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But it _is_ safe, and it's actually a very, very, very :) nice trick to use in C/Lua interfaces.
It is safe to use the returned string, simply because the garbage collector never runs until you return from your C function back to Lua. Thus, you can: get a string, pop it, use the pointer.
If you still need the string later, after returning from the C function, then there's no guarantees (since the garbage collection eventually runs). 

-asko


I know this is not always safe because the garbage collector might
take the string after it is popped off this stack but in this case the 
string still exists in the table and will not be collected until
"table" is collected (table is still on the stack).  So is this
example safe or is the string from lua_getfield actually a different
string than the one in the table?




Beric Holt kirjoitti 28.2.2006 kello 5.58:


The best option is to strcpy the string if you intend to use it longer
than the life of the function. As David points out, this doesn't apply 
if you are finished with it by the time the function ends and you pop
the value

Cheers
Beric Holt
http://buzzrick.emedia.net.nz

-----Original Message-----
From: David Morris-Oliveros [mailto:david.morrisoliveros@teambondi.com] 
Sent: Tuesday, 28 February 2006 4:54 p.m.
To: Lua list
Subject: Re: String lifetime (C API)
I don't know the answer to this, but i would always make the assumption 
that it's not safe. The Lua guys have always said that the
implementation of the VM is not something static, and might change in
the future, so, even if it may be safe to do so right now, i wouldn't
bet on it working in the future.

But, to my knowledge you can do this:

int SomeFunction( lua_State * L )
{
    lua_getfield( L, 1, "name" );
    const char * name = lua_tostring( L, -1 );

    UseItInAFunction( name );

    lua_pop( L, 1 );
    return 0;
}

// David Morris-Oliveros
// Camera & Lua Coder
// david@teambondi.com



Chris wrote:
Is it safe to use string values retrieved from a table even though the 


value is popped off the stack?  For example:

Lua side:

table = { name="test1234" }

C side:

/* assume "table" is passed as argument to this function */
int SomeFunction(lua_State* L)
{
   lua_getfield(L, 1, "name");
   const char* name = lua_tostring(L, -1);
   lua_pop(L, 1);

   ... use "name" char* variable...
}

I know this is not always safe because the garbage collector might
take the string after it is popped off this stack but in this case the 


string still exists in the table and will not be collected until
"table" is collected (table is still on the stack).  So is this
example safe or is the string from lua_getfield actually a different
string than the one in the table?

--
// Chris