如何用等效的strcat（）代替memcpy（）？

Question

I have the following code that works properly inserting a substring into a specific part of the original string. How can I do the same thing using strcat() instead of memcpy() ?

void insertString(char original[], int start, int length, char toInsert[]){
  size_t insertlen = strlen(toInsert);
  size_t origlen = strlen(original);
  char *m = malloc(origlen - length + insertlen);
  memcpy(m, &original[0], start);
  memcpy(m+start, &toInsert[0], insertlen);
  memcpy(m+start+insertlen, &original[start+length], origlen-length+insertlen);
  strcpy(original,m);
}

Where char original[] is the original string, int start is the element which the substring should start at, int length is the length of the substring being removed from original[] and char toInsert[] is the substring being inserted.

Example to clarify :

ex. if original[] = default string 1 , start = 5 , length = 6 , toInsert[] = hello world

the resuling m would equal 'defauhello worlding 1' since the substring being replaced in this example is 'lt str' starting at 5 chracters in and is 6 characters long. How can I do the same thing with strcat() instead of memcpy() ?

Answer 1

You can use strncat and do it in the below way. But, please note that the below doesn't handle error conditions and other constraints which you have as a part of this problem.

void insertString(char original[], int start, int length, char toInsert[]){
  size_t insertlen = strlen(toInsert);
  size_t origlen = strlen(original);
  size_t newBuffLen = origlen - length + insertlen + 1
  char *m = malloc(newBuffLen);
  memset(m, 0, newBuffLen);
  strncat(m, original, start);
  strncat(m, toInsert, insertlen);
  strncat(m, original + start+ length, origlen-start-length);
  strcpy(original,m);
}

Answer 2

I have replaced it with the following and it works perfectly

void insertString(char original[], int start, int length, char toInsert[]){
  size_t insertlen = strlen(toInsert);
  size_t origlen = strlen(original);
  char holder[origlen - length + insertlen];
  strcpy(holder,"");
  strncat(holder,original,start);
  strcat(holder,toInsert);
  strncat(holder, &original[start+length], origlen-length+insertlen);
  strcpy(original,holder);
}

Thank you all for your help!