std::promise throws Unknown error, unless calling sleep

Question

I have this code:

#include <future>
#include <thread>

int main()
{
    std::promise<void> p;
    p.set_value();
    p.get_future().get();

    return 0;
}

And after compiling it with gcc it throws std::system_error:

$ g++ -o foo foo.cpp -std=c++11 -lpthread
$ ./foo
terminate called after throwing an instance of 'std::system_error'
  what():  Unknown error -1

What is weird, adding zero-second sleep before creating the promise, prevents the exception:

int main()
{
    std::this_thread::sleep_for(std::chrono::milliseconds(0));
    std::promise<void> p;
    p.set_value();
    p.get_future().get();

    return 0;
}

$ g++ -o foo foo.cpp -std=c++11 -lpthread
$ ./foo
$

I tried gcc 4.8.5 and 5.4.0, same results. Why does it behave like that?

I cant reproduce it: https://wandbox.org/permlink/FDfl2jIkEeAXIbKj — Amadeus, Jan 22 '18 at 12:26
Is there any reason you felt the need to omit the version of GCC you can reproduce this on? Kind of important information... — user167921, Jan 22 '18 at 12:46
Possible duplicate of [Unknown exception from std::promise](https://stackoverflow.com/questions/43928715/unknown-exception-from-stdpromise) — ks1322, Jan 22 '18 at 12:48
It is wrong to build with `-lpthread`. You should with `-pthread`. — ks1322, Jan 22 '18 at 12:49
@ks1322 thanks, it seems that it was about `lpthread` vs `pthread` — Tomasz Maciejewski, Jan 22 '18 at 12:55
@TomaszMaciejewski I've edited your question back. On StackOverflow we avoid to answer the question in the question itself ;). — YSC, Jan 22 '18 at 13:02

YSC · Accepted Answer · 2018-01-22T13:06:40.400

10

This error comes from your compilation. It should be:

 g++ -o foo foo.cpp -std=c++11 -pthread

The <thread> library needs this special flag -pthread but you provided -lpthread. The former compile your translation unit with the full thread support. The later only links the library, without defining the needed macros and needed tools.

On coliru:

with -pthread: http://coliru.stacked-crooked.com/a/a53bed6696bb8d83
without: http://coliru.stacked-crooked.com/a/fd972e1556f8c060

edited Jan 22 '18 at 13:06

answered Jan 22 '18 at 12:47

YSC

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@infinitezero it might be due to my first version which was unperfect (was it worth a downvote though, who knows?). – YSC Jan 22 '18 at 13:26

std::promise throws Unknown error, unless calling sleep

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