Weird strlen behavior

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  • Chris
    #1

    Weird strlen behavior

    Hi there,

    I have been reading in this group for a while really enjoy this pool
    of infinite c wisdom. :)

    Anyway, I have question now. I have this little piece of code that is
    giving me a headache.

    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>

    /* Definitions */
    #define STRING_LENGTH 12

    /* Variables */
    char str[STRING_LENGTH];
    char *str1;
    char *str2 = &str[0];
    unsigned int i;

    /* Main */
    int main(void)
    {
    str1 = "Hello World!";

    printf("String length: %d\n",strlen(st r1));

    for(i = 0u; i < strlen(str1); i++)
    {
    *str2++ = *str1++;
    printf("%d\n",i );
    }

    puts(str);

    /* Wait for key */
    getc(stdin);
    return EXIT_SUCCESS;
    }

    The problem is the strlen(str1) expression. The code compiles fine
    (with latest MinGW under XP) but running the executable it gives me
    the following output:

    String Length: 12
    0
    1
    2
    3
    4
    5
    Hello

    It seems to break out of the loop early, after 6 iterations instead of
    12.

    I replace the expression with something like this:

    unsigned int length;

    ....

    length = strlen(str1);

    for(i = 0u; i < length; i++)
    {
    *str2++ = *str1++;
    printf("%d\n",i );
    }

    ....

    Then it works fine and runs 12 times and copies the entire string.

    Any idea whats the problem here?

    Thanks,
    Chris

    Tags: None
    • Eric Sosman
      #2
      Re: Weird strlen behavior

      Chris wrote:
      Hi there,
      >
      I have been reading in this group for a while really enjoy this pool
      of infinite c wisdom. :)
      >
      Anyway, I have question now. I have this little piece of code that is
      giving me a headache.
      >
      #include <stdio.h>
      #include <string.h>
      #include <stdlib.h>
      >
      /* Definitions */
      #define STRING_LENGTH 12
      >
      /* Variables */
      char str[STRING_LENGTH];
      char *str1;
      char *str2 = &str[0];
      unsigned int i;
      >
      /* Main */
      int main(void)
      {
      str1 = "Hello World!";
      >
      printf("String length: %d\n",strlen(st r1));
      >
      for(i = 0u; i < strlen(str1); i++)
      {
      *str2++ = *str1++;
      printf("%d\n",i );
      }
      >
      puts(str);
      >
      /* Wait for key */
      getc(stdin);
      return EXIT_SUCCESS;
      }
      >
      The problem is the strlen(str1) expression. The code compiles fine
      (with latest MinGW under XP) but running the executable it gives me
      the following output:
      >
      String Length: 12
      0
      1
      2
      3
      4
      5
      Hello
      >
      It seems to break out of the loop early, after 6 iterations instead of
      12.
      >
      I replace the expression with something like this:
      >
      unsigned int length;
      >
      ...
      >
      length = strlen(str1);
      >
      for(i = 0u; i < length; i++)
      {
      *str2++ = *str1++;
      printf("%d\n",i );
      }
      >
      ...
      >
      Then it works fine and runs 12 times and copies the entire string.
      >
      Any idea whats the problem here?
      You keep changing what str1 points to, and hence
      changing the result of strlen(str1):

      i = 0, strlen("Hello world!") = 12
      i = 1, strlen("ello world!") = 11
      i = 2, strlen("llo world!") = 10
      i = 3, strlen("lo world!") = 9
      i = 4, strlen("o world!") = 8
      i = 5, strlen(" world!") = 7
      i = 6, strlen("world!" ) = 6, that's all she wrote.

      Incidentally, you should be glad that your loop stopped early.
      The destination array str is twelve characters long, which is
      enough room for a string of *eleven* characters plus the zero
      end-marker. In your modified code you copy twelve characters
      from "Hello world!" to the destination, but there is no end-
      marker and the destination thus doesn't hold a proper string.
      Pass this malformed string to a function expecting a well-
      formed string (puts(), for example), and there's no telling
      what will happen.

      --
      Eric Sosman
      esosman@acm-dot-org.invalid

        Comment

        • Chris
          #3
          Re: Weird strlen behavior

          On Mar 3, 2:34 pm, Eric Sosman <esos...@acm-dot-org.invalidwrot e:
          You keep changing what str1 points to, and hence
          changing the result of strlen(str1):
          >
          i = 0, strlen("Hello world!") = 12
          i = 1, strlen("ello world!") = 11
          i = 2, strlen("llo world!") = 10
          i = 3, strlen("lo world!") = 9
          i = 4, strlen("o world!") = 8
          i = 5, strlen(" world!") = 7
          i = 6, strlen("world!" ) = 6, that's all she wrote.
          Doh! Thanks.
          Incidentally, you should be glad that your loop stopped early.
          The destination array str is twelve characters long, which is
          enough room for a string of *eleven* characters plus the zero
          end-marker. In your modified code you copy twelve characters
          from "Hello world!" to the destination, but there is no end-
          marker and the destination thus doesn't hold a proper string.
          Pass this malformed string to a function expecting a well-
          formed string (puts(), for example), and there's no telling
          what will happen.
          Thats correct. I always forget that little invisible sucker...

          Thanks for the quick response.

          Chris

            Comment

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