http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter4.html
On the UNIX system, for
example, the cc command mentioned in Chapter 1 does the job. Suppose that the three functions are stored in three files
called main.c , getline.c , and strindex.c . Then
the command cc main.c getline.c strindex.c compiles the three files, placing the resulting object code in files main.o , getline.o , and strindex.o , then loads them all into an executable file called a.out .