Negative numbers work fine when the return value is an int. But what do
you do when returning a pointer? Either you have an integer return value
and a pointer to pointer as an output parameter, or you return NULL and
indicate the error via errno
Eric Baldeschwieler wrote:
I'd vote against errno, because I don't see why we need it. Why not
just return the error as a negative number? Adding a global just
complicates the code and introduces an opportunity for further error.
What am I missing?
On May 2, 2006, at 11:19 PM, Devaraj Das wrote:
In our case, the components involved are the C API library, JNI layer
and
Java APIs. In all these, we have control over errno. For example, if a
particular C API uses a third party library function that might
return error
and hence set errno, we know about it already. Depending on the
error, we
take a decision whether to proceed further in the API implementation
code or
return an error to the client invoking the API. This includes the
functions
in the JNI library which the API implementation calls. In the Java
world, we
deal with exceptions and don't bother about errno. So for example, if
a Java
method, invoked through JNI from a C API, throws an exception, the C API
implementation will get the exception object and depending on that
the API
implementation will set a meaningful errno and return a (-1) or NULL to
signify that an error occurred. As I said earlier, this includes the
case
where the JNI function itself fails (for some reason like out-of-
memory or
something).
As an aside, the JNI layer doesn't generate errno-s.
-----Original Message-----
From: Konstantin Shvachko [mailto:[EMAIL PROTECTED]
Sent: Wednesday, May 03, 2006 2:40 AM
To: [email protected]
Subject: Re: C API for Hadoop DFS
Don't think errno is a particularly good idea for several reasons.
It is not common to set errno codes.
If a system library function uses errno, and we overwrite its value to
return
something dfs related, the library function behavior becomes
unpredictable.
This could be hard to debug.
We have a JNI layer between our C library and Java, which also might
generate
errno-s overwriting the values we were trying to bring back from Java.
--Konstantin
Doug Cutting wrote:
The spec says:
/** All APIs set errno to meaningful values */
So callers should always check errno after each call. Whether this is
the best way to handle errors in C can be debated, but an error
mechanism was in fact specified.
Doug
Konstantin Shvachko wrote:
I think this a very important issue raised by David.
IMO __ALL__ functions should return an integer value indicating
success (=0) or failure (<0).
Unless we want to use C style Exceptions, otherwise we won't be able
to identify what went
wrong if anything.
NULL or bool is not enough in most cases, since we need to
distinguish e.g. between
timeout (when we retry) and "file not found" cases.
The actual return objects should be passed as outputs parameters.
E.g.
dfsFS dfsConnect(char *host, tPort port);
will become
tCompletionCode dfsConnect(char *host, tPort port, dfsFS fileSystem );
where tCompletionCode could be integer for now. Or we can define a
structure
{ int errCode; char *errDescription; }
to return the actual error descriptions along with the error code.
--Konstantin
Devaraj Das wrote:
Do dfsConnect and dfsOpenFile return NULL on failure?
Yes.
Shouldn't dfsSeek, dfsRename, dfsCreateDirectory and
dfsSetWorkingDirectory each have a return value to indicate success
or failure? Or are they assumed to never fail?
Yes these functions should have return values. I will update the API
spec.
Thanks for pointing this out.
-----Original Message-----
From: David Bowen [mailto:[EMAIL PROTECTED] Sent: Monday, May
01, 2006 8:13 AM
To: [email protected]
Subject: Re: C API for Hadoop DFS
I'm curious about error handling.
Do dfsConnect and dfsOpenFile return NULL on failure?
Shouldn't dfsSeek, dfsRename, dfsCreateDirectory and
dfsSetWorkingDirectory each have a return value to indicate success
or failure? Or are they assumed to never fail?
- David
