Why this code of CPP async gives error however the function without async works well?

Question

The code below gives the following error, however the function without threads and async works well:

main.cpp:38:93: error: no matching function for call to ‘async(std::launch, void (&)(sqlite3*, std::string*, std::ofstream&), sqlite3*&, std::string*, std::ofstream&)’
   38 | ultFromDB = std::async(std::launch::async, writeToDB, DB, &z, logFile);

Can you please show the source of error?

// C library headers
#include <stdio.h>
#include <string.h>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>

// db
#include <sqlite3.h>

// Linux headers
#include <fcntl.h> // Contains file controls like O_RDWR
#include <errno.h> // Error integer and strerror() function
#include <termios.h> // Contains POSIX terminal control definitions
#include <unistd.h> // write(), read(), close()

// multithreading
#include <chrono>
#include <thread>
#include <future>

using namespace std;

void writeToDB(sqlite3* DB,std::string* state, ofstream& logFile)
{
    // do some work
}

int main()
{
    std::ofstream logFile;
    logFile.open ("a.txt");
    sqlite3* DB; 
    for(int i = 0; i < 10; i++)
    {
        string z = "123";
        std::future<void> resultFromDB = std::async(std::launch::async, writeToDB, DB, &z, logFile);
    }
}

I understand that there is something about pointers and params, but I am not advanced in CPP. Thank you.

EDIT: After I replaced logFile with std::ref(logFile), I get this error:

/usr/bin/ld: /tmp/ccl0RarK.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Does this answer your question? [Passing arguments to std::async by reference fails](https://stackoverflow.com/questions/18359864/passing-arguments-to-stdasync-by-reference-fails). — François Andrieux, Aug 10 '20 at 13:56
Aside: Luckily for you, `resultFromDB`'s destruction at the end of the block waits on that future, otherwise `state` in `writeToDB` might become invalid from under you. Unfortunately that means you do each write sequentially. — Caleth, Aug 10 '20 at 14:07
@Caleth That is a very good point, passing by value might be appropriate here. Perhaps `std::shared_ptr` for logFile. — Quimby, Aug 10 '20 at 14:10

score 2 · Accepted Answer · edited Aug 10 '20 at 14:01

2

std::thread and std::async do not accept references implicitly, you have to wrap them in std::ref:

std::async(std::launch::async, writeToDB, DB, &z, std::ref(logFile));

edited Aug 10 '20 at 14:01

Marek R

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answered Aug 10 '20 at 13:56

Quimby

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Ok, this solved the problem, but now, I get this: `/usr/bin/ld: /tmp/ccl0RarK.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5' /usr/bin/ld: /usr/lib/libpthread.so.0: error adding symbols: DSO missing from command line collect2: error: ld returned 1 exit status` – jokey Aug 10 '20 at 14:06
1

On linux with gcc you have to explicitly link with POSIX threads. Add `-pthread` option to the compiler. – Quimby Aug 10 '20 at 14:08
Are these proccesses totally independent from each other? – jokey Aug 10 '20 at 14:28
What processes? `std::async` launches threads or reuses them from a thread pool. – Quimby Aug 10 '20 at 14:56

Why this code of CPP async gives error however the function without async works well?

1 Answers1