www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Why sometimes stacktraces are printed and sometimes not?

reply JN <666total wp.pl> writes:
What makes the difference on whether a crash stacktrace gets 
printed or not?

Sometimes I get a nice clean stacktrace with line numbers, 
sometimes all I get is "segmentation fault error -1265436346" 
(pseudo example) and I need to run under debugger to get the 
crash location.
Sep 29 2021
parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 9/29/21 6:57 AM, JN wrote:

 What makes the difference on whether a crash stacktrace gets printed or 
 not?
 
 Sometimes I get a nice clean stacktrace with line numbers, sometimes all 
 I get is "segmentation fault error -1265436346" (pseudo example) and I 
 need to run under debugger to get the crash location.


segmentation faults are memory access errors. It means you are accessing 
a memory address that is not valid for your application. If you are 
accessing the wrong memory, it means something is terribly wrong in your 
program.

Note that on Windows in 32-bit mode, I believe you get a stack trace. On 
Linux, there is the undocumented `etc.linux.memoryhandler` which allows 
you to register an error-throwing signal handler.

Signals are not really easy to deal with in terms of properly throwing 
an exception. This only works on Linux, so I don't know if it's possible 
to port to other OSes. I've also found sometimes that it doesn't work 
right, so I only enable it when I am debugging.

-Steve
Sep 29 2021
next sibling parent Steven Schveighoffer <schveiguy gmail.com> writes:
On 9/29/21 8:15 AM, Steven Schveighoffer wrote:

 On Linux, there is the undocumented `etc.linux.memoryhandler`


Sorry, it's `etc.linux.memoryerror`

Here is the code: 
https://github.com/dlang/druntime/blob/master/src/etc/linux/memoryerror.d

-Steve
Sep 29 2021
prev sibling next sibling parent bauss <jj_1337 live.dk> writes:
On Wednesday, 29 September 2021 at 12:15:30 UTC, Steven 
Schveighoffer wrote:

 On 9/29/21 6:57 AM, JN wrote:

 What makes the difference on whether a crash stacktrace gets 
 printed or not?
 
 Sometimes I get a nice clean stacktrace with line numbers, 
 sometimes all I get is "segmentation fault error -1265436346" 
 (pseudo example) and I need to run under debugger to get the 
 crash location.


 segmentation faults are memory access errors. It means you are 
 accessing a memory address that is not valid for your 
 application. If you are accessing the wrong memory, it means 
 something is terribly wrong in your program.

 Note that on Windows in 32-bit mode, I believe you get a stack 
 trace. On Linux, there is the undocumented 
 `etc.linux.memoryhandler` which allows you to register an 
 error-throwing signal handler.

 Signals are not really easy to deal with in terms of properly 
 throwing an exception. This only works on Linux, so I don't 
 know if it's possible to port to other OSes. I've also found 
 sometimes that it doesn't work right, so I only enable it when 
 I am debugging.

 -Steve


You might also mention that even if you had a stacktrace where 
the error happened, that's usually not where the error was 
caused. It's most likely a completely different place in the code.

The only time where you somewhat can be sure where it happens is 
when you try to access ex. a reference type that hasn't been 
instantiated.


violation, but you get a NullReferenceException.

It's not a concept D has, so it defaults to segfault/access 
violation.

Which means you're in really deep water when you encounter one 
because you have no idea what caused it and where it was caused.
Sep 30 2021
prev sibling parent wjoe <invalid example.com> writes:
On Wednesday, 29 September 2021 at 12:15:30 UTC, Steven 
Schveighoffer wrote:

 On 9/29/21 6:57 AM, JN wrote:

 What makes the difference on whether a crash stacktrace gets 
 printed or not?
 
 Sometimes I get a nice clean stacktrace with line numbers, 
 sometimes all I get is "segmentation fault error -1265436346" 
 (pseudo example) and I need to run under debugger to get the 
 crash location.


 segmentation faults are memory access errors. It means you are 
 accessing a memory address that is not valid for your 
 application. If you are accessing the wrong memory, it means 
 something is terribly wrong in your program.

 [...]


So on Linux, I don't know the behavior on other OSs, the kernel 
sends SIGSEGV to your process which, if unhandled, simply 
terminates your program.
It's an abnormal termination and thus the D runtime or whatever 
library that in a normal case takes care of printing the traces 
doesn't get a chance to do so anymore.

You also change the signal in your handler to get a core dump, 
look here 
http://www.alexonlinux.com/how-to-handle-sigsegv-but-also-generate-core-dump
Sep 30 2021