what is this flag ? i have no idea what is this and from where to learn to use this. from where can i learn about this ? will that codeforces link guide me properly ?
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SIGABRT error is being returned in output console when i use
#pragma GCC optimize “trapv” on teh top of my code
with this flag i m getting something like this but i cannot figure out why ?
prog.cpp: In function ‘std::basic_ostream<_CharT, _Traits>& std::flush(std::basic_ostream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits<char>]’:
prog.cpp:150:1: internal compiler error: Segmentation fault
}
^
Please submit a full bug report,
with preprocessed source if appropriate.
See <file:///usr/share/doc/gcc-6/README.Bugs> for instructions.plz help me anyone
can you plz help ?? i don’t how to use these flags. PLz respond
There’s an example of using them here.
If you’re getting an internal compiler error, then there’s not much I can recommend except “upgrade your compiler” (GCC 6 is pretty old).
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can’t i do this on codechef’s or codeforces’s compiler ?
You can, but you’ll get much less useful information about what’s going on, and won’t be able to use a debugger. I wouldn’t bother, personally: just sort out your local build environment and use that.
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Can you share some syntax on how to use these flags in a c++ code ?
And also if possible a small example of code along with with the output error message, so that i can learn how to use this.
Also i don’t know anything about pragma.
Passing a bunch of debug flags to g++:
[simon@simon-laptop][13:36:04]
[~/devel/hackerrank/otherpeoples]>g++ rudyiscrazy1-DRCHEF.cpp -g3 -O3 -Wall -Wextra -DONLINE_JUDGE -D_GLIBCXX_DEBUG -fsanitize=undefined -ftrapv
rudyiscrazy1-DRCHEF.cpp: In function ‘int main()’:
rudyiscrazy1-DRCHEF.cpp:41:14: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
scanf("%d",&N) ;
~~~~~^~~~~~~~~
rudyiscrazy1-DRCHEF.cpp:42:15: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
scanf ("%ld",&x) ;
~~~~~~^~~~~~~~~~
rudyiscrazy1-DRCHEF.cpp:53:19: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
scanf ("%ld",&val) ;
~~~~~~^~~~~~~~~~~~
[simon@simon-laptop][13:36:12]
[~/devel/hackerrank/otherpeoples]>echo "3
> 5 5
> 1 2 3 4 5
> 5 1
> 40 30 20 10 50
> 3 10
> 20 1 110" | ./a.out
rudyiscrazy1-DRCHEF.cpp:122:41: runtime error: signed integer overflow: -6148914691236495360 * 2 cannot be represented in type 'long int'
rudyiscrazy1-DRCHEF.cpp:14:27: runtime error: signed integer overflow: 5764607523034234880 * 2 cannot be represented in type 'long int'
/usr/include/c++/7/debug/safe_iterator.h:327:
Error: attempt to decrement a dereferenceable (start-of-sequence) iterator.
Objects involved in the operation:
iterator "this" @ 0x0x7ffcb476d060 {
type = __gnu_debug::_Safe_iterator<std::_Rb_tree_const_iterator<long>, std::__debug::multiset<long, std::less<long>, std::allocator<long> > > (mutable iterator);
state = dereferenceable (start-of-sequence);
references sequence with type 'std::__debug::multiset<long, std::less<long>, std::allocator<long> >' @ 0x0x7ffcb476d300
}
Aborted (core dumped)
If you can’t do that, you can make sure the first two lines in your .cpp file are:
#define _GLIBCXX_DEBUG // Iterator safety; out-of-bounds access for Containers, etc.
#pragma GCC optimize "trapv" // abort() on (signed) integer overflow.
(this doesn’t include the -fsanitize=undefined flag, alas - not sure how to do that), but as I mentioned, this seems to give much less useful information, so I’d stick with compiling locally and passing the flags to gcc.
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hey i got few things cleared from your local compiling ! so there are two places where i am getting int overflow and at one place i m decrementing a iterator pointing to start of the container.
but there is new problem eating my head -
#pragma GCC optimize “trapv”
does this give an error message similar to the fashion in which we get when we use #define _GLIBCXX_DEBUG
or it just tells us a SIGBART error
herre is what i get when i compiled my code with just #pragma GCC optimize “trapv”
That just gives SIGABRT on signed integer overlow. It’s mainly useful for deciding whether your WA is due to integer overflow or not; -fsanitize=undefined is probably better for local debugging.
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okay things are getting clearer, Thnx a lot !
By the way, what does this part means, " ignoring return value of ‘int scanf(const char*, …)’, declared with attribute warn_unused_result [-Wunused-result] ", while you are compiling ??
scanf has a return value, but you are ignoring it (as do about 99% of C programmers in the world :))
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so i started using VS code from today and set it up for debugging from
this official page : Get Started with C++ and Mingw-w64 in Visual Studio Code
and when i tried to locally compile a small code to check whether the flags are working i got this :
can you help me figure it out why ??
No, sorry - I don’t use Windows.
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after setting up local compiling trough my windows cmd terminal and g++ compiler my compiling failed when i used this flag -fsanitize=undefined
when i used this command :
C:\Users\Humble Fool\Desktop\dummy>g++ main.cpp -g -D_GLIBCXX_DEBUG -ftrapv
the .cpp file compiled properly !
but when i used this command :
C:\Users\Humble Fool\Desktop\dummy>g++ main.cpp -g -D_GLIBCXX_DEBUG -fsanitize=undefined -ftrapv
I got this error -
C:/mingw-w64/x86_64-8.1.0-win32-seh-rt_v6-rev0/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lubsan
collect2.exe: error: ld returned 1 exit status
few people on stackoverflow were saying that maybe my compiler doesn’t supports it ! but if that’s the case then how is it working on your machine ?
It’s not working because the linker can’t find libubsan.so. Maybe you should shift to MSVC because it has support for AddressSanitizer. Hope this helps. 
in case Windows annoys you too much, Linux welcomes you 
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what is MSVC ??
@ssjgz can you give me some links to resources where i can learn how to setup my machine for being able to debug c++ code just like you did ??