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

char *fnReplace(char sString[], char sPoor[], char sGreat[]); // Function to replace the words

int main()
{
    char sString[] = "I am a poor programmer";
    printf("Input string: %s\n", sString);
    printf("Replaced substring: %s\n", fnReplace(sString, "poor", "great"));
    return 0;
}

char *fnReplace(char sString[], char sPoor[], char sGreat[])
{
    int iStLen  = strlen(sString); // In this variable we save the length of sString[] (without the '\0')
    int iPoorLen = strlen(sPoor);  // In this variable we save the length of sPoor[] (without the '\0')
    int iGreatLen = strlen(sGreat); // In this variable we save the length of sGreat[] (without the '\0')
    char *pPointer = NULL; // We use this pointer for the loop.
    for (pPointer = sString; pPointer = strstr(pPointer, sPoor); ++pPointer)
    {
      if (sPoor != sGreat) // Here we check if the words are with different length (in this case they are) and make the array "bigger", so we can save and see whole of it.
      {
        memmove(pPointer + iGreatLen, pPointer + iPoorLen, iStLen - (pPointer - sString) + iGreatLen);
        memcpy(pPointer, sGreat, iGreatLen);
      }
    }
    return sString;
}