CODEWITHSUNDEEP
c++multithreadingc++11g++pthreads
user5255670
user5255670

Reputation:

g++ - Why do I have to pass "-pthread" option while using std::thread?

I'm just trying to understand a concept used by g++. Here my very simple std::thread application:

#include <iostream>
#include <thread>

void func() {
    std::cout << "Running..." <<  std::endl;
}

int main()
{
    std::thread t(func);
    t.join();
    return 0;
}

I'm able to compile it on macOs/Xcode 9.0 setup with following command:

g++ main.cpp -std=c++11

But I'm unable to compile it on Linux with the same command, as far as I know I have to pass -pthread option too. Otherwise it gives me following error:

gcc version 7.1.1 20170622 (Red Hat 7.1.1-3)

main.o: In function `std::thread::thread<void (&)()>(void (&)())':
/usr/include/c++/5/thread:137: undefined reference to `pthread_create'

I think it's illogical and I shouldn't even know that it's implementing the std::thread class via pthread. Why do I have to pass -pthread option and link against pthread library? Isn't C++11 supposed to abstract me away from platform specific details? Or do I have any other alternative libraries such as pthread that I can link against for my std::thread usage? Or should I report that as a bug?

Thanks.

Upvotes: 16

Views: 5099

Answers (3)

eshangui
eshangui

Reputation: 21

I am moving pthread to std:thread in C++11 and have encountered the same phenomenon you've been met, and I found this artical -- it may be the proper answer: https://developers.redhat.com/articles/2021/12/17/why-glibc-234-removed-libpthread#

A quick conclusion: It depends on the version of glibc. glibc with versions prior to 2.34 will require the -lpthead flag even the code does not use pthread explicitly.

To check the version of glibc we can use ldd --version command, on my Ubuntu 20.04, it returns like this: ldd (Ubuntu GLIBC 2.31-0ubuntu9.9) 2.31, so I still have to add the -lpthread flag to use std:thread.

Upvotes: 2

Seth Johnson
Seth Johnson

Reputation: 15172

According to GCC's concurrency page, it's necessary to provide additional options to the compiler based on the features being used. You can verify that your version of GCC's threads rely on POSIX threads:

$ gcc -v 2>&1 | grep "Thread model"
Thread model: posix

See this bug report for a justification for the behavior:

The problem is that not all targets need -pthread, or some which do need it spell it differently, and not all platforms define _REENTRANT when the option is used, so there's no reliable way to do what you're asking for.

Upvotes: 2

Surt
Surt

Reputation: 16099

pthread is an industry standard over the OS specific threads, using the OS specific calls.

std::thread is an abstraction in C++ that could be implemented using pthread or the OS's native threads. But to make it work on as many OS's as possible fast the std-library implementer could just implement it in posix as they should be good to go on all compliant OS's.

There are exceptions, some windows only std-library uses windows native threads instead.

Upvotes: 0

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