asprintf bugs

[Jeff Johnston]

Eric Blake wrote:
> Jeff Johnston <jjohnstn <at> redhat.com> writes:
> 
>> Thanks for the analysis Eric.  Patch checked in.
>>
>> Regarding minimizing the asprintf default buffer size, yes, it would be 
>> worth pursuing.  I made up a trial patch.  It simply checks for __SMBF 
>> inside __smakebuf.  If already set, this implies that the asprintf 
>> family is using the routine for the first time.  Try it out and let me 
>> know what you think.
>>
> 
> Looks correct to me.  By lowering the default initial buffer size, it means 
> that a 1024 byte result from asprintf will call realloc up to 7 times, but my 
> gut feeling is that typical usage of asprintf does not often deal with strings 
> this long (otherwise the off-by-one error probably would have been caught much 
> sooner).  And your patch still guarantees that asprintf(&s,"") still allocates 
> (something to make sure of, since that execution path does not go through 
> __sfvwrite_r for a realloc but asprintf must always allocate).
> 
> My original thoughts had been along the lines of making __smakebuf skip 
> allocation altogether if __SSTR, but then asprintf would also have to be 
> modified to check for f._w of 0 and do a zalloc(1) in that case; not to mention 
> that growth from 0 to 64 bytes requires up to 9 additional realloc calls.  64 
> is a good default, since in some malloc implementations (at least in glibc 
> malloc) it is the largest "small" chunk of memory, anything larger switches 
> over to "large" chunk handling.  So I like your patch better than my original 
> idea.
> 

Ok, great.  Patch checked in.

-- Jeff J.