Many GDB warnings "Internal error: pc 0x100 in read in CU, but not in symtab"

[iainism]

I have followed the set-up and all the suggestions in #27 however my GDB console is filled with warning messages, e.g.

$ rust-gdb ../../../target/thumbv7em-none-eabihf/debug/examples/init
GNU gdb (Ubuntu 15.1-1ubuntu1~24.04.1) 15.1
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Reading symbols from ../../../target/thumbv7em-none-eabihf/debug/examples/init...
(gdb) target remote :1337
Remote debugging using :1337
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
0x00000100 in cortex_m_rt::{impl#1}::fmt (self=<error reading variable: Cannot access memory at address 0x20020040>, 
    f=<error reading variable: Cannot access memory at address 0x20020044>)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
(gdb) monitor reset halt
Resetting and halting target
Target halted
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
(gdb) b main
Breakpoint 1 at 0x178: file mdbook/src/05-meet-your-software/examples/init.rs, line 10.
Note: automatically using hardware breakpoints for read-only addresses.
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)
warning: (Internal error: pc 0x100 in read in CU, but not in symtab.)

I have run cargo install --locked probe-rs-tools and I have RUST_GDB=gdb-multiarch in my environment, so I'm not sure what else I may need to try.

Versions:

$ rustup --version
rustup 1.29.0 (28d1352db 2026-03-05)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: the currently active `rustc` version is `rustc 1.96.0 (ac68faa20 2026-05-25)`

$ probe-rs --version
probe-rs 0.31.0 (git commit: crates.io)

$ cargo --version
cargo 1.96.0 (30a34c682 2026-05-25)

$ rust-gdb --version
GNU gdb (Ubuntu 15.1-1ubuntu1~24.04.1) 15.1
Copyright (C) 2024 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

[BartMassey]

This is normal on some systems: things should nonetheless work as described. Are you seeing symptoms beyond these messages?

[BartMassey]

To be more clear: once you start the program running, these messages should go away. They are from the fact that the program initially stops in memory the debugger doesn't know about.

[iainism]

Hi, I also have a smattering of SIGINT messages throughout the GDB interface, e.g.

(gdb) next

Program received signal SIGINT, Interrupt.
init::__cortex_m_rt_main () at mdbook/src/05-meet-your-software/examples/init.rs:18

Would it be a bad idea to add a small note about 'possible/expected' warnings to Chapter 5?

[BartMassey]

@iainism Not sure what that is: maybe trying to step over a breakpoint? In general, it would be great to document this stuff, especially if we can narrow down the specific causes so that we can be specific about what to expect.

[iainism]

Sorry for the delayed follow-up, I've just come back to this, and now I can't even connect via rust-gdb after running cargo embed [...] ... it looks as though I have some serious debugging to do.