Re: how to make a string outlive the invocation of a C function?

[Jesús Ruiz de Infante <jesus.ruizdeinfante@...>]

Gé Weijers wrote:
> Every process should get a copy of your global variables, not share them 
> with other processes. Global variables are shared between (Posix) 
> threads that are part of the same process, not between processes (the 
> library loader usually uses copy-on-write pages to implement this). I 
> would still prefer a reentrant solution though, so storing it in the 
> module environment is the right thing to do.
You are right. Static variables contained in DLLs are not shared by the 
processes linked against them. At least not on Linux. Is this point 
standarized?


> Your solution is a little complicated, you could just store the ident 
> string passed as an argument itself, not a userdata object, why keep two 
> copies of the same thing around when Lua string are immutable? When you 
> need it you just use lua_tolstring to get the same string pointer again 
> (and again). So your code simplified to:
syslog() uses the pointer to ident set by the previous call to 
openlog(). There is no explicit ident parameter passed to syslog() [1].
So there is actually no need for lua_tolstring.

It should be guaranteed however that the ident pointer passed to 
openlog() points "for ever" to the ident string. In addition, storing a 
pointer to an internal Lua string is not a good idea, even if it is 
stored in the module enviroment (?). Hence the userdata.

> > static int l_openlog(lua_State *L)
> > {
> > ...
> > /* handle ident option */
> > size_t len;
> > const char *ident = luaL_checklstring(L, 1, &len);
>    lua_pushvalue(L, 1);
> > /* store it in our environment table */
> > lua_setfield(L, LUA_ENVIRONINDEX, "ident");
> > openlog(ident, option, facility);
> > return 0;
> > }
>
> If you're worried about efficiency you could store the string at integer 
> index 1, but you'll bog down the syslog daemon long before that :-/ 
> (syslogd tends to do an 'fsync' call after every write)
My intention regarding the call to lua_setfield() above is just to 
ensure that the ident string is referenced by Lua and does not get 
garbage collected.
So there is no "ident" lookup when calling syslog(), just in closelog(),
which should happen only once ;-)

> Gé
>
> --
> Gé Weijers
> ge@weijers.org <mailto:ge@weijers.org>
[1] http://www.hmug.org/man/3/syslog.php
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Mit freundlichen Grüßen

Jesus Ruiz de Infante
(Entwicklung)
HALE electronic GmbH
Eugen-Müller-Straße 18, 5020 Salzburg, Austria
Tel:  +43 (662) 439011 0
Fax:  +43 (662) 439011 9
jesus.ruizdeinfante@hale.at
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