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Compiling multiple files

It is usually very simple to compile a program that has been divided across multiple source files. Instead of typing

gcc -o executable sourcefile.c

you would type

gcc -o executable sourcefile_1.c sourcefile_2.c ... sourcefile_n.c

For example, if you were building a simple database program called mydb, the command line might look something like this:

gcc -o mydb main.c keyboard_io.c db_access.c sorting.c

Of course, if (say) db_access.c were lengthy, it might take a long time to compile your program every time you executed this command, even if you only made a small change in one of the other files. To avoid this, you might want to compile each of the source files into its own object file, then link them together to make your program. If you did, each time you made a small change in one file, you need only recompile that single file and then link the object files together again, potentially a great savings in time and patience. Here is how to generate a permanent object file for db_access.c.

gcc -c db_access.c

This would generate a permanent object code file called db_access.o, indicated by the suffix .o. You would perform this step when needed for each of the source code files, then link them together with the following command line:

gcc -o mydb main.o keyboard_io.o db_access.o sorting.o

You might even put the various commands into a shell file, so that you wouldn't need to type them repeatedly. For example, you could put the last command line into a shell file called build, so that all you would need to do to build your executable from object code files is type the following line.

./build

For programs on a very small scale, this approach works quite well. If your project grows even slightly complex, however, you will have a hard time keeping track of which object files are "fresh" and which need to be recreated because the corresponding source files have been changed since their last compilation. That's where the GNU utility make comes in. (See Writing a makefile.)