RE: Strange C programming problem

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> 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

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