strtod() and errno question.

strtod() and errno question.

[J.]

Tuesday, January 10 22:35:50

Hi people,

I have a question regarding errno and strtod() error checking. I have a
function [listed below] that extracts a number from a string. In case of
an error, strerror() in conjunction with errno is used to display a user
friendly error message. However on a strtod() error; `Success' is
returned.

Error - strtod(ca4)`c': Success

Since I am a bit confused by the strtod() manual page, what would be the 
correct way to handle this ?

Thankx !

J.

int extrnumber(char *str) {
 int retv = 0;
 char *nptr = NULL;

 retv = strtod(str, &nptr);
 if(retv == 0 && nptr != NULL) {
  fprintf(stderr, "%s: Error - strtod(%s)`%c': %s\n", PACKAGE, 
    str, *nptr, strerror(errno));
  exit(EXIT_FAILURE);
 }

 return retv;
}

RE: strtod() and errno question.

[James Stevenson]

Hi,

Try taking a copy of errno before calling fprintf.
See if that helps any.

	James

> -----Original Message-----
> From: linux-c-programming-owner@vger.kernel.org [mailto:linux-c-
> programming-owner@vger.kernel.org] On Behalf Of J.
> Sent: 10 January 2006 21:43
> To: linux-c-programming@vger.kernel.org
> Subject: strtod() and errno question.
> 
> Tuesday, January 10 22:35:50
> 
> Hi people,
> 
> I have a question regarding errno and strtod() error checking. I have a
> function [listed below] that extracts a number from a string. In case of
> an error, strerror() in conjunction with errno is used to display a user
> friendly error message. However on a strtod() error; `Success' is
> returned.
> 
> Error - strtod(ca4)`c': Success
> 
> Since I am a bit confused by the strtod() manual page, what would be the
> correct way to handle this ?
> 
> Thankx !
> 
> J.
> 
> int extrnumber(char *str) {
>  int retv = 0;
>  char *nptr = NULL;
> 
>  retv = strtod(str, &nptr);
>  if(retv == 0 && nptr != NULL) {
>   fprintf(stderr, "%s: Error - strtod(%s)`%c': %s\n", PACKAGE,
>     str, *nptr, strerror(errno));
>   exit(EXIT_FAILURE);
>  }
> 
>  return retv;
> }
> 
> -
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Re: strtod() and errno question.

[Glynn Clements]

J. wrote:

> I have a question regarding errno and strtod() error checking. I have a
> function [listed below] that extracts a number from a string. In case of
> an error, strerror() in conjunction with errno is used to display a user
> friendly error message. However on a strtod() error; `Success' is
> returned.

Note that strtod() only sets errno on overflow or underflow.

If you want to check for failure to convert (because no prefix of the
string is a valid floating-point number), you need to check whether
"nptr == str".

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: strtod() and errno question.

[J.]

On Wed, 11 Jan 2006, Glynn Clements wrote:
> J. wrote:
> 
> > I have a question regarding errno and strtod() error checking. I have a
> > function [listed below] that extracts a number from a string. In case of
> > an error, strerror() in conjunction with errno is used to display a user
> > friendly error message. However on a strtod() error; `Success' is
> > returned.
> 
> Note that strtod() only sets errno on overflow or underflow.
> 
> If you want to check for failure to convert (because no prefix of the
> string is a valid floating-point number), you need to check whether
> "nptr == str".
> 
> Glynn Clements <glynn@gclements.plus.com>

Hi, 

Thank you for all your answers. At first glance extracting a number from a
string seems rather easy, but it can be a hassle. I checked the source code 
of some other programs and I changed the function to the following. I hope
that will suffice validating user input.

Thankx again.

J.

#include <limits.h>

int extrnumber(char *str) {
 long lval = 0;
 int retv = 0;
 char *nptr = NULL;

 lval = strtol(str, &nptr, 10);
 if(str == '\0' || *nptr != '\0') {
  fprintf(stderr, "%s: Error - conversion(%s): strtol(`%c')\n", 
    PACKAGE, str, *nptr);
  exit(EXIT_FAILURE);
 }
 if((errno == ERANGE && (lval == LONG_MAX || lval == LONG_MIN)) ||
    (lval > INT_MAX || lval < INT_MIN)) {
  fprintf(stderr, "%s: Error - strtol(%s): %s\n", 
    PACKAGE, str, strerror(errno));
  exit(EXIT_FAILURE);
 }

 retv = lval;
 return retv;
}