C++: Core dump with packaged_task

Question

I got a strange core dump which I copied from a part of code in http://en.cppreference.com/w/cpp/thread/packaged_task,

#include 
#include 

        

Answer 1

I tried your first code snippet using the following version of g++:

g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

and compiled with the following

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

and I can run the program with no problems, and getting the following output:

$ ./test_threads
task_lambda: 512

If I then use the -lpthread as you did with the following

g++ -o test_threads test_threads.cpp -std=c++11 -lpthread  

I get

$ ./test_threads terminate called after throwing an instance of 'std::system_error'
what(): Unknown error -1
[1] 7890 abort ./test_threads

So please use -pthread as a flag and not -lpthread.

This behavior is also mentioned in the following
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59830

There's a difference between -pthread and -lpthread.
Looking at the man page for g++

-pthread
Adds support for multithreading with the pthreads library. This option sets flags for both the preprocessor and linker.

To have a look to what flags are activated for both one can check with the following:

g++ -dumpspecs | grep pthread
g++ -dumpspecs | grep lpthread

As one can clearly see, there are some preprocessor macros that are not activated if one is using -lpthread.