Need help with mock substr function and output

[blazen]

Here is a simple substring type function that I wrote in C.

char *substr(char arr[], int pos1, int pos2)
{ 
  char str[100];
  char *sub;
  
  for (int i = 0; i <= (pos2 - pos1); i++)
  {
    str[i] = arr[i+pos1];
  }
  
  sub = str;
  printf("Sub: %s\n", sub);  // debugging purposes
  
  return sub;
}

Here is my function call in main.

int main(int argc, char *argv[])
{
  char str[] = "Programming in C!";
  char *sub = substr(str, 0, 1);

  printf("String: %s", sub);
  return 0;
}

And here are my commands and output.

% cc -v -o search searchstr2.c
% search

Sub: Prúü
String:

Here's what I want the output to be is:

Sub: Pr
String: Pr

I can't figure this out and it's driving me bonkers. Any help would be greatly appreciated.

Kind Regards,
--
Jay

[ArkM]

Your substr function returns a pointer to its local variable str. This automatic storage variable are discarded immediatly after return, so this pointer value is senseless one. The classical novice's error ;)
The second defect: you forgot to append terminated null byte at the end of the formed substring.
The 3rd defect: no parameters check at all (positions >= 0, pos1 <= pos2 etc) .
Of course, you can declare str as a static variable (bad solution: every substr call reuses the same static storage), or you can add yet another pointer parameter for extracted substring...

[blazen]

Hi,

Thanks for the quick reply and wake up call! "I see said the blind man" :). That actually makes sense. I made some changes and all seems well now. I'll add some error checking and comments later as this is only a very very small piece of a humongous program. Please let me know if you (or anyone else) see anything else that may be harmful as I'm sure there is.

Thanks again for making my first post a great experience.

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

char *substr(char* dest, const char* str, int startpos, int endpos)
{ 
  /* error checking will be added */

  for (int i = 0; i <= (endpos - startpos); i++)
  {
    dest[i] = str[i + startpos];
    dest[i + 1] = '\0';
  }

  printf("Sub: %s\n", dest); /* debugging purposes */
  
  return dest;
}

int main(int argc, char *argv[])
{
  char *str = "Programming in C!";
  char *sub = malloc(strlen(str));
  int p1 = 0, p2;

  printf("String: %s\n", substr(sub, str, 0, 1));

  return 0;
}

Kind Regards,
--
Jay

[ArkM]

No need in dest[i + 1] = '\0'; in the loop:

int i = 0, n = (endpos-startpos)+1;
if (dest && str && startpos >= 0) { // prevent future troubles!
    if (startpos < strlen(str) && n > 0) {
        str += startpos;
        for (i = 0; i < n && str[i]; ++i) /* break if str ended! */
            dest[i] = str[i];
    }
    dest[i] = '\0';
}

No need in printf "debug" statement in substr body.
No need in dynamic allocated memory in main (and you forgot to add 1 for null terminator).