Dear Gophers I am running into a a weird error in C code that only materializes when calling it from Go using cgo. I hope you could help me understand this behaviour and why calling runtime.Gosched() seems to resolve the issue. I worry that this may not be the right fix, so I want to understand what is going on.
I communicate to a USB HID device and get an error every once in a while, only when using cgo, in around 5% of the messages sent to the device, non-deterministically. The messages sent and received are all the same and work most of the time. The C functions I am calling are hid_write and hid_read from here: https://github.com/libusb/hidapi/blob/4ebce6b5059b086d05ca7e091ce04a5fd08ac3ac/mac/hid.c If I call hid_write and hid_read in a loop in a C program, everything works as expected: ``` #include "hidapi.h" #include "hidapi_darwin.h" int run(hid_device *handle) { int res; const uint8_t write[64] = "<some 64 byte msg>"; const uint8_t expected[64] = "<expected 64 byte response>"; uint8_t read[64] = {0}; while(1) { res = hid_write(handle, write, sizeof(write)); if(res < 0) return -1; res = hid_read(handle, read, sizeof(read)); if(res < 0) return -1; if (memcmp(read, expected, 64)) return -1; } return 0; } ``` I ported the above to Go using cgo like below. I link `hid.o`, the same object file used in the C program, to make sure the same code is running to rule out differences in compilation: ``` package main import ( "bytes" "encoding/hex" ) /* #cgo darwin LDFLAGS: -framework IOKit -framework CoreFoundation -framework AppKit hid.o #include "hidapi.h" #include "hidapi_darwin.h" */ import "C" func main() { dev := C.hid_open(...) if dev == nil { panic("no device") } write := []byte("<some 64 byte msg>") expected := []byte("<expected 64 bytes response>") b := make([]byte, 64) for { written := C.hid_write(dev, (*C.uchar)(&write[0]), C.size_t(len(write))) if written < 0 { panic("error write") } read := C.hid_read(dev, (*C.uchar)(&b[0]), C.size_t(len(b))) if read < 0 { panic("error read") } if !bytes.Equal(b, expected) { panic("not equal") } } } ``` The Go program errors on hid_write with a "generic error" error code non-deterministically in about 5% of the messages sent. The USB device receives the message and responds normally however. I randomly tried adding `runtime.Gosched()` at the bottom of the Go-loop, and the issue disappeared completely. I am very confused why that would help, as the Go program has, as far as I can tell, no threads and no goroutines (except main). Other things I have checked: - If I move the loop from Go to C by calling `C.run(dev)` (the looping function defined above) from Go, there is no issue. - LockOSThread: if the loop runs in a goroutine and that goroutine switches OS threads, the issue reappears after some time (not immediately after a thread switch - the error happens non-deterministically) even if `runtime.Gosched()` is called. `runtime.LockOSThread()` is needed to fix it in that case. Since the goroutine is locked to an OS thread during the execution of a C function call anyway, this indicates that either hidapi or the macOS HID functions rely on thread-local vars across multiple C calls in some way, which seems a bit crazy. - In the above code (no goroutines), I checked that the OS thread ID (pthread_self()) is constant for all calls, and yet the issue appears unless runtime.Gosched() is called, which seems to contradict the previous point - I tried multiple Go versions between 1.16 and 1.20 and multiple macOS SDK versions between 10.13 and 13.1, all with the same problem (and same working fix). - only macOS is affected - on linux and Windows, there is no similar issue (these platforms use different C code to interact with the device). Does anyone have any insight into why invoking the scheduler could be necessary here or what could be going on in general? My worry is that using `runtime.LockOSThread()` and `runtime.Gosched()` are not proper fixes. Best, Marko -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAGgfmPPpi9SXtuKn%3DBmVzM%3D8_%3Dc6-F7G-k41wyUGEQ9UBHs-rQ%40mail.gmail.com.
