read double from, inconsistent results with some values

Alessio T · 2013-11-09T00:59:19

I was just playing with visual studio 2013 rtm and compile some old demos to see if there were some changes to the standard library (like "max()" and "min()" functions moved to <algorithm>....) And I had a little surprise with a sample code for write and read binary files with the standard library: using the following double values (111.111, 222.222, 333.333, 444.444), when I read back the data I have completely different data (like -6.27744e+066 -6.27744e+066 -6.27744e+066 -6.27744e+066). It is also strange that the values are the same in debug mode in 32 bit and 64 bit configuration, but they change in release mode in 64 bit and 32 bit configurations. Changing the float precision model ( "precise" 80 bit, strict and fast) doesn't solve the problem. Note also the sample works well if I change the double data to other values... Maybe that is due to some "nice" behaviour of the IEEE standard? ... I don't remember nothing about that in the floating point model, to me that seems just a bug (code or compiler... dunno XD ) Here is the code: #include <fstream> #include <iostream> int main() { double a[ 4 ] = { 111.111, 222.222, 333.333, 444.444 }; double* i = new( double[ 4 ] ); std::fstream file( "file", std::ios::out | std::ios::binary ); file.write( ( char* )&a, sizeof( a ) ); file.close(); file.open( "file", std::ios::in | std::ios::binary ); file.read( ( char* )&*i, sizeof ( double )* 4 ); file.close( ); for( int j = 0; j < 4; ++j ) { std::cout << i[ j ] << " "; } std::cout << std::endl; return( 0 ); }

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Started by Alessio1989 November 08, 2013 10:18 PM

12 comments, last by Brother Bob 10 years, 5 months ago

Alessio1989

4,648

Author

November 09, 2013 12:34 AM

Alessio, you still have the third incorrect parameter in your call to open(). You need to OR the last two parameters there as well.

thank you. now it works with that values too.

-6.27744e+066

That looks a lot like accidental/uninitialised memory reinterpreted as a double - the upper bytes are 0xCDCDCDCE...

Are you sure you are running the exact same program you have reproduced here? Apart from the missing bitwise-OR already mentioned, there isn't anything particularly wrong with your code snippet, and a compiler bug is beyond unlikely.

it was the comma in the input-mode of open()... anyway it is so strange, the comma don't brake me the read-back if I use other floating values.. Probably since ios openmodes are macros, some "funny" things happened...

"Recursion is the first step towards madness." - "Skegg?ld, Skálm?ld, Skildir ro Klofnir!"
Direct3D 12 quick reference: https://github.com/alessiot89/D3D12QuickRef/

swiftcoder

18,997

November 09, 2013 12:51 AM

it was the comma in the input-mode of open()... anyway it is so strange, the comma don't brake me the read-back if I use other floating values.. Probably since ios openmodes are macros, some "funny" things happened...

Weird. That just results in a compile error on Clang.

Tristam MacDonald. Ex-BigTech Software Engineer. Future farmer. [https://trist.am]

Alessio1989

4,648

Author

November 09, 2013 12:53 AM

it was the comma in the input-mode of open()... anyway it is so strange, the comma don't brake me the read-back if I use other floating values.. Probably since ios openmodes are macros, some "funny" things happened...

Weird. That just results in a compile error on Clang.

Pure Microsoft magic

"Recursion is the first step towards madness." - "Skegg?ld, Skálm?ld, Skildir ro Klofnir!"
Direct3D 12 quick reference: https://github.com/alessiot89/D3D12QuickRef/

Brother Bob

10,350

November 09, 2013 12:59 AM

The library that ships with MSVC apparently has an extra implementation defined parameter for (I guess) internal use. Since it has a default value, the public interface remains unchanged, but unexpected things happen when you call it the wrong way.

read double from, inconsistent results with some values

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read double from, inconsistent results with some values

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