CODEWITHSUNDEEP
makefilebuildgnu-makebuild-automation
Zeta.Investigator
Zeta.Investigator

Reputation: 983

How GNU make manages file version control?

I'm learning GNU make . Suppose I have a hello_world.c file and a Makefile:
hello_world.c:

#include <stdio.h>                        

int main() {                              
        printf("Hello World!\n");  
        return 0;                         
}

Makefile:

hello: hello_world.c
        gcc hello_world.c -o hello_world

Now, I think hello is my target and hello_world.c is its dependency. If make somehow detects that hello_world.c is newer than its object-file, it executes the corresponding command.

1- How does make manage file version control and how does it detect something is newer than something else and needs updating?
2- If I change hello_world.o using an editor and corrupt the file, it obviously does not execute but make hello reports that nothing needs to be done! I mean, make only checked that the dependency is older than the target and exited doing nothing. I think it should have detected that this target is not the one corresponding to its latest invocation. Somehow, it should have compared the "combination of dependency AND target" history instead of just comparing dependency's history w.r.t. target.
3- Is this the limitation of make? How can I circumvent this issue? Because there maybe some external apps messing with the target of my make operation.

Upvotes: 0

Views: 700

Answers (3)

R Sahu
R Sahu

Reputation: 206667

1- How does make manage file version control and how does it detect something is newer than something else and needs updating?

You already know the answer to this question. You said it in your next paragraph: "I mean, make only checked that the dependency is older than the target and exited doing nothing." That's right. make updates dependent targets when the dependencies of targets are newer.

2- If I change hello_world.o using an editor and corrupt the file, it obviously does not execute but make hello reports that nothing needs to be done! I mean, make only checked that the dependency is older than the target and exited doing nothing. I think it should have detected that this target is not the one corresponding to its latest invocation. Somehow, it should have compared the "combination of dependency AND target" history instead of just comparing dependency's history w.r.t. target.

You are asking make to do a lot more than what it was intended to do.

3- Is this the limitation of make? How can I circumvent this issue? Because there maybe some external apps messing with the target of my make operation.

From your point of view, it does seem like it is a limitation of make. However, I want to point out that you are sabotaging the workings of make by updating a target manually.

How can you circumvent it?

  1. Don't manually modify targets that are built by make.

  2. Manually update the time stamp of one of the dependencies and then run make. You can use the command touch for that.

  3. Provide a dummy target named clean that will remove all the dependent targets. Then, run make clean followed by make.

  4. Provide a dummy target named rebuild. Forcefully build whatever you need to build in that target. Then, run make rebuild.

Upvotes: 1

Gerhardh
Gerhardh

Reputation: 12404

How does make manage file version control and how does it detect something is newer than something else and needs updating?

Very simple: make does not care about file versions.

Detecting if something is newer than something else is simply done by comparing time stamps from file system.

If I change hello_world.o using an editor and corrupt the file, it obviously does not execute but make hello reports that nothing needs to be done! I mean, make only checked that the dependency is older than the target and exited doing nothing. I think it should have detected that this target is not the one corresponding to its latest invocation. Somehow, it should have compared the "combination of dependency AND target" history instead of just comparing dependency's history w.r.t. target.

You did not tell make anything about your .o file. Why should make check the timestamp of that file? Checking the dependency is exactly what make is expected to do. How should this tool possibly know that a file calles hello_world.o is involved in the process if you don't tell it? There is no magic happening but only the rules of your Makefile are followed.

Is this the limitation of make? How can I circumvent this issue? Because there maybe some external apps messing with the target of my make operation.

You can specify hierarchical dependencies:

all: hello

hello: hello_world.o
    <gcc linker command...>

hello_world.o: hello_world.c
    gcc hello_world.c -o hello_world

Upvotes: 1

Some programmer dude
Some programmer dude

Reputation: 409346

The make program simply compare the modification time-stamps of the target (hello) and the dependency (hello_world.c) files.

If the time-stamp of a dependency file is newer than the targets, then it execute the commands.

Upvotes: 1

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