Printf sets errno to 25?

[Doug Gwyn <gwyn>]

> The problem is a common misunderstanding about the use of errno. The value
> in errno is useful if, and only if, the call has returned a value indicating
> failure. This value is commonly -1, and sometimes NULL for functions 
> returning pointers. The Intro(2) manual page says: "Errno is not cleared on
> successful calls, so it should be tested only after an error has been indicated"
> 
> The documentation I have for printf further states that it, fprintf and sprintf
> return the number of characters transmitted, or a negative value if an output
> error was encountered.

Close, but not quite.  `errno' is only set by unsuccessful SYSTEM CALLS
(and some math library routines, alas).  Therefore, failure of printf()
does not imply that errno will correctly indicate the reason for failure.