jl_yield() C API

[stevengj]

It would be nice to have a jl_yield() function in the C API, equivalent to yield() in Julia, so that long-running C programs could give some cycles to the Julia event loop (see e.g. JuliaLang/IJulia.jl#153 or JuliaPy/PyPlot.jl#41 for why this is important even if you just want occasional status updates to be user-visible).

@cbecker mentioned that he tried jl_eval_string("yield()"), but that it crashed Julia upon return. @JeffBezanson, is there something special that needs to be done in order to yield inside of a ccall?

[JeffBezanson]

jl_eval_string("yield()") should work fine now. I also added jl_yield.

[cbecker]

Great! I tried it and it works like a charm. The only issue is that the text is only updated every two lines in IJulia, but I suppose that is a different issue related to IJulia itself. An example library call below for future reference
(note that JL_SET_STACK_BASE must not be used here)

#include <stdio.h>
#include <julia.h>

void test()
{
    int i=0;
    for (i=0; i < 10; i++)
    {   
        jl_printf(JL_STDOUT, "hehe\n");
        jl_flush_cstdio();
        jl_yield();

        system("sleep 0.5");
    }
}

To test, compile as library, then ccall() it from julia.

[stevengj]

Most likely it is grouping the outputs because there are multiple tasks running: the IJulia event loop as well as a task for outputting stdout and another for outputting stderr, for example. When you call jl_yield it only wakes up one of them, so it doesn't wake up the stdout task on every iteration.

In general, I would tend to call jl_yield more often than you print output, instead of always at the same time. You could probably call jl_yield every 10-50ms or so without having any noticable performance impact on your C code.