UNIX for Dummies Questions & Answers

Hello,

I'm a recent convert to UNIX and I'm attempting to understand exactly how the make utility is working under the hood.

Now, I understand that each rule has a target, dependencies, and update command, but the thing I'm confused about is exactly how the utility is determining when to build a target. From the man page, I thought I deduced that the only times that a target is updated is when the target does not exist in the file system or one of the dependencies is newer than the target file.

However, what I'm confused about is whether the utility does something different for the primary target being built (i.e. the target specified by the user at the command line or if no target specified, the first one in the makefile)

For example, if I have a makefile with the following:
-------------------------------------------------

Code:

test: dep1.o dep2.o
[tab]commands to update test here

dep1.o: dep1.c
[tab]commands to update dep1.o here

dep2.o: dep2.c
[tab]commands to update dep2.o here

-------------------------------------------------

If I create dummy files using cat to produce test, dep1.o, and dep2.o, making sure to create test after dep1.o and dep2.o, make still appears to go through and build each dependency. Does it just unconditionally build the primary target every time (ignoring timestamp comparison for it), and then for all dependency rules that were in test, it goes through and does the timestamp comparisons for those?

Any clarifications on this would be super helpful, thanks!

In this makefile, test has dependencies on dep1.o and dep2.o, dep1.o has a dependency on dep1.c, and dep2.o has a dependency on dep2.c. If your commands to update dep1.o and dep2.o don't create dep1.c and dep2.c, respectively, before updating dep1.o and dep2.o then dep1.o and dep2.o will never be considered up-to-date and will have to be rebuilt every time you check to see if test is up-to-date.

Normally, for something like this you would start out with dep1.c and dep2.c and make would use something like cc -c dep1.c to create dep1.o, cc -c dep2.c to create dep2.o, and the commands to update test would then link dep1.o, dep2.o, and whatever startup code is needed to produce test. Then subsequent invocations of make wouldn't need to do anything until you updated one of your C source files.