On Sat, Jun 09, 2018 at 10:06:55PM +0530, Sukrit Bhatnagar wrote:
Hi, I am starting this discussion thread as a continuation of my GSoC weekly meeting with Erik and Pavel on 8th June. I was going through src/util/virstring.c for adding cleanup macros and saw that virStringListFree takes on char ** as an argument, and equivalently, we declare a list of strings as char **. For the cleanup function defined by VIR_DEFINE_AUTOPTR_FUNC, it is required that the associated type has a name like virSomethingPtr. It was also discussed that there are similar issues with DBus types, but VIR_AUTOFREE can work there as we use VIR_ALLOC. I honestly don't know much about that. We discussed that we have two solutions: - Create a virSomethingPtr by typedef-ing char** As Pavel told, GLib has typedef gchar** GStrv; which is used together with g_auto and it has g_strfreev(gchar **str_array) which is the same as we have virStringListFree() I have tried adding the following in src/util/virstrnig.h, and it seems to work fine: typedef char **virStringList; VIR_DEFINE_AUTOPTR_FUNC(virStringList, virStringListFree) We can use it as: VIR_AUTOPTR(virStringList) lines = NULL; There may be other scalar and external types where this problem occurs, and it is not good to create a typedef for each of them, but maybe we can make an exception for char ** and create a type for it. - Overload VIR_AUTOFREE macro by making it variadic As Erik told, we could make VIR_AUTOFREE a variadic macro whose varying parameter can be the Free function name. If left blank, we use virFree. I went ahead with trying it and after reading some posts on StackOverflow, I came up with this: #define _VIR_AUTOFREE_0(type) __attribute__((cleanup(virFree))) type #define _VIR_AUTOFREE_1(type, func) __attribute__((cleanup(func))) type #define _VIR_AUTOFREE_OVERLOADER(_1, _2, NAME, ...) NAME #define VIR_AUTOFREE(...) _VIR_AUTOFREE_OVERLOADER(__VA_ARGS__, _VIR_AUTOFREE_1, _VIR_AUTOFREE_0)(__VA_ARGS__) The required functionality is working as expected; passing only one argument will use virFree, and passing two arguments will use the function specified as 2nd argument. Passing more than 2 arguments will result in an error. The macros with _ prefix are meant to be for internal use only. Also, @func needs to be a wrapper around virStringListFree as it will take char ***, not just char **. We probably need to define a new function. Here we are specifying the Free function to use at the time of usage of the VIR_AUTOFREE macro, which may make the code look bad: VIR_AUTOFREE(char **, virStringListSomethingFree) lines = NULL; Suggestions and opinions are welcome.
Just my two cents, but I like the second variant more. For few reasons: - If we typedef char ** to something, then all users of that function will need to cast it back to char ** since they will be accessing the underlying strings (char *), even if there is a macro or a function for that, it seems the code will be less readable. - We are using a trick similar to the second variant in tests/virmock.h, although for a different purpose. See VIR_MOCK_COUNT_ARGS - With the first approach we're going to have to create unnecessary types and possibly lot of them. Have a nice day, Martin
Thanks, Sukrit -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
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