multithreading - Passing string as Thread start routine parameter : C++ -

referring http://www.yolinux.com/tutorials/linuxtutorialposixthreads.html#scheduling

i trying create 2 threads in c++ , trying pass string parameter thread start routine. thread start routine parameter can (void *) type per definition:

int pthread_create(pthread_t * thread,                         const pthread_attr_t * attr,                        void * (*start_routine)(void *),                         void *arg); 

but below error:

$ make g++ -g -wall trial.cpp -o trial trial.cpp: in function `int main()': trial.cpp:22: error: cannot convert `message1' type `std::string' type `void*' trial.cpp:23: error: cannot convert `message2' type `std::string' type `void*' makefile:2: recipe target `trial' failed make: *** [trial] error 1 

the code is

#include <iostream> #include <pthread.h> #include <string>    using namespace std;  void *print_message_function( void *ptr );   int main() {     pthread_t thread1, thread2;      string message1 = "thread 1";     string message2 = "thread 2";      int  iret1, iret2;        iret1 = pthread_create( &thread1, null, print_message_function, (void*) message1);      iret2 = pthread_create( &thread2, null, print_message_function, (void*) message2);         pthread_join( thread1, null);      pthread_join( thread2, null);         cout << "thread 1 returns: " <<  iret1 << endl;      cout << "thread 2 returns: " << iret2 << endl;       return 0;     } void *print_message_function( void *ptr )  {      cout << endl <<  ptr << endl;      //return 0;  } 

is there way pass string (void *) parameter ? or c style strings can used multithreading parameters - given in reference code @ link.

the argument needs pointer, , try pass object it.

you have 2 choices, either pass pointer std::string object, or pass pointer underlying string. recommend first:

 iret1 = pthread_create(&thread1, null, print_message_function, &message1); 

then have modify thread function, otherwise print pointer , not string points to:

void* print_message_function(void* ptr) {     std::string str = *reinterpret_cast<std::string*>(ptr);      std::cout << str << std::endl;      return nullptr; } 

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unless it's requirement use posix threads, rather recommend threading functionality in c++ standard library:

#include <iostream> #include <string> #include <thread>  void print_message_function(const std::string& msg);  int main() {     std::string message1 = "thread 1";     std::string message2 = "thread 2";      std::thread thread1(print_message_function, std::cref(message1));     std::thread thread2(print_message_function, std::cref(message2));      thread1.join();     thread2.join(); }  void print_message_function(const std:string& msg) {     std::cout << msg << std::endl; }