title: why we use unix system interface in c programming?
body: <p>there is a part about UNIX system functions in R&amp;K C programming guide.</p>
  
  <p>there are system calls like <code>read()</code>, <code>write()</code>, <code>open()</code> or etc ,while we can use <code>scanf()</code>, <code>printf()</code>, <code>fopen()</code>  or other c library functions to do the same work.</p>
  
  <p>so what do we need C library functions instead of them?</p>
  
  <p>why do we need to know and use UNIX system functions in C programming while we  have that functions in C library?</p>