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error: Improve error.h's big comment

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Add headlines to the big comment.

Explain examples for NULL, &error_abort and &error_fatal argument
better.

Tweak rationale for error_propagate_prepend().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200707160613.848843-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
This commit is contained in:
Markus Armbruster 2020-07-07 18:05:30 +02:00
parent 47ff5ac81e
commit 9aac7d486c
1 changed files with 36 additions and 15 deletions
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51
include/qapi/error.h
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@ -15,6 +15,8 @@
/*
* Error reporting system loosely patterned after Glib's GError.
*
* = Creating errors =
*
* Create an error:
* error_setg(&err, "situation normal, all fouled up");
*
@ -27,6 +29,8 @@
* error_setg(&err, "invalid quark\n" // WRONG!
* "Valid quarks are up, down, strange, charm, top, bottom.");
*
* = Reporting and destroying errors =
*
* Report an error to the current monitor if we have one, else stderr:
* error_report_err(err);
* This frees the error object.
@ -40,6 +44,30 @@
* error_free(err);
* Note that this loses hints added with error_append_hint().
*
* Call a function ignoring errors:
* foo(arg, NULL);
* This is more concise than
* Error *err = NULL;
* foo(arg, &err);
* error_free(err); // don't do this
*
* Call a function aborting on errors:
* foo(arg, &error_abort);
* This is more concise and fails more nicely than
* Error *err = NULL;
* foo(arg, &err);
* assert(!err); // don't do this
*
* Call a function treating errors as fatal:
* foo(arg, &error_fatal);
* This is more concise than
* Error *err = NULL;
* foo(arg, &err);
* if (err) { // don't do this
* error_report_err(err);
* exit(1);
* }
*
* Handle an error without reporting it (just for completeness):
* error_free(err);
*
@ -47,6 +75,11 @@
* reporting it (primarily useful in testsuites):
* error_free_or_abort(&err);
*
* = Passing errors around =
*
* Errors get passed to the caller through the conventional @errp
* parameter.
*
* Pass an existing error to the caller:
* error_propagate(errp, err);
* where Error **errp is a parameter, by convention the last one.
@ -54,11 +87,10 @@
* Pass an existing error to the caller with the message modified:
* error_propagate_prepend(errp, err,
* "Could not frobnicate '%s': ", name);
*
* Avoid
* error_propagate(errp, err);
* This is more concise than
* error_propagate(errp, err); // don't do this
* error_prepend(errp, "Could not frobnicate '%s': ", name);
* because this fails to prepend when @errp is &error_fatal.
* and works even when @errp is &error_fatal.
*
* Create a new error and pass it to the caller:
* error_setg(errp, "situation normal, all fouled up");
@ -70,15 +102,6 @@
* handle the error...
* }
*
* Call a function ignoring errors:
* foo(arg, NULL);
*
* Call a function aborting on errors:
* foo(arg, &error_abort);
*
* Call a function treating errors as fatal:
* foo(arg, &error_fatal);
*
* Receive an error and pass it on to the caller:
* Error *err = NULL;
* foo(arg, &err);
@ -86,8 +109,6 @@
* handle the error...
* error_propagate(errp, err);
* }
* where Error **errp is a parameter, by convention the last one.
*
* Do *not* "optimize" this to
* foo(arg, errp);
* if (*errp) { // WRONG!