# Improving on \[\*c\]u8 when calling a C function that allocates a string? **URL:** \ **Category:** Help\ **Created:** [December 17, 2023, 2:08pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489 "2023-12-17T14:08:08Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![mtlynch](https://ziggit.dev/user_avatar/ziggit.dev/mtlynch/32/1254_2.png) [@mtlynch](https://ziggit.dev/u/mtlynch)\ **Post date:** [December 17, 2023, 2:08pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/1 "2023-12-17T14:08:08Z")
Hi All, I’m a Zig beginner, and I’ve been [learning about using Zig](https://mtlynch.io/tags/zig/) to maintain C code. I’m stuck on a typing issue, and I’m wondering if anyone can help. I have a C function whose signature looks like this: ```c void foo(char** c); ``` The function allocates and populates a null-terminated string, and the caller is responsible for freeing the memory. I can call the function from Zig like this: ```zig const std = @import("std"); const lib = @cImport({ @cInclude("foo.c"); }); pub fn main() !void { var x: [*c]u8 = null; lib.foo(&x); std.c.free(x); } ``` But I’d like to avoid the `[*c]u8` type, as I know that the output parameter from C is a many-item, null-terminated pointer. My intuition is to define the type like this: ```zig var x: [*:0]u8 = undefined; lib.foo(&x); ``` But that gives me a compilation error: ```plaintext error: expected type '[*c][*c]u8', found '*[*:0]u8' ``` I’ve tried a few different variations, but the Zig compiler rejects all of them. Am I missing a more obvious type here? ---
**Author:** ![Luke](https://ziggit.dev/letter_avatar_proxy/v4/letter/l/7ab992/32.png) [@Luke](https://ziggit.dev/u/Luke)\ **Post date:** [December 17, 2023, 2:12pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/2 "2023-12-17T14:12:54Z")
Maybe something like this: ```zig [*:null]?[*:0]const u8 ``` There was a related discussion on this recently: [Null terminated array of strings from C function - #4 by castholm](https://ziggit.dev/t/null-terminated-array-of-strings-from-c-function/2459/4) ---
**Author:** ![mtlynch](https://ziggit.dev/user_avatar/ziggit.dev/mtlynch/32/1254_2.png) [@mtlynch](https://ziggit.dev/u/mtlynch)\ **Post date:** [December 17, 2023, 2:18pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/3 "2023-12-17T14:18:13Z")
Thanks for the suggestion! That unfortunately doesn’t compile either: ```zig var x: [*:null]?[*:0]const u8 = undefined; lib.foo(&x); ``` ```plaintext error: expected type '[*c][*c]u8', found '[*:null]?[*:0]const u8 ``` I think the issue is that in the other thread, the user was working with an array of strings, whereas in this case, the `foo` function just populates a single string. ---
**Author:** ![Luke](https://ziggit.dev/letter_avatar_proxy/v4/letter/l/7ab992/32.png) [@Luke](https://ziggit.dev/u/Luke)\ **Post date:** [December 17, 2023, 2:34pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/4 "2023-12-17T14:34:42Z")
Ok, what about then?: ```zig var x: ?[*:0]u8 = null; ``` ---
**Author:** ![LucasSantos91](https://ziggit.dev/user_avatar/ziggit.dev/lucassantos91/32/440_2.png) [@LucasSantos91](https://ziggit.dev/u/LucasSantos91)\ **Post date:** [December 17, 2023, 2:47pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/5 "2023-12-17T14:47:06Z")
Your type is correct. ```zig var x: [*:0]u8 = undefined; lib.foo(@ptrCast(&x)); ``` ---
**Author:** ![mtlynch](https://ziggit.dev/user_avatar/ziggit.dev/mtlynch/32/1254_2.png) [@mtlynch](https://ziggit.dev/u/mtlynch)\ **Post date:** [December 17, 2023, 2:49pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/6 "2023-12-17T14:49:21Z")
> [@Luke](#): > > Ok, what about then?: > > ` var x: ?[*:0]u8 = null;` Ah, that did it. Thanks so much! ---
**Author:** ![mtlynch](https://ziggit.dev/user_avatar/ziggit.dev/mtlynch/32/1254_2.png) [@mtlynch](https://ziggit.dev/u/mtlynch)\ **Post date:** [December 17, 2023, 2:50pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/7 "2023-12-17T14:50:12Z")
> [@LucasSantos91](#): > > Your type is correct. > > ```zig > var x: [*:0]u8 = undefined; > lib.foo(@ptrCast(&x)); > > ``` Hmm, that gives me a segfault: ```bash $ zig build run Segmentation fault at address 0x0 ???:?:?: 0x7ffff7e72075 in ??? (libc.so.6) Unwind information for `libc.so.6:0x7ffff7e72075` was not available, trace may be incomplete ``` ---
**Author:** ![LucasSantos91](https://ziggit.dev/user_avatar/ziggit.dev/lucassantos91/32/440_2.png) [@LucasSantos91](https://ziggit.dev/u/LucasSantos91)\ **Post date:** [December 17, 2023, 2:53pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/8 "2023-12-17T14:53:39Z")
Well, that’s weird. At least you already have a solution. ---
**Author:** ![permutationlock](https://ziggit.dev/user_avatar/ziggit.dev/permutationlock/32/9958_2.png) [@permutationlock](https://ziggit.dev/u/permutationlock)\ **Post date:** [December 17, 2023, 3:36pm UTC](https://ziggit.dev/t/improving-on-c-u8-when-calling-a-c-function-that-allocates-a-string/2489/9 "2023-12-17T15:36:07Z")
> [@mtlynch](#): > > ```zig > var x: [*:0]u8 = undefined; > lib.foo(&x); > > ``` > > But that gives me a compilation error: > > `error: expected type '[*c][*c]u8', found '*[*:0]u8'` I ran into this recently too. The issue is that if `x: ?*[*:0]u8` was allowed to coerce to `y: [*c][*c]u8 = x`, then the type system would have to allow `y.* = null`, violating the guarantee that `x.?.*` is not null. So you have to add an extra optional `x: ?*?[*:0]u8`. It seems that this is a limitation you have to live with when using `@cImport`, and to get the “correct” types you need to manually declare the signature of `foo` (which I generally prefer over `@cImport` anyway). ```zig extern fn foo(c: ?*[*:0]u8) void; ```