Re: confusing messages from clang

[Dimitry Andric]

On 20 Feb 2016, at 08:33, Alex Denisov <1101.deb...@gmail.com> wrote:
>> On 20 Feb 2016, at 01:57, Steve Kargl  
>> wrote:
>> 
>> If anyone is interesting fixing FreeBSD's C compiler, it
>> would be appreciated.
...
>> foo.c:21:1: error: use of undeclared identifier 'corrupt'; did you mean 
>> 'crypt'?
>> corrupt:
>> ^~~
>> crypt
>> /usr/include/unistd.h:450:7: note: 'crypt' declared here
>> char*crypt(const char *, const char *);
>>^
>> foo.c:21:8: error: expected ';' after return statement
>> corrupt:
>>  ^
>>  ;
>> foo.c:14:12: error: use of undeclared label 'corrupt'
>> goto corrupt;
>>  ^
...
> I’ve submitted the bug to LLVM’s bugzilla: 
> https://llvm.org/bugs/show_bug.cgi?id=26678
> 
> Also, it looks like OS agnostic. At least I see the same behaviour on OSX as 
> well.

Yes, the messages could definitely be improved.  But for a compiler, it
is sometimes hard to guess the 'intent' of the code, though. :)

Note that gcc (I used 6.0 from ports) gives similarly confused messages:

foo.c: In function 'foo':
foo.c:21:1: error: 'corrupt' undeclared (first use in this function)
 corrupt:
 ^~~
foo.c:21:1: note: each undeclared identifier is reported only once for each 
function it appears in
foo.c:21:1: warning: 'return' with a value, in function returning void
foo.c:5:1: note: declared here
 foo(int i)
 ^~~
foo.c:21:8: error: expected ';' before ':' token
 corrupt:
^
foo.c:14:6: error: label 'corrupt' used but not defined
  goto corrupt;
  ^~~~

In gcc's case, it appears to read the statement as "return corrupt:",
causing it to first complain about the undeclared identifier, and then
about a missing semicolon.  Later, the label is of course still not
defined, so it complains again.

-Dimitry



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Re: confusing messages from clang

[David Chisnall]

C compilers are always doing best effort attempts to report when you feed them 
code that is not valid C.

For example, in this case:

On 20 Feb 2016, at 00:57, Steve Kargl  wrote:
>   if (i > 0)
>  goto corrupt;

This is valid, as long as you have a label called corrupt to look for.  You do 
not, however, because:

>   return;
> 
> whoops:
>   printf("whoops\n");
>   return
> 
> corrupt:
>   printf("corrupt\n”);

The statement:

> return corrupt: printf("corrupt\n");


is just confusing.  It appears to be trying to return the value in corrupt 
(which is not an identifier that corresponds to any valid variable) and then 
has some trailing characters after the end of the statement.  Fortunately, the 
compiler tells you exactly what is wrong:

First it says:

> foo.c:21:1: error: use of undeclared identifier 'corrupt'; did you mean 
> 'crypt'?
> corrupt:
> ^~~

Here, it is telling you that the value passed to your return statement is an 
undeclared identifier.  Then it tells you that you have more tokens after the 
end of your return statement:

> foo.c:21:8: error: expected ';' after return statement
> corrupt:
>   ^
>   ;

I am slightly surprised that there’s no warning that a return statement with a 
value is invalid in a function that returns void, but perhaps that’s because 
after finding two things wrong with one statement it gives up.

The correct fix, of course, is to insert the missing semicolon after the return 
at the end of line 19.  If you had tried compiling the same thing with gcc 5, 
then you would have noticed that you get very similar error messages (though 
gcc doesn’t attempt to provide a fixit hint and does warn that you have a 
return statement returning a value from a function that returns void).

David

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Re: confusing messages from clang

[Alex Denisov]

Hi Steve,

I’ve submitted the bug to LLVM’s bugzilla: 
https://llvm.org/bugs/show_bug.cgi?id=26678

Also, it looks like OS agnostic. At least I see the same behaviour on OSX as 
well.
--
AlexDenisov
Software Engineer, http://lowlevelbits.org

> On 20 Feb 2016, at 01:57, Steve Kargl  
> wrote:
> 
> If anyone is interesting fixing FreeBSD's C compiler, it
> would be appreciated.
> 
> % cat foo.c
> #include 
> #include 
> 
> void
> foo(int i)
> {
>   if (i < 0)
>  goto whoops;
> 
>   if (i == 0)
>  printf("foo\n");
> 
>   if (i > 0)
>  goto corrupt;
>   return;
> 
> whoops:
>   printf("whoops\n");
>   return
> 
> corrupt:
>   printf("corrupt\n");
> }
> 
> % cc -c foo.c
> foo.c:21:1: error: use of undeclared identifier 'corrupt'; did you mean 
> 'crypt'?
> corrupt:
> ^~~
> crypt
> /usr/include/unistd.h:450:7: note: 'crypt' declared here
> char*crypt(const char *, const char *);
> ^
> foo.c:21:8: error: expected ';' after return statement
> corrupt:
>   ^
>   ;
> foo.c:14:12: error: use of undeclared label 'corrupt'
>  goto corrupt;
>   ^
> 3 errors generated.
> 
> 
> No, I did not mean crypt.  'corrupt' is a defined label.
> The missing semicolon occurs in line 19, but the line is
> not properly identified in error output.
> 
> --
> Steve
> ___
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confusing messages from clang

[Steve Kargl]

If anyone is interesting fixing FreeBSD's C compiler, it
would be appreciated.

% cat foo.c
#include 
#include 

void
foo(int i)
{
   if (i < 0)
  goto whoops;

   if (i == 0)
  printf("foo\n");
   
   if (i > 0)
  goto corrupt;
   return;

whoops:
   printf("whoops\n");
   return

corrupt:
   printf("corrupt\n");
}

% cc -c foo.c
foo.c:21:1: error: use of undeclared identifier 'corrupt'; did you mean 'crypt'?
corrupt:
^~~
crypt
/usr/include/unistd.h:450:7: note: 'crypt' declared here
char*crypt(const char *, const char *);
 ^
foo.c:21:8: error: expected ';' after return statement
corrupt:
   ^
   ;
foo.c:14:12: error: use of undeclared label 'corrupt'
  goto corrupt;
   ^
3 errors generated.


No, I did not mean crypt.  'corrupt' is a defined label.
The missing semicolon occurs in line 19, but the line is
not properly identified in error output.

-- 
Steve
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