Stdout and Stderr, and printf

• When your program starts, there are three file descriptor open by default.
  • More about file descriptors later, but for now, they are an index into a kernel table.
  • See page 95 if you wish.
• In C++ , these get mapped to fd 0, 1 and 2.
  • cin is mapped to fd 0
  • cout is mapped to fd 1
  • cerr is mapped to fd 2
• This is a result of the action of fork
• cerr and cout are normally mapped to the output device
• cerr is intended to be for error messages, especially if you want them separated from the other output.

       cout << "This is to cout" << endl;
       cerr << "This is to cerr " << endl;
      

• You can redirect output as we have discussed

  $ output
  $ output > foo
  $ output 2> bar
  $ output 1> foo 2> bar
      

• In c, the output statement is printf, sprintf, fprintf, ...
  • There is a man page
  • printf(format, exp1, exp2, ...)
  • sprintf(c-string, format, exp1, exp2, ...)
  • fpritnf(stream, format, exp1, exp2, ...)
  • The format is a string,
    • %d is replaced with an integer expression
    • %s is replaced with a string expression
    • %f with a float
    • MUCH MUCH MORE.