Specifying dependency between files: makefiles

Prelude to programming project management: time stamps of files

Fact:

Each file has a modification timestamp

Use:

ls -l -rwxr-xr-x 1 cheung staff 6492 Nov 14 19:39 hello* -rw-r--r-- 1 cheung staff 92 Nov 14 19:39 hello.c -rw-r--r-- 1 cheung staff 708 Nov 14 19:39 hello.o

to see the modification time

Make: a UNIX utility to help you manage "building" your project files

The UNIX make utility:

The make application in UNIX can use the time stamps in files to determine the last modification time on a file
A special file called a "Make file" contains:
The make utility will use the file dependencies and the file time stamps to determine which files are out-of-date

The "Make file"

"Make file":

The name of a Make file must be equal to:

makefile or: Makefile

The Make file contains:

the dependency relationship between files, and
the command(s) used to update a target file when it's depend-on file(s) is/are updated
(I call these command: maintenance commands)

Dependency between files

Dependency between files

Let's look at a simple example where we compile a single-file C program using the multi-files C program compilation method

Comment:

A C project will normally consists of multiple C program files, so we should use the multi-files C program compilation method to build the project.
To illustrate the Make utility in UNIX, I will use a single-file C program, but compile it with the multi-files C program compilation method

Suppose we have a C program that consists of one single program file:

File: "hello.c" #include <stdio.h> int main( int argc, char* argv[] ) { printf( "Hello World !\n" ); }

We compile the input file hello.c using the following commands:

Command: gcc -c hello.c Result: hello.c ----> hello.o Command: gcc -o hello hello.o Result: hello.o ----> hello (executable command in UNIX)

Therefore:

hello.o depends on (= is produced using) hello.c hello depends on (= is produced using) hello.o

Summary of the file dependency and the commands used to update the target files:

Targets: --------- hello depends on hello.o Command to update hello when hello.o is changed: gcc -o hello hello.o hello.o depends on hello.c Command to update hello.o when hello.c is changed: gcc -c hello.c

How to use dependency relationships between files to maintain a C project

Important facts concluded from the file dependency relationships:

When a file hello.o depends on (= is prodcued using) another file hello.c, then:

The file hello.o can only be produced when the file hello.c exists !!!
Therefore:
If the file hello.c is updated, we must re-produce the file hello.o !!!

Example:

When we update (= edit) the (source file) hello.c, then we must update (= re-compile !) the object file hello.o
We do this by running the compile command:
which will create (= generate) the object file hello.o