS.
(.com is another extension used for a DOS executable filename.) In UNIX, it's not necessary to include such an extension to an executable filename.
Later, you'll learn that in many cases, there may be several object files that have to be linked together in order to make an executable program.
Note that the object file and executable file are both machine-dependent. You cannot simply move an executable file, without recompiling the source code, from the current computer platform to another one that is operated by a different operating system even though the source code of the executable file, presumably written in ANSI C, is machine independent (that is, portable).
What's Wrong with My Program?
When you finish writing a C program and start to compile it, you might get some error or warning messages. Don't panic when you see error messages. We're human beings. Everybody makes mistakes. Actually, you should appreciate that your compiler catches some errors for you before you go any further.
Usually, your compiler can help you check the grammar of your C program and make sure you've followed the C programming rules properly. For instance, if you forget to put the ending brace on the main() function in line 8 of Listing 2.1, you'll get an error message something like this: syntax error : end of file found.
Also, the linker will issue an error message if it cannot find the missing code for a needed function in the libraries. For instance, if you misspell printf() as pprintf() in the program of Listing 2.1, you'll see an error message: `_pprintf': unresolved external (or something similar).
All errors found by the compiler and linker must be fixed before an executable file (binary code) can be made.
- 14 views
