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Implement `CMSG_*`-equivalent functionality to `std.c`, enabling the effective use of `std.c.cmsghdr`. #30629

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opened 2025年12月30日 20:58:24 +01:00 by pltrz · 2 comments

pltrz commented

2025年12月30日 20:58:24 +01:00

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This comment brought up this concern, and sums it up pretty well. Unfortunately it wasn't made into it's own issue.

To send and receive file descriptors in C, you need to interface with the libc cmsghdr struct, and you're supposed to do so via the CMSG_* macros which are defined in <sys/socket.h> in the C standard library.

I see that there is already a std.c.cmsghdr struct (see also: std.c.posix_cmsghdr, std.os.linux.cmsghdr).

But, there isn't a way to interface with this API in Zig (as far as I understand).

This comment explains some stuff as well on the topic.

I put together the list of the macros from <sys/socket.h>:

CMSG_DATA(): Defined in POSIX at least since Issue 6
CMSG_NXTHDR(): Defined in POSIX at least since Issue 6
CMSG_FIRSTHDR(): Defined in POSIX at least since Issue 6
CMSG_SPACE(): Defined in POSIX at least since Issue 8
CMSG_LEN(): Defined in POSIX at least since Issue 8
CMSG_ALIGN(): Linux Extension, since v2.1.15

Some further reading:

[This comment](https://github.com/ziglang/zig/pull/24603#issuecomment-3136040106) brought up this concern, and sums it up pretty well. Unfortunately it wasn't made into it's own issue. To send and receive file descriptors in C, you need to interface with the libc `cmsghdr` struct, and you're supposed to do so via the `CMSG_*` macros which are defined in `<sys/socket.h>` in the C standard library. I see that there is already a `std.c.cmsghdr` struct (see also: `std.c.posix_cmsghdr`, `std.os.linux.cmsghdr`). But, there isn't a way to interface with this API in Zig (as far as I understand). [This comment](https://github.com/ziglang/zig/pull/25118#issuecomment-3246571861) explains some stuff as well on the topic. I put together the list of the macros from `<sys/socket.h>`: - `CMSG_DATA()`: Defined in POSIX at least since Issue 6 - `CMSG_NXTHDR()`: Defined in POSIX at least since Issue 6 - `CMSG_FIRSTHDR()`: Defined in POSIX at least since Issue 6 - `CMSG_SPACE()`: Defined in POSIX at least since Issue 8 - `CMSG_LEN()`: Defined in POSIX at least since Issue 8 - `CMSG_ALIGN()`: Linux Extension, since v2.1.15 ## Some further reading: - [ZIG: GH: Comment that describes this issue](https://github.com/ziglang/zig/pull/24603#issuecomment-3136040106) - [ZIG: An example of another user's Zig abstraction for using the `cmsg_hdr` struct](https://github.com/gdnsd/gdnsd/blob/3.99-alpha/gks/ancil.zig) - [ZIG: GH: PR #25118](https://github.com/ziglang/zig/pull/25118#issuecomment-3246571861) - [POSIX: ISSUE 6: socket.h](https://pubs.opengroup.org/onlinepubs/009695399/basedefs/sys/socket.h.html) - [POSIX: ISSUE 7: socket.h](https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/basedefs/sys_socket.h.html) - [POSIX: ISSUE 8: socket.h](https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_socket.h.html) - [LINUX: CMSG_ALIGN patch](https://ftp.funet.fi/index/Linux/kernel/v2.1/patch-html/patch-2.1.15/linux_include_linux_socket.h.html) - [LINUX: v2.1.14: socket.h](https://elixir.bootlin.com/linux/2.1.14/source/include/linux/socket.h) - [LINUX: v2.1.15: socket.h](https://elixir.bootlin.com/linux/2.1.15/source/include/linux/socket.h) - [MUSL: socket.h](https://git.musl-libc.org/cgit/musl/plain/include/sys/socket.h) - [GLIBC: socket.h](https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=blob_plain;f=include/sys/socket.h;hb=HEAD) - [RUST: cmsghdr](https://docs.rs/libc/latest/libc/struct.cmsghdr.html)

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2026年01月05日 13:09:52 +01:00

ElSebas41 commented

2026年07月05日 18:53:47 +02:00

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I am working with unix sockets for one of my projects and I encountered this problem.
In Zig 0.16 the Io operations moved to the Io interface. The Io.net.UnixAddress struct has the method connect which gives you a Io.net.Stream struct wich represents the connection and has a reader and writer.

