Re: Newbie: bug in argument casting to digit

From: Al Bowers (xabowers_at_rapidsys.com)
Date: 08/16/04

  • Next message: Thomas Stegen: "Re: Why index starts in C from 0 and not 1" 
    Date: Mon, 16 Aug 2004 15:04:15 -0400

    Jobs'R'Us wrote:
    > Hello,
    >
    > I have a bug in a simple validation for an argument that must be an
    > int from 1 to 99.
    >
    > If in the command prompt I enter the following, the program behaves
    > perfectly:
    > programname filename.txt doghouse 5
    >
    > If in the command prompt I enter the following, the program still
    > behaves perfectly, it explains that the digit ("5all") is incorrect:
    > programname filename.txt doghouse 5all
    >
    > The bug is, if I enter the following, the program incorrectly accepts
    > "5a" and interprets it exactly like like "5":
    > programname filename.txt doghouse 5a
    >
    > Someone kindly point out what I'm doing wrong?
    >

    > else if(strlen(argv[3]) > 2)
    You are only checking the length of the string. You also
    need to test to make sure the characters of the string are
    digits.
    Try:
    else if(strlen(argv[3]) > 2 || !isdigit((unsigned char)argv[3][0] ||
                       !isdigit((unsigned char)argv[3][1])

    > }

    > {
    > printf("Sorry, the number is wrong, must be a digit from 1 to
    > 99...\n\n");
    > printf("Usage: blah blah blah\n\n");
    > return 3;
    > }

    > /* Program continues... */
    >

    You also have the option of using of of the strto* functions
    provided by the Standard. With this program, you
    might use function strtoul to validate that the string is valid.
    Example of using strtoul is function ValidUINT defined below.

    #include <stdlib.h>
    #include <limits.h>
    #include <errno.h>

    int ValidUINT(const char *src, unsigned *result)
    {
            char *s;
            unsigned long tmp;

            errno = 0;
            tmp = strtoul(src,&s,10);
            if(s == src || errno == ERANGE || *s != '\0' ||
                    tmp > UINT_MAX) return 0;
            if(result) *result = tmp;
            return 1;
    }

    You can use this function as follows;

    change seracheddigit to
    unsigned searcheddigit;

    else if(!ValidUINT(argv[3],&searchddigit) || searcheddigit > 99)
       /* Not valid or greater than 99 */
    else /* Use searcheddigit. It is in range */ 

    -- 
Al Bowers
Tampa, Fl USA
mailto: xabowers@myrapidsys.com (remove the x to send email)
http://www.geocities.com/abowers822/
  • Next message: Thomas Stegen: "Re: Why index starts in C from 0 and not 1"

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