16:08 <rivten> hey everybody ! I hope you are doing fine :) I'm pretty new to Zig and I've got a quick question (hope this is a good place to ask :P). I'm debugging some memory allocation (with FixedBufferAllocator) and, in the process of understanding what's going on, I'm stuck on understandling this lign of the allocator : https://github.com/ziglang/zig/blob/master/std/mem.zig#L105. Like : why using undefined parameter and what is even ([*]u8)(undefined)[0..0] ? Thanks a

16:08 <rivten> lot :)

16:13 <Sahnvour> rivten, the underlying allocator interface is based on two functions: `reallocFn` and `shrinkFn`. reallocFn is used to increase the allocation size (which is potentially 0 for a new allocation) and shrink to reduce it. in this case, as `alignedAlloc` is known to be called when the user wants a new allocation, this calls reallocFn to query for a new allocation of size `byte_count` but has to pass the "old" (in this case,

16:13 <Sahnvour> undefined) allocation to satisfy the interface

16:15 <Sahnvour> and this is a good place to ask :)

16:21 <daurnimator> rivten: passing undefined like that means we don't care what we pass as that parameter

16:22 <daurnimator> note that in debug builds, your program will panic if it tries to use (e.g. compare/add) an undefined value.

16:23 <daurnimator> rivten: `([*]u8)(undefined)[0..0]` is a way to get a 0 length slice.

16:25 <daurnimator> to decompose it: `[*]u8(undefined)` an undefined pointer to an unknown number of u8 values. => `[0..0]` => take a 0 length slice of it. so now we have a slice of u8 with an undefined pointer, and a length of 0.

16:56 <rivten> ok thanks everyone ! what's weird is that : if undefined means "it shouldn't be used in the called function", then the realloc, on a FixedBufferAllocator, we do use the old_mem and old_align params which are passed as undefined https://github.com/ziglang/zig/blob/master/std/heap.zig#L314 hmmmm

16:57 <scientes> rivten, llvm will optimize out those things

16:58 <scientes> when it know val is undefined

17:00 <rivten> oh ok. So if my program fails on the OutOfMemory error, then it's the compile time pass that was not able to properly resolve the memory allocation ? (otherwise it would have been optimized out ?)

17:01 <scientes> undefined behavior is undefined

17:03 <Sahnvour> rivten, OutOfMemory error is more likely to come from a programming mistake, I don't think the optimization pass should have any impact on the observable result of your code

17:07 <daurnimator> huh? no

17:08 <daurnimator> rivten: you get OutOfMemory if there wasn't enough memory available to satisfy the request; whether that's because your fixedbuffer is too small; your OS failed the mmap() call; or if everything is just too fragmented

17:08 <daurnimator> OutOfMemory is rarely if ever a programming mistake

17:10 <Sahnvour> asking too much memory from a fixed buffer is a programming mistake

17:11 <rivten> hmm ok ! I will keep reviewing my code then :P it's weird but I do give a _very_ high fixed buffer size, but still get an error. Will keep debugging. Thanks a lot everybody ! :)

17:12 <jjido> rivten using newruntime?

17:13 <jjido> oops wrong channel

19:05 <donpdonp> im trying to alloc ram for a new struct, but gettting an error I dont understand

19:06 <donpdonp> pub const Actor = struct {...}; if (allocator.alloc(Actor, 1)) |actor| { ...} => error: expected optional type, found '@typeOf(std.mem.Allocator.alloc).ReturnType.ErrorSet![]thread.Actor'

19:06 <donpdonp> A!B type seems like it is an 'optional type'

19:15 <tyler569> ?T is the optional type (a value of T or null), A!B is the error union type (a value of type B or an error of set A)

19:19 <donpdonp> isnt expr in 'if(expr) |value| { }' expected to be an error union?

19:19 <tyler569> I think you might have to provide an else clause for error unions, since zig makes strong guarantees about handling error conditions

19:20 <tyler569> it could just be else { unreachable } if you want to assert that that will never happen and get a stack trace in debug mode if it does

19:20 <donpdonp> ah that was the problem, thx!

