cgo: support second return value (errno) #4585

aykevl · 2024-11-06T14:07:46Z

This adds support for code like this:

val, err := C.some_func()
if val == -1 {
    println("failed:", err.Error())
}

That is, the second return value is the errno value (a syscall.Errno value) after the call. Because of how C errno works, you can't just check errno: you have to check the normal return value to know whether the call succeeded or not, and only use errno when you know it failed and was therefore set by the call.

To make this work, I also had to implement some CGo features that weren't implemented yet (mainly for WASI which likes to use rather indirect errno values). I also changed the syscall package for wasm/js because the errno values in there didn't actually match the values in wasi-libc.

The reason I did this was to be able to run tinygo test os/syscall. That test needs some more CGo stuff but this is one of the prerequisites.

github-actions · 2024-11-06T14:25:14Z

Size difference with the dev branch:

Binary size difference

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  85924   85924      0   0.00%    5140    5140      0   0.00% tinygo build -size short -o ./build/test.hex -target=challenger-rp2040 ./examples/net/ntpclient/
 296644  296644      0   0.00%   13788   13788      0   0.00% tinygo build -size short -o ./build/test.hex -target=pyportal -stack-size 8kb ./examples/net/http-get/
 119376  119376      0   0.00%    7848    7848      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-nano33 -stack-size 8kb ./examples/net/tcpclient/
 245516  245516      0   0.00%   10488   10488      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/websocket/dial/
 103112  103112      0   0.00%    9816    9816      0   0.00% tinygo build -size short -o ./build/test.hex -target=metro-m4-airlift -stack-size 8kb ./examples/net/socket/
 338452  338452      0   0.00%   15396   15396      0   0.00% tinygo build -size short -o ./build/test.hex -target=matrixportal-m4 -stack-size 8kb ./examples/net/webstatic/
 111588  111588      0   0.00%    7772    7772      0   0.00% tinygo build -size short -o ./build/test.hex -target=arduino-mkrwifi1010 -stack-size 8kb ./examples/net/tlsclient/
 154216  154216      0   0.00%    6540    6540      0   0.00% tinygo build -size short -o ./build/test.hex -target=nano-rp2040 -stack-size 8kb ./examples/net/mqttclient/natiu/
 117048  117048      0   0.00%   13132   13132      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webclient/
 286136  286136      0   0.00%   17812   17812      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/webserver/
 287132  287132      0   0.00%   18212   18212      0   0.00% tinygo build -size short -o ./build/test.hex -target=wioterminal -stack-size 8kb ./examples/net/mqttclient/paho/
5332918 5332918      0   0.00%  811790  811790      0   0.00%

aykevl · 2024-11-06T14:48:19Z

Looks like I need to fix some issues in the cgo tests.

dgryski · 2024-11-06T20:23:57Z

Does this increase or decrease our upstream Go compatibility?

aykevl · 2024-11-07T07:27:54Z

Increase, because Go also supports this feature.
From the docs:

Any C function (even void functions) may be called in a multiple assignment context to retrieve both the return value (if any) and the C errno variable as an error (use _ to skip the result value if the function returns void). For example:
n, err = C.sqrt(-1)
_, err := C.voidFunc()
var n, err = C.sqrt(1)

(We don't support the void return value yet, but if the C function returns a value it should now match upstream Go).

aykevl · 2024-11-07T07:51:35Z

The testing failure with the cgo package should be fixed now.

aykevl · 2024-11-07T15:25:21Z

There are some more errors. Should be easy to fix, I'll do that another day.

aykevl · 2024-11-08T14:48:26Z

From CI:

--- expected
+++ actual
@@ -75,8 +75,8 @@
 len(C.GoBytes(nil, 0)): 0
 len(C.GoBytes(C.CBytes(nil),0)): 0
 rountrip CBytes: hello
-EINVAL: true
-EAGAIN: true
+EINVAL: false
+EAGAIN: false
 copied string: foobar
 CGo sqrt(3): +1.732051e+000
 C   sqrt(3): +1.732051e+000

Well that's just great. The reason is that while mingw-w64 defines actual error values, the Go stdlib syscall package fakes these error values on Windows. I wonder what's up with that.

aykevl · 2024-11-08T15:01:10Z

Apparently it's a known issue in the Go toolchain as well: golang/go#23468

I guess I can fix it by modifying __get_errno to map the C errno value to the fake syscall.Errno value on Windows.

eliasnaur

Comment from a cursory review.

eliasnaur · 2024-11-09T14:52:51Z

cgo/libclang.go

+			sourceBuf.WriteString(spelling)
+		}
+	}
+	C.clang_disposeTokens(tu, rawTokens, numTokens)


Nit: convert to earlier defer?

aykevl · 2024-11-11T08:47:47Z

Updated, hopefully this makes most errno values on Windows actually work.

aykevl · 2024-11-15T10:52:09Z

This PR would be useful to simplify #4603.

dgryski · 2024-11-15T18:42:07Z

This is still failing CI with what looks like legit errors.

aykevl · 2024-11-16T09:28:35Z

Yes, I need to fix that. Specifically, I think it's the third commit (syscall: use wasi-libc tables for wasm/js target).
I extracted #4607 to make this PR smaller and easier to debug.

aykevl · 2024-11-18T10:58:21Z

Also extracted that third commit into #4611

Making this work on all targets was interesting but there's now a test in place to make sure this works on all targets that have the CGo test enabled (which is almost all targets).

aykevl · 2024-11-19T06:54:55Z

Rebased so it's just the addition of the errno return value.

dgryski

LGTM

aykevl force-pushed the cgo-errno-return branch from 3d92f43 to 5e678b6 Compare November 7, 2024 07:51

aykevl force-pushed the cgo-errno-return branch from 5e678b6 to 0c78bd3 Compare November 8, 2024 10:02

eliasnaur reviewed Nov 9, 2024

View reviewed changes

aykevl force-pushed the cgo-errno-return branch 2 times, most recently from 44231e3 to 8a7017c Compare November 11, 2024 08:47

aykevl force-pushed the cgo-errno-return branch from 8a7017c to 5617dc8 Compare November 15, 2024 11:02

aykevl mentioned this pull request Nov 16, 2024

cgo: support more macros #4607

Merged

aykevl mentioned this pull request Nov 18, 2024

syscall: refactor wasm environment variables and constants #4611

Merged

cgo: support errno value as second return parameter

c6c9e0a

Making this work on all targets was interesting but there's now a test in place to make sure this works on all targets that have the CGo test enabled (which is almost all targets).

aykevl force-pushed the cgo-errno-return branch from 5617dc8 to c6c9e0a Compare November 19, 2024 06:54

dgryski approved these changes Nov 19, 2024

View reviewed changes

aykevl merged commit 6593cf2 into dev Nov 20, 2024
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aykevl deleted the cgo-errno-return branch November 20, 2024 06:54

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cgo: support second return value (errno) #4585

cgo: support second return value (errno) #4585

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cgo: support second return value (errno) #4585

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