format! macro in no_std, no alloc environment

[ewrogers]

I feel like a bit of a Rust newbie asking something so trivial, but I am trying to figure out a way to do sprintf(...) style string formatting similar to the format!() macro.

Every example of this uses println!() or some other macro that does not rely on allocation and can just write data to some other stream behind the scenes. In my case I need to display a formatted string to a ssd1309 LCD using the embedded-graphics crate and Text rendering.

I thought a library like ufmt + str-buf might solve the issue but I can't get it to compile for the life of me. I considered writing my own buffer/formatter but it felt like I was re-inventing a wheel that certainly had to exist already.

What is the general convention for needing to do string formatting in a no_std environment that may not have an allocator available to it. I know I could bring in esp-idf-hal but I am trying to keep it as lightweight as possible.

[jessebraham]

I'm not sure this is possible without allocations; there may be some way but I'm not familiar with any off the top of my head.

We do provide the esp-alloc crate which will allow you to perform allocations using esp-hal and only requiring core + alloc. This gives you access to collections, strings, etc. and in turn the format! macro.

The format_args! macro is available in core and in some instances can be used instead (IIRC format! just ends up calling format_args! anyway) however this macro returns the type core::fmt::Arguments, not a String or &str. Some APIs are able to consume this type, however.

[ewrogers]

Thanks for that tip! I thought I had to bring in the entire esp-idf-hal to get alloc support, I may end up going this route just getting esp-alloc. That should allow me to use ufmt or other similar libraries.

[bjoernQ]

Another option would be the Heapless crate

[ewrogers]

Awesome suggestion, I think this is what I will go with. They even have a ufmt-impl feature that lets me use ufmt::writeln! so that should be sufficient. Appreciate the help!