Segmentation fault with strcat and malloc in C

Question

I'm still pretty new to programming, but I'm trying to write a function that duplicates letters in a given string. It takes a series of integer inputs for how many of each letter to include, then concatenates each letter to the end of the stretched string. I need to dynamically allocate the memory.

This is what I have:

char * stretch (char * aString) {

   int length;
   int i, k, checking;
   int amount[10];
   int aTotal = 0;
   char *stretched;
   char ch;

   length = strlen(aString);

   for (i = 0; i < length; i++) {

      printf("Enter an amount: ");
      scanf("%d", &checking);

      while (checking <= 0) {

         printf("\nPositive integers only\n");

         printf("Enter an amount: ");
         scanf("%d", &checking);
      }
      amount[i] = checking;
   }

   for (i = 0; i < length; i++) {
      aTotal = aTotal + amount[i];
   }

   stretched = malloc(sizeof(char) * total + 1);
   stretched[aTotal] = '\0';

   for (i = 0; i < length; i++) {

      ch = aString[i];
      for (k = 0; k < amount[i]; k++) {
         strncat(stretched, &ch, 1);
      }
      printf("stretched: %s", stretched);
   }
   return stretched;
}

I end up with a segmentation fault, but 'stretching' the string does work, the print statements show that stretched contains the right data (ie. if the string is "Hi!" and the input is 2 1 3, then at the end of the loop stretched contains "HHi!!!"). However I can't print stretched outside of the for loop, but I can print just a basic string to show it leaves the loop.

Running gdb returned

Program received signal SIGSEGV, Segmentation fault.
__strcpy_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S:716
716 ../sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S: No such file or directory.

I'll be honest I have no idea what it means.

I've tried everything I could think of and most of the solutions I could understand online. I've initialized stretched a bunch of ways, and used malloc, calloc, realloc, strcpy, strcat.

Any ideas for what's causing the segmentation fault, and how to fix it? Thank you!! :)

EDIT: I ended up fixing it, I used memset after the malloc and then the next problem was actually in my main(), I was trying to strcpy the returned value into a char*. Thanks for helping!

Answer 1

In order to use strcat family of functions, you need to have a null terminated string. Your stretched array is filled with garbage and then a null terminator at the end of this garbage array, which could contain anything (including the value 0 elsewhere).

Instead of stretched[total] = '\0'; you need to do stretched[0] = '\0'; .

Also you must obviously ensure that this array is large enough to contain all data, including a null terminator. (Each strcat call will add one at the end of the new string.)