Question

I have this C function which attempts to tell me if a sub string is contained in a string.

int sub_string(char parent [1000], char child [1000]){

  int i;
  i = 0;
  int parent_size = (int) strlen(parent);
  int child_size = (int) strlen(child);
  char tempvar [child_size];

  int res;
  res  = 1;

  while(i<(parent_size - child_size + 1) && res != 0){

   strncpy(tempvar, parent + i, child_size);

   if(strcmp(tempvar, child)==0){
      res = 0;
   }
   i++;

   memset(tempvar, 0, child_size);
  }

  memset(tempvar, 0, sizeof(tempvar));
  return res;
}

Now the strange thing is, when I pass a string “HOME_DIR=/tmp/” and “HOME_DIR” it returns a 0 the first time round, but after I call this function again, it returns a 1 to say it hasn’t found it!!

I am guessing this is a memory issue, but I can’t tell where, I would appreciate any help on this.

Answer 1

Is there any reason you can’t use the strstr function? Otherwise there are some things you should clean up in your code. For starters since you are limiting the length of the arrays coming in to 1000 characters you should use strnlen instead of strlen with a limit of 1000. You should also create you should zero out the tempvar array before you start copying into it. If parent is not null terminated you could run off the end of the array in your while loop. I would also suggest using strncmp and giving a length limit (in general if you are using the C string library you should use the ‘n’ version of the functions i.e. strnlen instead of strlen so that you put a bounding length on the operation, this helps to protect buffer overflows and potential security holes in your code).