A few Qs on the -Wfoo options

A few Qs on the -Wfoo options

[ballsystemlord]

-Wreal-q-constant   What's a 'q' exponent-letter?

-Wline-truncation    What is truncation of source lines?
-Winvalid-offsetof   What/how do you offset of macro and how does this
affect anything?
-Wsurprising           Could you explain what "suspicious" constructs
are/look like? What's the criteria?

-Wfunction-elimination  shouldn't it be noticeable when I am making a
useless function call?

-Wimplicit-interface         What's an implicit-interface?
-Wimplicit-procedure        What's a procedures?
-Winvalid-memory-model    atomic memory model?
-Warray-temporariesarray   What temporaries?
-Wcharacter-truncation     How can a character expression get truncated?
-Wcast-align                   Why's that a problem?

-Wcoverage-mismatch note this may result in poorly optimized code. Why
poorly??

-Wlogical-op   Why would it give false positives? What's the criteria under
which it
makes these assumptions?


-Wmudflap  mudflap?


-Wsuggest-attribute=[pure|const|noreturn]
const? pure? noreturn? How can functions be of this type?


-Wstack-protector how can a function get smashed?





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Re: A few Qs on the -Wfoo options

[Ian Lance Taylor]

On Wed, Nov 20, 2013 at 11:46 AM, ballsystemlord <doark@mail.com> wrote:
> -Wreal-q-constant   What's a 'q' exponent-letter?

That is a Fortran option.  Did you check the Fortran documentation?

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gfortran/Error-and-Warning-Options.html

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gfortran/_003ccode_003eQ_003c_002fcode_003e-exponent_002dletter.html


> -Wline-truncation    What is truncation of source lines?

This is also a Fortran option and has to do with the way Fortran
works.

> -Winvalid-offsetof   What/how do you offset of macro and how does this
> affect anything?

http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Warning-Options.html


I'm not going to answer any of your questions.  Please look at the
docs and come back if you still have questions.

Ian

Re: A few Qs on the -Wfoo options

[ballsystemlord]

Ok, I've reread them. I could not find the definition of some of them because
they are only mentioned int the info manual under their '--no-'
counterparts, so when I searched for them with grep (because I did not
remember seeing them before because they were not there,) nothing came up.
I'm still interested in learning a little more about a few though.

-Wimplicit-interface               What's an explicit interface?
-Wimplicit-procedure             What's an explicit interface?
-Winvalid-memory-model      I could not find it mentioned in the info
manual.
-Wcharacter-truncation         "Warn when a character assignment will
truncate the assigned string." How can a string be truncated?
-Wlogical-op                        Why would it give false positives?
What's the criteria under which it makes these assumptions?
-Wstack-protector                 How can a function get smashed?




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Re: A few Qs on the -Wfoo options

[ballsystemlord]

Ok, I've reread the docs. I had not noticed some of the entry before because
I did not read the sections on FORTRAN and C++. I also greped to info manual
but most of the options only had documentation for there '--no-'
counterparts.



-Wline-truncation           How can it be truncated?
-Wimplicit-interface        What's an explicit interface?
-Wimplicit-procedure        What's an explicit interface?
-Winvalid-memory-model      I could not find it mentioned in the info
manual.
-Wcharacter-truncation      "Warn when a character assignment will truncate
the assigned string." How/why would a string be truncated?
-Wlogical-op                Why would it give false positives? What's the
criteria under which it makes these assumptions?
-Wstack-protector           How can a function get smashed? 



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Re: A few Qs on the -Wfoo options

[ballsystemlord]

Thanks, that clears it all up!



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Re: A few Qs on the -Wfoo options

[Ángel González]

On 17/12/13 18:43, ballsystemlord wrote:
> Ok, I've reread the docs. I had not noticed some of the entry before because
> I did not read the sections on FORTRAN and C++. I also greped to info manual
> but most of the options only had documentation for there '--no-'
> counterparts.
>
>
>
> -Wline-truncation           How can it be truncated?
Code of Fortran programs is truncated at a number of characters. Everything
after col 72 is a comment (there are options to change this limit, see 
-*ffixed-line-length*).

https://www.gnu.org/software/emacs/manual/html_node/emacs/Fortran-Columns.html

> -Wimplicit-interface        What's an explicit interface?
One defined between 'interface' and 'end interface'? These are more 
language questions than
compiler doubts (fortran, in this case)

> -Wimplicit-procedure        What's an explicit interface?
> -Winvalid-memory-model      I could not find it mentioned in the info
> manual.
Memory models are explained at 
http://gcc.gnu.org/onlinedocs/gcc-4.7.1/gcc/_005f_005fatomic-Builtins.html

> -Wcharacter-truncation      "Warn when a character assignment will truncate
> the assigned string." How/why would a string be truncated?
If you store into a variable able to store up to n characters data from 
another variable which can hold up to m, with m > n.

> -Wlogical-op                Why would it give false positives? What's the
> criteria under which it makes these assumptions?
It can give false positives if it's indeed what the author wanted. Note 
how in this
option it is trying to outsmart the programmer by noticing that he intended
to do something different than what he stated.
A example warned by -Wlogical-op would be: «if (var > 1 || var < 5) {»
Since var (assumed to be an int variable) will always be greater than 1 or
less than 5, there's something fishy with that if.

> -Wstack-protector           How can a function get smashed?
'-Wstack-protector'
      This option is only active when '-fstack-protector' is active.  (...)

So you go to:

'-fstack-protector'
      Emit extra code to check for buffer overflows, such as stack
      smashing attacks.  This is done by adding a guard variable to
      functions with vulnerable objects.  This includes functions that
      call 'alloca', and functions with buffers larger than 8 bytes.  The
      guards are initialized when a function is entered and then checked
      when the function exits.  If a guard check fails, an error message
      is printed and the program exits.


So in two words: "Buffer overflows"