two 4GB mallocs => SEGV

[Willem Jan Withagen]

Peter Pentchev wrote:

> On Tue, Oct 26, 2004 at 02:47:06PM +0200, Willem Jan Withagen wrote:
> 
>>Georgi Guninski wrote:
>>
>>
>>>on a freebsd amd64 box with more than 8GB swap i experience the following:
> 
> [snip]
> 
>>>amdkotef64# cat test.c
> 
> [snip 2 * 4GB malloc]
> 
>>>amdkotef64# gcc test.c
>>>amdkotef64# ./a.out 
>>>100000000
>>>503000
>>
>>>/: write failed, filesystem is full
>>>Segmentation fault
>>
>>These 2 lines stem from the fact that the program is being dumped and there 
>>is not enough space to dump the full size core of that program.
>>
>>what does swapinfo tell you during the the run of the program???
>>
>>I ran malloc tests in 4Gb with 5Gb of swap, which it nicely completely 
>>filled, and then in faulted, because there was no more space.
>>This behaviour can be set, I think. Check man 5 malloc.conf, and check 
>>/etc/malloc.conf to see if you've got the X-flag on.
> 
> 
> But shouldn't malloc() write out an 'out of memory' message before the
> core dump then?  There's no such message in Georgi's output.

That depends on the flags set with malloc.conf. The X-flag tells you how 
errors are handled. But you are right, it is strange:
	malloc should either return 0, or give a warning.

I think that std-malloc settings were AX for anything before -STABLE.

> It's true that when he asked me on Saturday about this, I completely
> forgot about malloc's X flag, but I don't think he's using it, and even
> if he were, there ought to be a message there.

That's why I send him to the man-page. He could even force the settings in the 
testprogram. And check the link!!! it is probably set to something, or 
something is assumed. So better be safe and force it one-selves.

Could be that the message does not arrive because the out-of-mem trap killed 
it first.... ???
Perhaps also check /var/log/messages. I remember seeing things there as well.

>>I have malloc.conf-> aj, and the program terminates in 3 seconds because it 
>>does not zero the memory.
>>
>>So pick and choose the behaviour you want.
> 
> 
> Sure, the question is if this is the result of 'X' or something else :)

Well given the fact that there are not yet that many big boxes, let alone 
people really wanting to allocate a 4Gb blob, I'm not going to bet the usual 
case of beer on it. :~)

--WjW