Stdio.write/writeln and flushing
Steven Schveighoffer via Digitalmars-d
digitalmars-d at puremagic.com
Thu Sep 8 06:22:10 PDT 2016
On 9/8/16 4:53 AM, Somebody wrote:
> If i write something like:
>
> writeln("what to do?");
> switch(readln[0 .. $ - 1])
> { //..
> }
> writeln("bye");
>
> ....that works just as it should at Windows, started from a command
> prompt. However, if I run it from GNU Emacs, I have to manually flush
> the output after each time I do it before taking input, or else the
> output does not show when it should.
>
> I wonder if write(...) and writeln(...) should automatically flush, to
> enhance portability? Of course, I may be issing missing something?
write and writeln depend on the behavior of FILE * for flushing, there
is no specific flush.
Note that FILE * examines the file descriptor and if it is detected as
an interactive descriptor, flush is done every newline. If not, then
flush is only done when the buffer is full. This is standard behavior
forever, and is done to avoid performance problems when piping the
result of a command to a file, for instance (flushing is expensive).
Your emacs "console" is not marked by the OS as interactive (or however
it's detected by FILE *, implementation defined), therefore flush does
not happen on newlines.
If you want to force newline flushing, use
http://dlang.org/phobos/std_stdio.html#.File.setvbuf with a mode
parameter of _IOLBF.
-Steve
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