How do I generate random string using the characters of length(len)? in C

Richard has given you the link you need: now all you have to do is use it.
How? Simple: You function gets two parameters, a pointer and a
Use a loop that iterates len times, and inside the loop call the rand function. That returns an integer value, so use the % operator (which returns a remainder) to give you a value between 0 and the length of charset minus one. That is your random index into charset - so fetch the character at the index, and add it to the output and increment the pointer.

You may need to add a null value after the loop if it's to be treated as a string, but that will depend on the defintion of your function. If there aren't enough characters in the memory area passed to your function, don't add it!