Re: [CentOS] Strange C programming problem
> Using an input of something other than 2.5 changes the middle two lines in > some way in which I haven't yet discerned a pattern, but the result is still > highly bogus. Adding -std=gnu99 to the compile makes it work, it seems by selecting a different strtof implementation in /usr/include/stdlib.h. Maybe the gcc folks are interested, I dunno. -Andy ___ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos
RE: [CentOS] Strange C programming problem
> I've got this little program I wrote to test something, and
> it keeps giving the wrong result. I'm not inexperienced in C,
> but I can't believe strtof (et al) are broken, so I must be
> doing something wrong. However, I've spent hours looking at
> this and comparing it to the man pages and don't see what I'm
> doing wrong. strtod() and strtold() also give equally wrong
> results. (the example program given on the strotd man page
> works fine, BTW.)
>
> Can someone wield a clue-bat please? :)
>
> Here's the program:
>
> #include
> #include
> #include
> #include
>
> int main (int argc, char ** argv)
> {
> float ldbl = 0.0;
> char * endp;
>
> printf ("%s\n", argv[1]);
>
> errno = 0;
> ldbl = strtof (argv[1], &endp);
> if (errno != 0)
> printf ("strtof failed! errno=%d\n", errno);
>
> printf ("%f\n", (double) ldbl);
> printf ("%f\n", (double) strtof (argv[1], (char **)NULL));
> printf ("%f\n", (double) atof (argv[1]));
>
> return 0;
> }
>
> Compile it with:
>
> cc -O0 -g -o x4 x4.c
>
> then run it like this:
>
> ./x4 2.5
>
> and I'd EXPECT it to produce this output:
>
> 2.5
> 2.5
> 2.5
> 2.5
>
> but it actually produces this:
>
> 2.5
> 1075838976.00
> 1075838976.00
> 2.50
>
> the typecase of the arg in the 3 printf calls makes no
> difference. Remove it and the results are the same.
>
> Using an input of something other than 2.5 changes the middle
> two lines in some way in which I haven't yet discerned a
> pattern, but the result is still highly bogus.
>
The following strtod line works fine on my system (CentOS 5, latest updates,
x86_64):
printf ("%lf\n", (double) strtod (argv[1], (char **)NULL));
For strtof, the SYNOPSIS in the man page mentions you need to add:
#define _ISO_C99_SOURCE
Or
#define _XOPEN_SOURCE 600
Either line should be added before ALL include files (note there is a
mistake in the synopsis. There should be no = sign in the define statement
for _XOPEN_SOURCE).
The above #define lines enforces C99 compatibility rules, which is the
revised ISO C standard which came out in 1999. As a previous responder
suggested, you can also specify -std=gnu99 or -std=C99 on the compile line.
Michael
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Re: [CentOS] Strange C programming problem
On Sat, Jul 14, 2007 at 12:52:09PM -0400, Michael Velez wrote:
>
>
>
> > I've got this little program I wrote to test something, and
> > it keeps giving the wrong result. I'm not inexperienced in C,
> > but I can't believe strtof (et al) are broken, so I must be
> > doing something wrong. However, I've spent hours looking at
> > this and comparing it to the man pages and don't see what I'm
> > doing wrong. strtod() and strtold() also give equally wrong
> > results. (the example program given on the strotd man page
> > works fine, BTW.)
> >
> > Can someone wield a clue-bat please? :)
> >
> > Here's the program:
> >
> > #include
> > #include
> > #include
> > #include
> >
> > int main (int argc, char ** argv)
> > {
> > float ldbl = 0.0;
> > char * endp;
> >
> > printf ("%s\n", argv[1]);
> >
> > errno = 0;
> > ldbl = strtof (argv[1], &endp);
> > if (errno != 0)
> > printf ("strtof failed! errno=%d\n", errno);
> >
> > printf ("%f\n", (double) ldbl);
> > printf ("%f\n", (double) strtof (argv[1], (char **)NULL));
> > printf ("%f\n", (double) atof (argv[1]));
> >
> > return 0;
> > }
> >
> > Compile it with:
> >
> > cc -O0 -g -o x4 x4.c
> >
> > then run it like this:
> >
> > ./x4 2.5
> >
> > and I'd EXPECT it to produce this output:
> >
> > 2.5
> > 2.5
> > 2.5
> > 2.5
> >
> > but it actually produces this:
> >
> > 2.5
> > 1075838976.00
> > 1075838976.00
> > 2.50
> >
> > the typecase of the arg in the 3 printf calls makes no
> > difference. Remove it and the results are the same.
> >
> > Using an input of something other than 2.5 changes the middle
> > two lines in some way in which I haven't yet discerned a
> > pattern, but the result is still highly bogus.
> >
>
> The following strtod line works fine on my system (CentOS 5, latest updates,
> x86_64):
>
> printf ("%lf\n", (double) strtod (argv[1], (char **)NULL));
>
> For strtof, the SYNOPSIS in the man page mentions you need to add:
>
> #define _ISO_C99_SOURCE
>
> Or
>
> #define _XOPEN_SOURCE 600
>
> Either line should be added before ALL include files (note there is a
> mistake in the synopsis. There should be no = sign in the define statement
> for _XOPEN_SOURCE).
>
> The above #define lines enforces C99 compatibility rules, which is the
> revised ISO C standard which came out in 1999. As a previous responder
> suggested, you can also specify -std=gnu99 or -std=C99 on the compile line.
>
> Michael
Sorry, I forgot to mention that I'm using Centos 4.5. And the man
page here doesn't mention those #define settings. I'll give it a try,
thanks!
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