From d76a3bf5c46d9e981faf09e6194c795ee54e5ae4 Mon Sep 17 00:00:00 2001 From: Markus Armbruster Date: Wed, 3 Feb 2016 19:03:48 +0100 Subject: HACKING: Add a section on error handling and reporting MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Inspired by an RFC PATCH from Lluís Vilanova. Signed-off-by: Markus Armbruster Message-Id: <1454522628-28294-3-git-send-email-armbru@redhat.com> Reviewed-by: Lluís Vilanova --- HACKING | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) (limited to 'HACKING') diff --git a/HACKING b/HACKING index 12fbc8afe4..058aa8fd49 100644 --- a/HACKING +++ b/HACKING @@ -157,3 +157,58 @@ painful. These are: * you may assume that integers are 2s complement representation * you may assume that right shift of a signed integer duplicates the sign bit (ie it is an arithmetic shift, not a logical shift) + +7. Error handling and reporting + +7.1 Reporting errors to the human user + +Do not use printf(), fprintf() or monitor_printf(). Instead, use +error_report() or error_vreport() from error-report.h. This ensures the +error is reported in the right place (current monitor or stderr), and in +a uniform format. + +Use error_printf() & friends to print additional information. + +error_report() prints the current location. In certain common cases +like command line parsing, the current location is tracked +automatically. To manipulate it manually, use the loc_*() from +error-report.h. + +7.2 Propagating errors + +An error can't always be reported to the user right where it's detected, +but often needs to be propagated up the call chain to a place that can +handle it. This can be done in various ways. + +The most flexible one is Error objects. See error.h for usage +information. + +Use the simplest suitable method to communicate success / failure to +callers. Stick to common methods: non-negative on success / -1 on +error, non-negative / -errno, non-null / null, or Error objects. + +Example: when a function returns a non-null pointer on success, and it +can fail only in one way (as far as the caller is concerned), returning +null on failure is just fine, and certainly simpler and a lot easier on +the eyes than propagating an Error object through an Error ** parameter. + +Example: when a function's callers need to report details on failure +only the function really knows, use Error **, and set suitable errors. + +Do not report an error to the user when you're also returning an error +for somebody else to handle. Leave the reporting to the place that +consumes the error returned. + +7.3 Handling errors + +Calling exit() is fine when handling configuration errors during +startup. It's problematic during normal operation. In particular, +monitor commands should never exit(). + +Do not call exit() or abort() to handle an error that can be triggered +by the guest (e.g., some unimplemented corner case in guest code +translation or device emulation). Guests should not be able to +terminate QEMU. + +Note that &error_fatal is just another way to exit(1), and &error_abort +is just another way to abort(). -- cgit v1.2.1