SIGSEGV hangs instead of raising NilAccessDefect with `import segfaults` and gc:arc

[qb-0]

SIGSEGV is not catchable with arc and import segfaults.

Example

import segfaults

try:
  echo cast[ptr int](nil)[]
except NilAccessDefect:
  echo "SegFault!"

Current Output

Traceback (most recent call last)
Tests\segfaulttest.nim(4) segfaulttest
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

SegFault!

$ nim -v
Nim Compiler Version 1.4.2 [Windows: amd64]
Compiled at 2020-11-30

[timotheecour]

bad example because in devel this now gives static, not RT error (regardless of gc): Error: nil dereference is not allowed

however there is indeed a bug

when true:
  import segfaults
  var a: ptr int
  try:
    echo a[]
  except NilAccessDefect:
    echo "SegFault!"

default gc: prints SegFault! gc:arc: hangs

And yes, segfaults is useful and needs to be supported, it's useful in cases where untrusted code should not cause a program to abort if it can (eg: repls, dll plugins)

[Araq]

And yes, segfaults is useful and needs to be supported, it's useful in cases where untrusted code should not cause a program to abort if it can (eg: repls, dll plugins)

Feel free to submit a patch then.

[johnnovak]

And yes, segfaults is useful and needs to be supported, it's useful in cases where untrusted code should not cause a program to abort if it can (eg: repls, dll plugins)

Or for applications shipped to users, where you want to capture errors in a crash log with full stack traces, that the user can the submit in a bug report.

[timotheecour]

note that you'll get a stacktrace without segfaults if you compile with --stacktrace

[johnnovak]

note that you'll get a stacktrace without segfaults if you compile with --stacktrace

Yeah I know, but it just gets dumped to stderr, or in case of a Windows app, it gets shown in a popup dialog box. What I want is to write the stacktrace into a log file, so I need a mechanism to hook into this thing and change the behaviour of just dumping the stacktrace to stderr/displaying it in an alert box.

[timotheecour]

can you try unhandledExceptionHook and onUnhandledException?

(we should still fix the bug though)

[ringabout]

It is related to goto exception and the example works with exceptions:setjmp.

[bung87]

change segfaults.nim

var SEGV_ACCERR {.importc, header: "<signal.h>".}: cint
...
if y.si_code == SEGV_MAPERR or y.si_code == SEGV_ACCERR:

compile with -d:noSignalHandler -d:nimPanics

you get Error: unhandled exception: Could not access value because it is nil. [NilAccessDefect]

lib/system/excpt.nim also set a signal handler, so noSignalHandler flag is required. Edit: turns out only -d:nimPanics required, it become unhandled exception.