19:37 <tgschultz> generally we use `try` and `catch` to unwrap error unions: `const instance = try allocator.alloc(Actor, 1);` or `const instance = allocator.alloc(Actor, 1) catch unreachable;` also, for single instances you can use alloc.create(T) and alloc.destroy(T);

19:48 <donpdonp> nod. thx.

19:50 <donpdonp> my struct has a field of []u8. im passing in a []u8 and warn says the addr is "[]const u8@204b40" which seems reasonable, but when I set my_struct.array = my_u8_arry, and warn/print the address of &my_struct.array, I get "[]const u8@7fff345cc5b8"

19:50 <donpdonp> i figured the address by my_struct.array would be the first address. also surprised by the difference in width of the address

19:59 <tgschultz> []u8 is a slice of u8 (ptr + len), not an array

19:59 <tgschultz> &my_struct.array gives the address of the slice, not what it points to

20:00 <tgschultz> my_struct.ptr would be where the slice points

20:00 <tgschultz> er: my_struct.array.ptr

20:05 <donpdonp> fn myfun(data: []const u8) how do i tell if data is an array or a slice?

20:07 <donpdonp> and similarly, struct { payload: []const u8 }, is that an array or a slice?

20:07 <Sahnvour> array types have their size inside the brackets, it is known at compile time

20:08 <Sahnvour> slices are denoted by `[]`

20:08 <donpdonp> oh! thx

20:09 <Sahnvour> you can take a look at https://ziglang.org/documentation/master/#Arrays (and note that initialization of an array does not force to specify its size, the compiler will infer it)

20:10 <donpdonp> yeah thats not confusing at all when the first example of an array uses what looks like a slice declaration :)

20:14 <tgschultz> there's an issue for that, actually.

20:18 <tgschultz> https://github.com/ziglang/zig/issues/1797

20:21 <donpdonp> intersting.

20:40 <donpdonp> how do i get the type of a variable? @typeName(my_var) complains my_var is not a type, which is understandable, im just not sure how to get at the type

20:41 <emekankurumeh[m]> @typeName(@typeOf(my_var))

20:41 <donpdonp> thx!

20:43 <donpdonp> warn("{} {}", actor, &actor); both print the struct. im trying to get the address of the struct

20:45 <emekankurumeh[m]> `warn("{} {p}", actor, &actor);` should work

20:47 <donpdonp> warn("typeOf {} {p}\n", @typeName(@typeOf(actor)), &actor); => typeOf Actor Actor{ .thread = 5, .read_fd = 0, .payload = }

20:47 <donpdonp> its still dumping the actor contents

20:47 <donpdonp> are there docs for the {} substitutions? i dont see them on https://ziglang.org/documentation/master/

20:49 <donpdonp> "{x}" @ptrToInt(&actor) works ok

20:51 <emekankurumeh[m]> `warn("{} {*}", actor, &actor);` should print the type and the address.

20:54 <tyler569> std isn't super well documented, but std/fmt.zig is pretty eazy to follow in my opinion. it implements a state machine, so you just look for the character to State mapping and follow that

20:54 <donpdonp> emekankurumeh[m]: {*} is great. looks like ruby.

21:01 <donpdonp> here's another puzzler. doit(number) => error: expected type 'c_ulong', found '*c_ulong'

21:01 <donpdonp> doit(*number) => error: expected type 'type', found '*c_ulong'

21:02 <donpdonp> oh i just remembered seeing in the docs, its number.*

21:14 <donpdonp> here's another one. im passing a struct to pthread_create, and the callback func is getting the right address in the void ptr, but when i look at the contents, its all zeros

21:15 <donpdonp> var actor = Actor@7ffedb6d2248 Actor{ .thread_id = c_ulong@7ffedb6d2290 ...}; pthread_create(..., &actor); thats fine

21:16 <donpdonp> callback is fn go(data: ?*c_void); Actor@7ffedb6d2248 Actor{ .thread_id = c_ulong@0 ...} (all data zeroed out)

21:16 <donpdonp> i had to use var data8 = @alignCast(@alignOf(thread.Actor), data); var actor = @ptrCast(*thread.Actor, data8);

21:17 <donpdonp> so maybe im not doing something right there.

22:28 <bketelsen> shritesh: epic wasm, my friend. That's awesome.