The problem is that the reader and writer both use the functions io.vtable.netRead and io.vtable.netWrite which doesn't manage the ancillary data at all. So now you must use the io.vtable functions that manage this ancillary data, manually.

In this case those functions are io.vtable.netSend and io.vtable.netOperate with the operation net_receive. Both sends and receive the data using the structs Io.net.OutgoingMessage and Io.net.IncomingMessage respectively.

These structs are not exactly posix.msghdr so is impossible to use the macros imported from <sys/socket.h> to iterate safely over the control: []u8 field inside Io.net.IncomingMessage and Io.net.OutgoingMessage.

I am thinking of implementing some sort of cross platform iterator which takes the control field of these structs, iterates on it safely, aligning memory correctly, bounds check, etc. And gives you a struct ControlMessage whith this data:

constControlMessage=struct{header:std.c.cmsghdr,data:[]u8};

This is only a fast thought implementation. We could discuss how to really implement this feature.
Also I can be the one implementing this too.

I am working with unix sockets for one of my projects and I encountered this problem. In Zig 0.16 the Io operations moved to the `Io` interface. The `Io.net.UnixAddress` struct has the method `connect` which gives you a `Io.net.Stream` struct wich represents the connection and has a reader and writer. The problem is that the reader and writer both use the functions `io.vtable.netRead` and `io.vtable.netWrite` which doesn't manage the ancillary data at all. So now you must use the `io.vtable` functions that manage this ancillary data, manually. In this case those functions are `io.vtable.netSend` and `io.vtable.netOperate` with the operation `net_receive`. Both sends and receive the data using the structs `Io.net.OutgoingMessage` and `Io.net.IncomingMessage` respectively. These structs are not exactly `posix.msghdr` so is impossible to use the macros imported from `<sys/socket.h>` to iterate safely over the `control: []u8` field inside `Io.net.IncomingMessage` and `Io.net.OutgoingMessage`. I am thinking of implementing some sort of cross platform iterator which takes the `control` field of these structs, iterates on it safely, aligning memory correctly, bounds check, etc. And gives you a struct `ControlMessage` whith this data: ```zig const ControlMessage = struct { header: std.c.cmsghdr, data: []u8 }; ``` This is only a fast thought implementation. We could discuss how to really implement this feature. Also I can be the one implementing this too.

ElSebas41 commented

2026年07月06日 22:40:52 +02:00

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Sorry I looked better at the standard library and the Io.net.Socket struct has the methods sendMany and receive/receiveManyTimeout so there's not only the raw io.vtable.netSend and io.vtable.netOperate functions.

But the problem is still the same. Neither Io.net.IncomingMessage nor Io.net.OutgoingMessage has any way to handle their control field.

Sorry I looked better at the standard library and the `Io.net.Socket` struct has the methods `sendMany` and `receive`/`receiveManyTimeout` so there's not only the raw `io.vtable.netSend` and `io.vtable.netOperate` functions. But the problem is still the same. Neither `Io.net.IncomingMessage` nor `Io.net.OutgoingMessage` has any way to handle their `control` field.

ElSebas41 referenced this issue

2026年07月08日 22:04:26 +02:00

Io.net: IncomingMessage and OutgoingMessage control fields lack cmsghdr alignment #36096

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Implement CMSG_*-equivalent functionality to std.c, enabling the effective use of std.c.cmsghdr. #30629

Some further reading:

Implement `CMSG_*`-equivalent functionality to `std.c`, enabling the effective use of `std.c.cmsghdr`. #30629