> 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 <stdio.h>
> #include <math.h>
> #include <stdlib.h>
> #include <errno.h>
>
> 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
> 1075838976.000000
> 1075838976.000000
> 2.500000
>
> 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