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It's cool to see things like interrupt.memfault.com/blog/trice (and zephyr's version) becoming common for C folks as well. As far as I can tell, the "deferred logging revolution" seems to have all started with

@FerrousSystems

' #knurling defmt tool, but there's always SOME prior art :)

interrupt.memfault.com

Tracing & Logging with the TRICE Library (Interrupts too!)

Best practices on embedded logging and tracing and an introduction to the open-source library Trice

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Cool! I went through a similar process. Eventually decided that adding complexity of an extra build step was not worth it so I started converting the performance sensitive statements to binary logs.

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I do wonder though. Is this an actual parser, or a regexp hack?

James "Undocumented Errata" Munns

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No idea how the TRACE library works. defmt works by placing the interned strings into an unused linker section, and decodes it automatically when you give it an ELF file and a stream of logs.

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Oh nice. I tried something like that for C but couldn't figure it out how to do it for const strings.

James "Undocumented Errata" Munns

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At least in Rust, with the benefit of macros, defmt is pretty well documented, even in impl details: defmt.ferrous-systems.com/interning.html

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Thanks. The issue I had is how to annotate a const string without first binding it to a variable like answer here:

stackoverflow.com

How to put c-style strings (function arguments) in the specific section?

I want to put some functions in a specific section named ".xip.text", but the read only data cannot put in that section. The following is my test code: #include void foo(void)

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But it might be possible to wrap it with a C expression statement in a macro, do a variable binding in that expression statement and add the annotation.

2:00 PM · Jan 13, 2022Twitter Web App

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I mean statement expression... gcc.gnu.org/onlinedocs/gcc

James "Undocumented Errata" Munns

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Yeah, something like that might work. I think defmt relies on the linker to gc all the strings out, after creating it as a static string in the specific section. I think the defmt section is expressly excluded from the final binary.

James "Undocumented Errata" Munns

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(Rust procedural macros are also much more powerful, basically compiler plugins, so doing shenanigans like generating random identifiers for each string and encoding as JSON is much easier)

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#define STR(str) ({ __attribute__((section(".fmtstring"))) static const char _str[] = str; _str;}) Then wrap every literal string with STR() macro.