How to print variables using writeln!(Console::writer(), ... )

[potto216]

How do you print variables using writeln!(Console::writer(), ... )?
I can successfully print string arguments with writeln!(Console::writer(), "This arg is: {}", "a string").unwrap();
But the app crashes when I try

let val1: u8 = 0x41; //letter 'A'
writeln!(Console::writer(), "This arg is u8:  {}", val1).unwrap();

With the error:

panicked at 'Process console had a fault', kernel/src/process_standard.rs:412:17
        Kernel version release-2.1-524-g053b90b48

As an aside, the purpose for printing variables is to debug code to access the UDP capsule. If there is a better way to display debug data I would appreciate knowing that. Thanks.

[hudson-ayers]

I transferred this discussion to the libtock-rs repo since it is a question about libtock-rs specifically.

Can you post the full panic print? The example code you shared is intended to work.

[potto216]

I restored the kernel to the original on master and simplified console to be

fn main() {
    let val1: u8 = 0x41; //letter 'A'
    writeln!(Console::writer(), "This arg is u8:  {}", val1).unwrap();
}

After connecting to the board I pressed the reset button on the board since the app had already crashed. It then started and crashed again. The output is below. BTW I have also tried different argument types, and only writeln!(Console::writer(), "This arg is: {}", "a string").unwrap(); works.

Which option? [0] 1
[INFO   ] Using "/dev/ttyACM0 - J-Link - CDC".
[WARNING] Found stale tockloader server, removing.
[WARNING] This may occur if a previous tockloader instance crashed.
[INFO   ] Listening for serial output.
␀Initialization complete. Entering main loop
NRF52 HW INFO: Variant: AAD0, Part: N52840, Package: QI, Ram: K256, Flash: K1024
This arg is u8:  
panicked at 'Process console had a fault', kernel/src/process_standard.rs:412:17
        Kernel version release-2.1-524-g053b90b48

---| No debug queue found. You can set it with the DebugQueue component.

---| Cortex-M Fault Status |---
Data Access Violation:              true
Memory Management Stacking Fault:   true
Instruction Bus Error:              true
Precise Data Bus Error:             true
Forced Hard Fault:                  true
Faulting Memory Address:            0x20007FF8
Bus Fault Address:                  0x20007FF8
Fault Status Register (CFSR):       0x00008392
Hard Fault Status Register (HFSR):  0x40000000

---| App Status |---
𝐀𝐩𝐩: console   -   [Faulted]
 Events Queued: 0   Syscall Count: 9   Dropped Upcall Count: 0
 Restart Count: 0
 Last Syscall: Subscribe { driver_number: 1, subdriver_number: 1, upcall_ptr: 0x0, appdata: 0 }
 Completion Code: None


 ╔═══════════╤══════════════════════════════════════════╗
 ║  Address  │ Region Name    Used | Allocated (bytes)  ║
 ╚0x20009000═╪══════════════════════════════════════════╝
             │ Grant Ptrs      112
             │ Upcalls         320
             │ Process         920
  0x20008AB8 ┼───────────────────────────────────────────
             │ ▼ Grant          76
  0x20008A6C ┼───────────────────────────────────────────
             │ Unused
  0x20008100 ┼───────────────────────────────────────────
             │ ▲ Heap            0 |   2412               S
  0x20008100 ┼─────────────────────────────────────────── R
             │ Data              0 |      0               A
  0x20008100 ┼─────────────────────────────────────────── M
             │ ▼ Stack         296 |    256 EXCEEDED!
  0x20007FD8 ┼───────────────────────────────────────────
             │ Unused
  0x20008000 ┴───────────────────────────────────────────
             .....
  0x00032000 ┬─────────────────────────────────────────── F
             │ App Flash      8096                        L
  0x00030060 ┼─────────────────────────────────────────── A
             │ Protected        96                        S
  0x00030000 ┴─────────────────────────────────────────── H

  R0 : 0xBAD00BAD    R6 : 0x20008078
  R1 : 0xBAD00BAD    R7 : 0x20008000
  R2 : 0xBAD00BAD    R8 : 0x20008078
  R3 : 0xBAD00BAD    R10: 0x00000000
  R4 : 0x07200190    R11: 0x200080D0
  R5 : 0x00000000    R12: 0xBAD00BAD
  R9 : 0x00000002 (Static Base Register)
  SP : 0x20007FD8 (Process Stack Pointer)
  LR : 0xBAD00BAD
  PC : 0xBAD00BAD
 YPC : 0x000302A2

 APSR: N 1 Z 0 C 1 V 1 Q 1
       GE 0 0 0 0
 EPSR: ICI.IT 0x42
       ThumbBit false !!ERROR - Cortex M Thumb only!

 Total number of grant regions defined: 14
  Grant  0 : --          Grant  5 : --          Grant 10 : --        
  Grant  1 : --          Grant  6 : --          Grant 11 : --        
  Grant  2 : --          Grant  7 : --          Grant 12 : --        
  Grant  3 0x1: 0x20008a6c  Grant  8 : --          Grant 13 : --        
  Grant  4 : --          Grant  9 : --        

 Cortex-M MPU
  Region 0: [0x20008000:0x20008800], length: 2048 bytes; ReadWrite (0x3)
    Sub-region 0: [0x20008000:0x20008100], Enabled
    Sub-region 1: [0x20008100:0x20008200], Disabled
    Sub-region 2: [0x20008200:0x20008300], Disabled
    Sub-region 3: [0x20008300:0x20008400], Disabled
    Sub-region 4: [0x20008400:0x20008500], Disabled
    Sub-region 5: [0x20008500:0x20008600], Disabled
    Sub-region 6: [0x20008600:0x20008700], Disabled
    Sub-region 7: [0x20008700:0x20008800], Disabled
  Region 1: Unused
  Region 2: [0x00030000:0x00032000], length: 8192 bytes; UnprivilegedReadOnly (0x2)
    Sub-region 0: [0x00030000:0x00030400], Enabled
    Sub-region 1: [0x00030400:0x00030800], Enabled
    Sub-region 2: [0x00030800:0x00030C00], Enabled
    Sub-region 3: [0x00030C00:0x00031000], Enabled
    Sub-region 4: [0x00031000:0x00031400], Enabled
    Sub-region 5: [0x00031400:0x00031800], Enabled
    Sub-region 6: [0x00031800:0x00031C00], Enabled
    Sub-region 7: [0x00031C00:0x00032000], Enabled
  Region 3: Unused
  Region 4: Unused
  Region 5: Unused
  Region 6: Unused
  Region 7: Unused

To debug, run `make lst` in the app's folder
and open the arch.0x30060.0x20008000.lst file.

I tried the suggestion make lst but I could not get it to work and could not find a makefile with the lst target. Also I found the file FaultDebuggingExample.md which describes a couple debugging techniques. I can try these, but they seem to low level for this problem.

[potto216]

I just noticed in the above log
│ ▼ Stack 296 | 256 EXCEEDED!
So I adjusted the console app stack size to
stack_size! {0x200}
And I got the expected output of
This arg is u8: 65

[hudson-ayers]

Great, glad you got it working! The default stack sizes for the examples are very conservative so I am not too surprised that a small change like this exceeded it.

Regarding your question about make lst, that actually only works for libtock-c apps -- we should update the panic printout to reflect that