calling pthread_create with a function which takes char pointer

Question

I have a function which takes string. Since pthread don't accept string , i make the function's parameter char pointer. Now I want to call that function with pthread_create but i can not do it. Problem arise because of void * i think. I searched it and make some casting but i cannot succeed. How can i fix it so that it can work under g++

#include <iostream>
#include <cstdlib>
#include <pthread.h>

using namespace std;

#define NUM_THREADS     5

void printString(char *x)
{
    cout << x << endl;
        pthread_exit(NULL);
}

int main ()
{
     pthread_t threads[NUM_THREADS];
     int rc;
     int i;
     string temp = "hello";

  char *bufferG;
  bufferG = new char[temp.size() + 1]; 
  std::copy(temp.begin(), temp.end(), bufferG); 
  bufferG[temp.size()] = '\0'; 

     for( i=0; i < NUM_THREADS; i++ ){
         cout << "main() : creating thread, " << i << endl;
         rc = pthread_create(&threads[i], NULL, printString,  &bufferG ); //(void *) &bufferG also doesn't work
     }
 pthread_exit(NULL);
 }

the error is : thread.cpp: In function ‘int main()’: thread.cpp:27:69: error: invalid conversion from ‘void ()(char)’ to ‘void* ()(void)’ [-fpermissive] /usr/include/pthread.h:225:12: error: initializing argument 3 of ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* ()(void), void*)’ [-fpermissive]

mockinterface · Accepted Answer · 2014-04-07 12:45:24Z

3

The argument expected by pthread_create is,

void *(*)(void *)

that is a pointer to function accepting a void pointer and returning a void pointer.

Change your method to have the following signature:

/*static*/ void* printString(void *x) { ... }

answered Apr 7, 2014 at 12:45

mockinterface

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2 Comments

oiyio Over a year ago

it can compile but cannot print "hello" whether cout << *x or cout << x

mockinterface Over a year ago

@user1308990 Don't pass &bufferG but rather bufferG in your pthread_create.

Dr. Debasish Jana · Accepted Answer · 2014-04-07 13:05:59Z

1

Ok, try this out, now the main thread joins on children to execute and also instead of printing single character within thread fn, we are now using the whole buffer instead

#include <iostream>
#include <cstdlib>
#include <pthread.h>

using namespace std;

#define NUM_THREADS     5

void * printString(void *x)
{
    cout << ((char *)x)<< endl;
}

int main ()
{
     pthread_t threads[NUM_THREADS];
     int rc;
     int i;
     string temp = "hello";

  char *bufferG;
  bufferG = new char[temp.size() + 1]; 
  std::copy(temp.begin(), temp.end(), bufferG); 
  bufferG[temp.size()] = '\0'; 


     for( i=0; i < NUM_THREADS; i++ ){
         cout << "main() : creating thread, " << i << endl;
         rc = pthread_create(&threads[i], NULL, printString,  bufferG ); 
     }
    for( i=0; i < NUM_THREADS; i++ ){
     pthread_join(threads[i], NULL);
    }
 pthread_exit(NULL);
 }

edited Apr 7, 2014 at 13:05

answered Apr 7, 2014 at 12:53

Dr. Debasish Jana

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4 Comments

oiyio Over a year ago

can be compiled but cannot print "HELLO"

Dr. Debasish Jana Over a year ago

@user1308990, try the edited anser now, should work as intended

Dr. Debasish Jana Over a year ago

@user1308990, if the answer is acceptable, you may please accept for my two cents

oiyio Over a year ago

two answer cannot be accepted simultaneously and since @mockinterface answered first it would be unfair for him

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