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I am doing a cross compilation test in Eclipse IDE for meta-toolchain made with Yocto, for arm cortex A9 processor. After performing hello world test, which ran successfully, I created a basic program to test pthreads.

    #include <iostream>
    #include <string.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <fcntl.h>
    #include <time.h>
    #include <pthread.h>

    #define MILLION 1000000         /* one million = 10^6*/
    #define sec_to_nsec 1000000000  /* ns to s conversion = 10^9 */
    #define FILENAME "Schd.txt"
    #define FLUSH_TIME 10.0
    #define SIG_LLP_TIMER       SIGRTMIN+1

    int     isr_idx;            /* counter of ISR occurred -- starts from 0 and increments at each interrupt*/

    volatile float  clk_k,              /* MY_CLOCK() value for the current sample*/
            clk_k_1;            /* MY_CLOCK() value for the previous sample*/

    /*clock and timer values*/
    struct itimerspec custom_itimerspec;
    timer_t timer_id;
    clockid_t USED_CLK;
    struct  timespec tv;

    float a_n;

    /*THREAD DATA*/
    pthread_t thread0;
    pthread_attr_t attr;
    struct sched_param param;
    using namespace std;



    void* thread_scheduler(){
        //function pointer
        //mainThread
        //make thread for scheduling
        //exit after max cycle
    }

    int main(void) 
    {

            cout << "Starting the program!" << endl; /* prints Hello World */

            cout<< "Creating a Thread to deploy" << endl;


            int  status;


            param.__sched_priority = 99;


                int retc;
                /*PTHREAD ATTR setup*/
                retc = pthread_attr_init(&attr);
                retc |= pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
                retc |= pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
                retc |= pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
                retc |= pthread_attr_setschedparam(&attr,&param);


                if (retc != 0) {
                    //fail
                    while(1){}
                }


                retc = pthread_create(&thread0, &attr, (void * (*)(void *))thread_scheduler, NULL);

            printf("Exiting here!");
            return 0;





    }

But I get this error, undefined reference to `pthread_create', followed with some make errors.

Though after doing some search I found that adding '-pthread' command in configure and autogen settings works for building the project, as described here. But I am puzzled why the compiler can't see these files even if this file is present in 'includes' in the drop down folder of project explorer.

piyj
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    `-pthread` is not for the compiler, but for the linker. What the compiler see or doesn't see has nothing to do with what the linker needs to see and link with. They are two independent compilation phases. – Sam Varshavchik Feb 23 '19 at 17:08
  • Why not use the `` facilities of the C++ Standard Library? Much easier to use than pthreads. –  Feb 23 '19 at 17:13
  • At least with recent versions of `glibc`, `std::thread` also requires `-pthread`, because `std::thread` exposes `pthread_create()` in the header files, pulling it into the app code. – Sam Varshavchik Feb 23 '19 at 17:16
  • Please show your command line, including a compile and link command. Also see [Significance of -pthread flag when compiling](https://stackoverflow.com/q/2127797/608639) and [Difference between -pthread and -lpthread while compiling](https://stackoverflow.com/q/23250863/608639). – jww Feb 23 '19 at 19:23
  • Thank you everyone for explaining. I understand that linker needs the '-pthread' command in the linked commandline to see the pthread_create function to link to the executable. Sorry to ask further questions on similar lines, but when I use semaphore functions or etc why I do not need to assign further linker commands? Or -pthread does that job already? @jww, I used the linker commandline to add the '-pthread' in both CFLAGS and CXXFLAGS, like the following.. CFLAGS=" -g -O0 -pthread -lrt --sysroot=/home/linux/Desktop/poky/build/tmp/sysroots/zc706" CXXFLAGS=" -g -O0 -pthread .. – piyj Feb 23 '19 at 21:23

1 Answers1

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The error about undefined reference is coming from linking step, not from compiling and assembling step, compile step would look for header files and its rightly finding the pthread.h from sysroot include directory as you see as well. After compiling, it has to invoke the linker to create the executable binary and thats where it fails.

When linking it need to add libpthread to linker commandline so linker can find the pthread_create function and link it into final executable, this is usually done via specifying LDFLAGS which then get appended to linker invocation.

compiler driver ( gcc ) can be used to drive both compiling and linking steps. so when you add -pthread option to compiler and compiler is also used to perform linking then it translates this option into -lpthread to linker cmdline which would then find libpthread and link it in.

Khem
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