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I'm trying to run a Rust Hello World on my STM32F401xB. I have a working C program for comparison (different functionality, but it verifies the flashing toolchain) and use the same openocd commands to flash both programs onto the controller.

When I flash the C program, I get:

...
** Verified OK **
adapter speed: 2002 kHz

When I flash the Rust program, I get the following:

...
** Verified OK **
adapter speed: 2002 kHz
target halted due to breakpoint, current mode: Thread
xPSR: 0x21000000 pc: 0x08002f42 msp: 0x2000ffa0

note the two extra lines. When I connect using gdb and continue, I get the Hello, World! printout through semihosting, so the program does in principle execute fine - it just pauses before main even though I have not set a breakpoint. My procedure for both binaries is the same. What could be going on? Why are the two binaries behaving differently?

  • I used svd2rust to create a stm32f40x crate from the official stm32f401.svd
  • I used the cortex-m-quickstart and basically the hello.rs example - configuring memory.x and adding the dependency to stm32f40x of course
  • there are no special configurations (that I would have identified) in my project, and the same behavior happens for debug and release builds.
Silly Freak
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    Show the disassembly of the code at the breakpoint. It is possible that the code contains a direct beakpoint instruction in a conditional of the form `if debugger attached then break` Also [this](https://michaelwoerister.github.io/2013/09/27/what-you-call-the-present.html) may be relevant - specifically the part about changing the start-on-startup symbol from `main` to `helloworld::main` – Clifford Jul 07 '18 at 04:53
  • The `cortex-m-quickstart` uses `#![no_main]` and `entry!(func);` to define the entry function into the binary, so I don't think the problem is with a symbol name. But I'm sure you're right that I'll see a breakpoint in the disassembly; probably from semihosting as Turbo J wrote. Thanks! – Silly Freak Jul 09 '18 at 09:15

1 Answers1

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I get the Hello, World! printout through semihosting

Check the semihosting documentation. Last time I checked, semihosting works using breakpoints on several ARM targets.

Your C program probably does not use semihosting at all, so no breakpoints are used.

Turbo J
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  • That must have been it. After removing semihosting, the message disappears! Lucky this detail appeared in the question... – Silly Freak Jul 08 '18 at 13:57