I am currently wrapping a proprietary library with cgo.
About error reporting, the library documentation says "Every function that
can fail returns at least boolean or pointer value that can be used to
detect error state (false and NULL means error, every other value means
success). If such error happens, details about it can be queried using
thelib_error(). The error information is thread-local."
So when calling a function from the library, in C I write about the
following code:
bool ret = thelib_dosomework(...);
if (!ret) {
// call has to be in same thread as call to thelib_dosomework() above
int errcode = thelib_error(buf, BUF_SIZ, null);
fprintf(stderr, "error %d: %s\n", errcode, buf);
return false;
}
My current approach to call a C function from the library and get the
corresponding error information looks like this:
func callIntoTheLib() error {
runtime.LockOSThread()
defer runtime.UnlockOSThread()
ret := C.thelib_dosomework(...)
if !ret {
return extractErrorFromTheLibError()
}
return nil
}
func extractErrorFromTheLibError() error {
var buf [1024]byte
var outsize C.size_t
errcode := C.thelib_error((*C.char)(unsafe.Pointer((&errbuf[0]))), C.
size_t(len(errbuf)), &outsize)
// not wrapping in struct type for brevity
return fmt.Errorf("error %d: %s", errcode, buf[:outsize])
}
Is this sensible? Looking at the runtime implementation,
LockOSThread/UnlockOSThread seem to be really simple as the only save a
couple of words in goroutine storage. I don't mind about using up an OS
thread as there will not be a large number of concurrent calls into the
library. Also, calls into C use up one thread anyway and the majority of
time will be spent in the payload call, not in thelib_error(), which is a
short and fast function.
Another approach I was thinking about was to copy the error message and
code to memory in C code, but with this approach I always have to have a
preallocated buffer for an error that might not happen.
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