Re: How far should one go preventing potential memory leaks?

Am 09.05.2016 um 15:37 schrieb Roberto Ierusalimschy <roberto@inf.puc-rio.br>:

I would like to open a discussion on best practices for preventing memory leaks in C programs using the Lua API.

Consider the following code snippet:

char *s = malloc(32);
snprintf(s, 32, "bumblebee");
lua_pushstring(L, s);
free(s);

lua_pushstring() could raise an error (e.g. under memory pressure) and then the subsequent free(s) would not be called, leaking the memory.

One could use lua_newuserdata() instead of malloc, but this works only if the memory is allocated by our own code, it does not work where the memory is allocated e.g. by a third party library.

So a possible way could be to create a metatable containing a __gc method, then allocating a small userdata value containging only a pointer to the allocated data. The __gc method could then free the memory, even if lua_pushstring() fails.

Is it worth the effort? How do others (you) handle this kind of possible errors?

My personal take on this is, btw: I don't care as I long as I don't reference NULL pointers. If we are under memory pressure and functions like lua_pushstring() are starting to fail, we will be in much deeper trouble anyway soon... ymmv, opinions very welcome.

I think both options are valid. If you are writing a public library,
I would go the first way; often this userdata has some useful meaning
to be exposed in Lua. As an example, streams (FILE*) in the I/O library
follow this pattern.

As we are talking about a public library indeed, I guess I will have to go the

char *s = lua_newuserdata(L, 32);
snprintf(s, 32, "bumblebee");
lua_pushstring(L, s);

This prevents a memory leak if lua_pushstring() raises an error, but I will end

up with a number of rather small allocations that are only used for a short

period of time. Wouldn't it be nice to have a explicit call in the Lua API to

free a userdata value immediately, e.g.

char *s = lua_newuserdata(L, 32);
snprintf(s, 32, "bumblebee");
lua_pushstring(L, s);