strange C program fgets -

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I was going through an open source codebase and I see the following: - 
 
 Code> char * buf; Four * line; #define BUFSIZE 5000 buf = malloc (BUFSIZE) line = buff; While (in fgets (line, (unsigned) (buf + BUFSIZE -line))! = NULL) {// do stuff // ....}   
 Why is another < Code> buf + BUFSIZE - line ? The argument given as  line  to  buf   
 
 
 The number of characters at the end of / code> your  // do stuff  potential increment  line   
  buf + BUFSIZE  one  After the  letter <4> >  buf   
  buf + BUFSIZE - line  There is an integral number pointing to / em>  line  to  buf + BUFSIZE  - and therefore the number of characters that you can safely  line      can be typed without any overflow  buf <

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