NotNull pointers

[Steven Schveighoffer]

On Wed, 31 Aug 2011 17:20:04 -0400, Robert Clipsham  
<robert at octarineparrot.com> wrote:

> On 31/08/2011 22:01, Steven Schveighoffer wrote:
>> You can catch sigsegv on linux. It takes one call to signal to register
>> a handler.
>
> Are you sure? I could have sworn it didn't work? If it does work, what  
> is the purpose of this library:
>
> http://libsigsegv.sourceforge.net/

I think that may be a way to override the OS's page fault handling.

Here's how a page fault (which is *NOT* a segmentation fault necessarily)  
works (this might be really badly described, it's been a while since I had  
to know this):

The cpu requests a certain memory address.
If the memory isn't in the CPU's cache, it requests the page from the MMU  
(memory management unit).
If the page isn't in memory, it triggers an interrupt to the OS for a page  
fault.
If the OS sees the page is cached on disk in the swap file, it loads the  
page from disk, and then gets it from the MMU.
If the page is not on disk, then the process is sent the SIGSEGV signal.

Maybe the library hooks the signal to do it's own retrieval of memory from  
some other cache.  No clue.

But this program works swimmingly:

import core.stdc.signal;
import std.stdio;

extern(C) void sigsegvhandler(int sig)
{
     writeln("in signal handler!");
     assert(0);
}

void main()
{
     signal(SIGSEGV, &sigsegvhandler);
     int * i;
     *i = 5;
}

output:
in signal handler!
core.exception.AssertError at testsignal(7): Assertion failure
----------------
./testsignal(onAssertError+0x2e) [0x8086b9e]
./testsignal(_d_assertm+0x16) [0x80844b6]
./testsignal() [0x8081e96]
./testsignal(sigsegvhandler+0x1e) [0x8081de2]
[0x8bf400]
./testsignal(_D2rt6dmain24mainUiPPaZi7runMainMFZv+0x1a) [0x808495e]
./testsignal(_D2rt6dmain24mainUiPPaZi7tryExecMFMDFZvZv+0x20) [0x80845f8]
./testsignal(_D2rt6dmain24mainUiPPaZi6runAllMFZv+0x32) [0x80849a2]
./testsignal(_D2rt6dmain24mainUiPPaZi7tryExecMFMDFZvZv+0x20) [0x80845f8]
./testsignal(main+0x94) [0x80845a4]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0xa79e37]
./testsignal() [0x8081d11]
----------------

-Steve