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RE: stack overflow bug in ofstream::operator<<
- From: "Dave Korn" <dave dot korn at artimi dot com>
- To: <cygwin-developers at cygwin dot com>
- Date: Tue, 28 Jun 2005 16:42:16 +0100
- Subject: RE: stack overflow bug in ofstream::operator<<
----Original Message----
>From: Christopher Faylor
>Sent: 28 June 2005 16:33
> On Tue, Jun 28, 2005 at 01:39:14PM +0200, Corinna Vinschen wrote:
>> We're using alloca/memcpy/write in writev so that writev is atomic.
>> If we would change that so that write is called multiple times, we'd
>> need to make write explicitely thread safe and each single write would
>> be slowed down. We also can't use WriteFileGather here, unfortunately.
>>
>> So, what we can do without slowing down write, is either to use malloc
>> instead of alloca, or use both, dependent on the number of bytes to be
>> written in relation to the size of the stack.
>>
>> Analogous for readv.
>
> We should probably just use malloc.
>
> cgf
... perhaps only if the size is above a reasonable limit (for stack-based
objects, probably somewhere in the range 16kB and 256kB would be a suitable
dividing line), otherwise keep the alloca; that way, most cases will retain
the current efficiency, and software that wants to write 2Mb strings
probably isn't in the middle of a time-critical loop anyway!
#define STACK_MAX_OBJECT_SIZE 65536
char *const buf = (char *) (tot > STACK_MAX_OBJECT_SIZE) ? malloc (tot) :
alloca (tot);
.... snip ....
int rv = write (buf, tot);
if (tot > STACK_MAX_OBJECT_SIZE)
free (buf);
return rv;
cheers,
DaveK
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