Use compile-time parameter inside program

Embedded
1

I'd like to pass the compile time as a parameter into my embedded program. How can I achieve that?
I think about running the program with something like cargo run -- "2024-04-21T10:06:20Z" to set the internal clock of the target to this timestamp.
But can be any parameter, it's not (yet) so much about the time stamp parsing.

Thanks a lot!

2

You can use a build script for this.

Cargo.toml:

[build-dependencies]
chrono = { version = "0.4", default-features = false, features = ["now"] }

build.rs:

fn main() {
    println!("cargo::rustc-env=BUILD_TIME={}", chrono::Utc::now());
}

main.rs:

fn main() {
    println!("Application built on {}", env!("BUILD_TIME"));
}
2 Likes
3

Great, thank You very much!

Is there a alternative way via cargo command line? Because I might want by to choose a different timestamp than "now", or also choose not to set the time at all (by leaving out the argument). I wanted to avoid to change a source or config file for this.

4

Set the environment variable on the command line and use the same env! macro to statically include it into the executable.

5

Yes, this is where I stumble upon:
With
BUILD_TIME="2024-04-21T10:06:20Z";cargo run
I get
environment variable 'BUILD_TIME' not defined at compile time
I think I must say, that I'm totally new to rust, so I'm not sure how to set an environment variable.

6

Remove the semicolon or export the var.

$ BUILD_TIME="2024-04-21T10:06:20Z" cargo run
   Compiling build-time v0.1.0 (C:\Users\jay\Desktop\build-time)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.32s
     Running `target\debug\build-time.exe`
Application built on 2024-04-21T10:06:20Z

$ export BUILD_TIME="2024-04-21T10:06:20Z"
$ cargo run
   Compiling build-time v0.1.0 (C:\Users\jay\Desktop\build-time)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.19s
     Running `target\debug\build-time.exe`
Application built on 2024-04-21T10:06:20Z

You will need a build script anyway to rebuild when the environment variable changes (but you can remove the [build-dependencies].

build.rs:

fn main() {
    println!("cargo::rerun-if-env-changed=BUILD_TIME");
}
2 Likes
7

Ahh, that's it. I was so close...
BUILD_TIME="2024-04-21T10:06:20Z" cargo run

Cargo rebuilds also without that line in build.rs.

Thank You very much!

1 Like
8

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