Subj : Re: reentrant fwrite/fputc/etc ?
To   : borland.public.cpp.borlandcpp
From : gary <news@cordelli.net>
Date : Fri Nov 14 2003 12:59 pm

i think maybe i didn't ask the question clearly enough, or perhaps you 
misunderstood what is meant by "reentrant".  there is no such thing as a 
function that is reentrant "some of the time".  it is possible, i grant, 
to describe a function as being "reentered" some of the time, as the 
condition is obviously dependent on the totality of the code and what is 
happening within threads/ISRs.  however, a function is either 
"reentrant" or it is "non-reentrant", meaning that it does or doesn't 
support being "reentered" without the second call screwing up what the 
first call was in the middle of doing.

for example, if you put a printf() or fwrite() call in an ISR, you are 
going to find yourself in trouble eventually.  the fact that it doesn't 
crash in the lab does not mean that it isn't going to crash in the field.

some C lib functions aren't even technically compatible with each other 
for the silly reason that they set the global "errno" variable on error. 
  so if your program is in the middle of a C lib call that has just set 
errno, but an ISR occurs before it can return and that ISR calls another 
C lib function that happens to set errno, upon return your main-line C 
lib function is going to appear to have set a different errno than it 
really did, which might well at some point make your program respond 
incorrectly to an error condition.

this is why you need to be careful what C lib functions you call inside 
an ISR.

in my case, the question was just about C lib functions that disturb the 
operation of other instances of themselves.  for example, those that use 
static buffers (like strtok(), etc) and those that use global buffers 
(like the _iob[] structures used by stream i/o functions).


Andrue Cope wrote:

> Gary,
> 
> 
>>does anyone know 
>>if Borland (or Microsoft or any other compiler vendor) bothers to 
>>publish such a list for their C libraries?
>>
>>
> 
> I don't know that there would be such a list. It may be that some of 
> the functions just happen to be reentrant some of the time.
> --
> Andrue Cope
> [Bicester, UK]
> 
>

.