Question

I have a loop that looks something like this

while(condition){
  read_some_data(source, buf, BUFSIZE);
  printf(buf);
  memset(buf, 0, BUFSIZE+1);
  //do stuff to affect condition that does not touch buf
}

buf is an char array of size BUFSIZE+1. The strange thing is that if I comment out the printf, the program executes perfectly with no segmentation fault. It is only when I try to print out buf that I get the problem. Also, the seg fault does not necessarily happen on the first iteration of the loop. It usually takes 6 or 7 iterations.

Also, there is no dynamic memory allocation in this program.

Answer 1

• Make sure that buf has a null (zero-valued) byte somewhere within the allocated range. That null byte is how printf can tell that it’s reached the end of a string; without it, it will keep reading past where it can safely do so.
• Make sure that buf doesn’t contain anything like %d that printf might take to indicate additional arguments. Better yet — just use printf("%s", buf), which completely eliminates any such risk.