Re: [Xen-devel] Error reporting capabilities for libxc

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Tue, Sep 26, 2006 at 01:40:01PM -0500, Hollis Blanchard wrote:
> On Fri, 2006-09-22 at 21:31 +0100, Daniel P. Berrange wrote:
> > So, I've prototyped a simple error reporting mechanism for libxc. The idea
> > is we first define an enum for the broad classes of errors which can occur.
> > As proof of concept I've defined one generic error code, and a special one
> > for invalid kernels
> > 
> >  typedef enum {
> >     XC_ERROR_NONE = 0,
> >     XC_INTERNAL_ERROR = 1,
> >     XC_INVALID_KERNEL = 2,
> >  } xc_error_code;
> > 
> > Next, I created an internal function  for setting the error code, and
> > providing an associated descritive message
> > 
> >   void xc_set_error(int code, const char *fmt, ...);
> > 
> > And simply re-defined the existing ERROR & PERROR macros to use this 
> > function passing the generic XC_INTERNAL_ERROR
> > 
> >  #define ERROR(_m, _a...) xc_set_error(XC_INTERNAL_ERROR, _m, ## _a)
> > 
> >  #define PERROR(_m, _a...) xc_set_error(XC_INTERNAL_ERROR, "%s (%d = %s)", 
> > _m, errno, strerror(errno))
> > 
> > In the various places which deal with validating the guest kernel I
> > changed calls to ERROR to call xc_set_error(XC_INVALID_KERNEL,...)
> > instead. We can define further error codes over time to reduce use
> > fo the genreic XC_INTERNAL_ERROR as desired.
> > 
> > In the public API defined a way to either fetch the last error, or
> > register a callback for receving errors:
> > 
> >   int xc_get_last_error_code(void);
> >   const char *xc_get_last_error_message(void);
> >   void xc_clear_last_error(void);
> >   const char *xc_error_code_to_desc(int code);
> > 
> >   typedef void (*xc_error_handler)(int code, const char *msg);
> >   void xc_default_error_handler(int code, const char *msg);
> >   xc_error_handler xc_set_error_handler(xc_error_handler handler);
> 
> Isn't the whole idea of "get_last_error" racy by design? I guess errno
> has the same problem, but I'm not quite clear how it's solved there (a
> magic array of errnos that is somehow sized to include all threads?)

Yeah, the libc errno  is kept in thread local storage, so we'll have to
do the same thing here to be thread-safe. The async callback function is of
course always thread safe, but a little more inconvient for the caller to
use so I wanted to allow the direct access too. 

Regards,
Dan.
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