1. CSC 253
Lecture 6

2. Declarations vs. Definitions
Why do we separate declarations from definitions?
You can’t call a function unless it’s been previously declared; suppose f calls g and g calls f …
You can compile with the declarations even if the code has been previously compiled.
You might not want them to unless they bought a source license.
What is the rule for placing declarations and definitions?
What is “module-based programming” in C?
Dividing functionality of the program into set of paired .h and .c files.

3. Our int_math module Let’s define a file that contains two functions
pow(int base, int exp)
fact(int n)
The first function is essentially the same as we wrote in the 3rd week.
Let’s start with the header file.

4. Our int_math.h
What idiom do we need to use at the beginning of the file?
Let’s write the rest of the code …

5. Out int_math.c Let’s check parameters for validity …
Let’s code fact() recursively.
Suppose we compile int_math.c; what will happen?

6. Separate Compilation
We can compile int_math.c even without writing a main program.
We just use the -c switch on our call to gcc
This produces an object file that can be used later.

7. The Main Program Now let’s write the main program …
(exercise)
We can run the program by listing our source file, along with the precompiled object file.
We can also compile both files together …

8. Here’s the shell commands we invoked
# Suppose we compile int_math.c without a flag that says not to link ...
lec6% gcc int_math.c
Undefined first referenced
symbol in file
main /afs/bp.ncsu.edu/contrib/gcc281/lib/gcc-lib/sparc-sun-\
solaris2.8/2.8.1/crt1.o
ld: fatal: Symbol referencing errors. No output written to a.out
# To avoid this error, we need to use a -c flag on the compile; that means "compile-only"
lec6% gcc -c int_math.c
# Let's see what file this created for us ..
lec6% ls -lt | head -5
total 16
-rw-r--r efg ncsu Sep 21 11:17 int_math.o
-rw-r--r efg ncsu Sep 21 11:15 int_math.c
-rw-r--r efg ncsu Sep 21 11:03 int_math.h
# Suppose we now compile (only) our main program
lec6% gcc our_main.c
pow /var/tmp/cchBaadV1.o
fact /var/tmp/cchBaadV1.o
# Oh, we need to tell the compiler about the code we already compiled ...
lec6% gcc our_main.c int_math.o
lec6% a.out
7 to the 7th power is
7! = 5040
# Now lets "rm int_math.o" and see what happens ...
lec6% rm int_math.o

9. Here’s the last part of the commands …
lec6% gcc -pedantic-errors our_main.c int_math.o
gcc: int_math.o: No such file or directory
# So if the .o file isn't there, we need to compile the source again ...
lec6% gcc -pedantic-errors our_main.c int_math.c
lec6% a.out
7 to the 7th power is
7! = 5040
# Or, we can name the binary file that the compile produces ...
lec6% gcc -pedantic-errors -o our_main our_main.c int_math.c
lec6% our_main
lec6%