[Solved] Replacing sub-string of a String with another String in c

The sprintf command has the following declaration:

sprintf(char *str, const char *format, ...);

your declaration in question is:

sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));

The sprintf command writes to the character pointer str, the values rep and p+strlen(orig) in accordance with the format string "%s%s". Essentially, it is simply combining (or concatenating) rep and p+strlen(orig) in buffer+(p-str).

Note that str (in the sprintf declaration) is of type char * meaning that buffer+(p-str) must also be type char * (a character pointer). buffer has a pointer address, say 1000 as an example. To that address, we will add the difference p-str (say 25). So the sprintf command will combine the two strings rep and p+strlen(orig) in memory beginning at the address 1000 + 25 = 1025

We also know from the format string "%s%s" that both rep and p+strlen(orig) will likewise be type char * pointers. The address of rep is whatever it is, but the next char * pointer p+strlen(orig) will be p (say 1500) + strlen(orig) (the length of orig assume is 20 chars. So sprintf will read the string value beginning at address 1520 along with the string rep into buffer at address buffer+(p-str) (or 1025 for this example).

Lets say the pointers point to the following strings:

rep = "In the beginning"
  p = "the sky was blue and there was light!"

Lets also say that strlen(orig) = 20

  p+strlen(orig) = p[20] = " there was light!"

So the resulting string in buffer at (my fake address 1025) is:

buffer+(p-str) = "In the beginning there was light!"