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digitalmars.D.learn - union/toString: crash/segfault: What's happening here?

reply kdevel <kdevel vogtner.de> writes:
crash.d
```
import std.stdio;

union U {
    float f;
    int i;
    string toString ()
    {
       string s;
       return s;
    }
}

void main ()
{
    U u;
    writeln (u);
}
```

$ dmd crash.d
$ ./crash

std.exception.ErrnoException /.../dmd2/linux/bin64/../../src/phobo
/std/stdio.d(2776):  (Bad address)
----------------
??:?  safe int std.exception.errnoEnforce!(int, 
"/.../dmd2/linux/bin64/../../src/phobos/std/stdio.d", 
2776uL).errnoEnforce(int, lazy immutable(char)[]) [0x43f20a]
??:?  safe void 
std.stdio.File.LockingTextWriter.put!(immutable(char)[]).put(immutable(char)[])
[0x4422a7]
??:?  safe void 
std.range.primitives.doPut!(std.stdio.File.LockingTextWriter, 
immutable(char)[]).doPut(ref std.stdio.File.LockingTextWriter, 
ref immutable(char)[]) [0x44224b]
??:?  safe void 
std.range.primitives.put!(std.stdio.File.LockingTextWriter, 
immutable(char)[]).put(ref std.stdio.File.LockingTextWriter, 
immutable(char)[]) [0x44222b]
??:? void 
std.format.formatObject!(std.stdio.File.LockingTextWriter, 
crash.U, char).formatObject(ref std.stdio.File.LockingTextWriter, 
ref crash.U, ref const(std.format.FormatSpec!(char).FormatSpec)) 
[0x44220a]
??:? void 
std.format.formatValue!(std.stdio.File.LockingTextWriter, 
crash.U, char).formatValue(ref std.stdio.File.LockingTextWriter, 
ref crash.U, ref const(std.format.FormatSpec!(char).FormatSpec)) 
[0x44219d]
??:? uint 
std.format.formattedWrite!(std.stdio.File.LockingTextWriter, 
char, crash.U).formattedWrite(ref 
std.stdio.File.LockingTextWriter, const(char[]), crash.U) 
[0x43e703]
??:? void std.stdio.File.write!(crash.U, char).write(crash.U, 
char) [0x43e3f5]
??:? void std.stdio.writeln!(crash.U).writeln(crash.U) [0x43e389]
??:? _Dmain [0x43e344]
Jan 11 2018
next sibling parent reply Nicholas Wilson <iamthewilsonator hotmail.com> writes:
On Friday, 12 January 2018 at 00:54:03 UTC, kdevel wrote:

 crash.d
 ```
 import std.stdio;

 union U {
    float f;
    int i;
    string toString ()
    {
       string s;
       return s;
    }
 }

 void main ()
 {
    U u;
    writeln (u);
 }
 ```

 $ dmd crash.d
 $ ./crash


because you don't initialise `s` in


    string toString ()
    {
       string s;
       return s;
    }


so it defaults to `string s = null;` thus giving a segfault.

try `string s = "";` instead.
Jan 11 2018
next sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 12 January 2018 at 01:45:37 UTC, Nicholas Wilson wrote:

 so it defaults to `string s = null;` thus giving a segfault.


null and "" are basically the same for strings. that's not the 
problem.
Jan 11 2018
prev sibling parent ag0aep6g <anonymous example.com> writes:
On 01/12/2018 02:45 AM, Nicholas Wilson wrote:

 because you don't initialise `s` in
 

    string toString ()
    {
       string s;
       return s;
    }


 
 so it defaults to `string s = null;` thus giving a segfault.
 
 try `string s = "";` instead.


A null string is a perfectly fine empty string. Printing it does not 
lead to a segfault. And the thrown error is not a segfault.

Actually, the problem seems to be that s is not default initialized. 
It's garbage. And garbage usually isn't a proper string, so writeln 
fails. Looks like a compiler bug.
Jan 11 2018
prev sibling parent reply Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 12 January 2018 at 00:54:03 UTC, kdevel wrote:

 $ dmd crash.d
 $ ./crash


Nicholas Wilson is right that you can use = "" to work around it, 
but with strings, null is supposed to behave the same way.

And this gives different (each wrong) behavior on -m32 vs -m64, 
which leads me to believe you actually found a compiler bug.

Calling u.toString directly also leads to random spam, which 
means it isn't even the library.

I'd file this as a compiler codegen bug.
Jan 11 2018
parent reply kdevel <kdevel vogtner.de> writes:
Thanks for the quick answer!

On Friday, 12 January 2018 at 02:16:39 UTC, Adam D. Ruppe wrote:

 On Friday, 12 January 2018 at 00:54:03 UTC, kdevel wrote:

 $ dmd crash.d
 $ ./crash


 Nicholas Wilson is right that you can use = "" to work around 
 it, but with strings, null is supposed to behave the same way.

 And this gives different (each wrong) behavior on -m32 vs -m64, 
 which leads me to believe you actually found a compiler bug.


dmd -O crash even produces a binary which segfaults on my machine.


 Calling u.toString directly also leads to random spam, which 
 means it isn't even the library.

 I'd file this as a compiler codegen bug.


https://issues.dlang.org/show_bug.cgi?id=18232
Jan 12 2018
next sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Jan 12, 2018 at 11:09:47AM +0000, kdevel via Digitalmars-d-learn wrote:
[...]

 On Friday, 12 January 2018 at 02:16:39 UTC, Adam D. Ruppe wrote:


[...]

 I'd file this as a compiler codegen bug.


 
 https://issues.dlang.org/show_bug.cgi?id=18232


Yep, definitely a codegen bug.  Apparently, local variables in union
member functions aren't initialized to .init as they should be.  Digging
into the compiler code right now to see if I can find where the problem
is...


T

-- 
Only boring people get bored. -- JM
Jan 12 2018
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Fri, Jan 12, 2018 at 10:49:45AM -0800, H. S. Teoh via Digitalmars-d-learn
wrote:

 On Fri, Jan 12, 2018 at 11:09:47AM +0000, kdevel via Digitalmars-d-learn wrote:
 [...]


[...]

 https://issues.dlang.org/show_bug.cgi?id=18232


 
 Yep, definitely a codegen bug.  Apparently, local variables in union
 member functions aren't initialized to .init as they should be.


[...]

Fix: https://github.com/dlang/dmd/pull/7687

Temporary workaround: explicitly initialize your local variables in
union member functions.


T

-- 
Life begins when you can spend your spare time programming instead of watching
television. -- Cal Keegan
Jan 12 2018