When casting some large long values to a double the resultant double values is 1 larger than the long. This can easily be seen with the value for LONG_MAX, but will even appear with long values as low as 23372036854775807 (and possibly lower). It is not consistent and does not affect all large longs, but does appear in a great number of them. ratscat> gcc -v Using built-in specs. Target: x86_64-redhat-linux Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --enable-plugin --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic --host=x86_64-redhat-linux Thread model: posix gcc version 4.1.2 20080704 (Red Hat 4.1.2-48) Program is: #include <stdlib.h> #include <stdio.h> #include <limits.h> #include <float.h> int main( int argc, char **argv ) { long infc = LONG_MAX; double real_size = (double) infc; printf( "infc %ld\nreal_size %lf\n", infc, real_size ); exit( 0 ); } ratscat> gcc -Wall truncfc.c ratscat> a.out infc 9223372036854775807 real_size 9223372036854775808.000000
This is correct as long is 64bits and the precision of a double is less than 64bits.
I think GCC (>= 4.4) should catch this -Wconversion is you drop the forced cast: long infc = LONG_MAX; double real_size = infc; No?
Let me try again: I think GCC (>= 4.4) should catch this using -Wconversion if you drop the explicit cast like in: long infc = LONG_MAX; double real_size = infc; Doesn't it?
I haven't tried it with anything greater than GCC 4.1.2. Therefore I do not know the answer. > From: gcc-bugzilla@gcc.gnu.org > To: dogbreath69@hotmail.com > Subject: [Bug c/47742] Large Values Increment By 1 When Casting Long to Double > Date: Tue, 15 Feb 2011 23:16:17 +0000 > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47742 > > --- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-02-15 23:16:09 UTC --- > Let me try again: > > I think GCC (>= 4.4) should catch this using -Wconversion if you drop the > explicit > cast like in: > > long infc = LONG_MAX; > double real_size = infc; > > Doesn't it? > > -- > Configure bugmail: http://gcc.gnu.org/bugzilla/userprefs.cgi?tab=email > ------- You are receiving this mail because: ------- > You reported the bug.