malloc in gethostbyname

Bernd Schmidt bernds_cb1 at t-online.de
Tue Jun 10 11:59:54 UTC 2008


For gethostbyname and related functions, I can see two alternatives to 
just reverting to pre-uc_malloc state, either of which would enable us 
to reduce the amount of static buffers.

Looking at some other C libraries, glibc still seems to use static 
buffers, but in the FreeBSD C library, there is a dynamic allocation: 
per thread, even, from what I can tell.  Of course they don't just exit 
when it fails, but they return NULL from gethostbyXXX (without setting 
h_errno or errno though, so it doesn't seem all that robust either).

I had the idea of adding a config option that only leaves in the _r 
variants of these functions and thus eliminates the static buffers. 
While investigating this, I've hit a slight obstacle and discovered a 
bug: getnameinfo, which is documented as one of the replacements for 
gethostbyXXX, uses gethostbyaddr internally, which of course means that 
it isn't reentrant.  In glibc, it uses temporary buffers on the stack 
and calls gethostbyaddr_r.  We could do the same; the advantage would be 
that we'd have reentrancy and we could ifdef out the non-reentrant 
functions, but we'd also enlarge the stack requirements which is 
unfortunate for nommu users.  OTOH, the other function in the pair, 
getaddrinfo, already uses local buffers on the stack, so it's not 
without precedent.

Comments?  Where should we go with this?


Bernd
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