Xlib : Segmentation fault on multithreading - c++

Question

My attempt is to write an Xlib wrapper for the purpose of implementing triple buffering methods. Using which a person needs only compute their display matrix and forward it to the API for displaying. I have two separate threads for handling events and display. The events thread seems to execute without any issue, however, the display thread, when used with standard Xlib functions such as XDrawRectangle, XFillArc, XSetForeground, etc. seems to cause a segmentation fault of an unknown nature.
This is my thread execution part :
int startx(){
pthread_t eventsThread, displayThread;
char msg1[15] ="Events Thread", msg2[15] = "Display Thread";
int pid1, pid2;
pid1 = pthread_create( &eventsThread, NULL, eventsHandler, (void*) msg1);
pid2 = pthread_create( &displayThread, NULL, displayHandler, (void*) msg2);
pthread_join(eventsThread, NULL);
pthread_join(displayThread, NULL);
return 0;
};
This is my displayHandler :
void *displayHandler(void* args){
cout<<connectionNumber<<endl;
Color c(50,50,250);
int width = 40, height = 60,x = 500, y = 100;
for(int i=0;i<1300;i++){
XSetForeground(display, xgraphics, c.decimal);
XDrawRectangle(display, mainWindow, xgraphics, x, y, width, height);
XFlush(display);
}
}
The eventsThread seems to be executing without error. Also, I have tried making the display function a part of the main program, with the same results.
If somebody could tell me an alternative/correct method to paint the window using matrices, it would be most appreciated.
Note : Color is a self made class for ease of colour computation.

Answer 1

This crashes for me before the howdy line. Uncommenting the return NULL; line makes it work.
#include <iostream>
#include <pthread.h>
void *displayHandler(void* args) {
char* txt = reinterpret_cast<char*>(args);
std::cout << txt << "\n";
// return NULL;
}
int startx(){
pthread_t displayThread;
char msg2[15] = "Display Thread";
int pid2;
pid2 = pthread_create( &displayThread, NULL, displayHandler, (void*) msg2);
pthread_join(displayThread, NULL);
return 0;
}
int main() {
startx();
std::cout << "howdy\n";
}

Answer 2

As Ted Lyngmo points out. The problem lay with the fact that Xlib has no thread safety implemented for writing to the display. So writing a mutex presented a solution.
If any of the event masks are set to write to the screen, separate threads for both become pointless. Instead making the masks toggle variables, allow them to work simultaneously.