RFC: add flush_cstdio function

[stevengj]

This adds flush_cstdio() function, which calls fflush(stdout) and fflush(stderr) in C. This is needed in order to flush any output from external C code.

(For example, I need this in IJulia in order to make sure external printf output is captured after evaluating an input cell.)

[vtjnash]

This might be incorrect, since the stream may be (should be?) non-blocking at the time: http://pubs.opengroup.org/onlinepubs/009696799/functions/fflush.html
@loladiro ping

It's possible you could write this directly in julia:
ccall(:fflush, Cint, (Ptr{Void},), cglobal(@osx? :__stdoutp : :stdout)))
but this would be broken on windows, where stdout is defined as &__iob_func()[1] and stderr is defined as &__iob_func()[2]

[Keno]

I intentionally don't put it into non-blocking mode since child processes may not know how to deal with that. I think it is reasonable to assume that the stream is blocking since anything else is bad form.

[stevengj]

Good to hear that we open stdout/stderr in the usual blocking mode. Okay to merge, then?

(It could throw an exception on an error, but I have a hard time imagining a circumstance when anything useful could be done with such an exception from flush_cstdio.)

[vtjnash]

indentation

[vtjnash]

OK. I thought libuv was doing it, but a review of the code reveals that it only does for internal (not spawn-safe) pipes and all sockets. Normal pipes and TTY's are fine (as are normal files).

(edit) LGTM. OK to merge.

[vtjnash]

@loladiro on a related note, can you put together a flush(::UVStream) function which blocks the task until all pending writes are finished?

[Keno]

@vtjnash I'll see what I can do.

[stevengj]

Fixed indentation.