[pthread - Solaris 8] pthread_mutex_lock / pthread_mutex_unlock weird pb

Question

Hi all,
I actually have the current problem : I create 2 threads, one thread works a lot and spend most of its time in the critical session and the other one sleeps most of the time (outside the critical section)
The problem is : the slow thread never get the mutex so it never access to the critical session and the big worker seems to unlock/relock to fast to let the other thread work.
The problem occurs under Solaris 8 but I have no problem under Solaris 10 (most of our workstations still work under Solaris 8 so I must make it work under Solaris 8)
Here is a very simple code (main.cpp) you can compile/try on your Sun Solaris 8 workstation :
#include <pthread.h>
#include <stdio.h>
#include <iostream.h>
#include <unistd.h>
pthread_mutex_t mutex;
extern "C" void bigWorkerThr(void pClosure)
     while(1)
          if(0 != pthread_mutex_lock(&mutex)) exit(0);
          char str[5];
          strcpy(str, "test"); // short time operation
          if(0 != pthread_mutex_unlock(&mutex)) exit(0);
          //usleep(1);
     return(NULL);
extern "C" void slowWorkerThr(void pClosure)
     while(1)
          sleep(1);
          if(0 != pthread_mutex_lock(&mutex)) exit(0);
          cerr << "HELLO WORLD" << endl; // display "HELLO WORLD" means it works
          if(0 != pthread_mutex_unlock(&mutex)) exit(0);
     return(NULL);
int main()
     cerr << "STARTING ..." << endl;
     pthread_mutex_init(&mutex, NULL);
     pthread_t bigWorkerThrID;
     if(0 != pthread_create(&bigWorkerThrID, NULL, bigWorkerThr, NULL)) exit(0); // big worker
     pthread_t slowWorkerThrID;
     if(0 != pthread_create(&slowWorkerThrID, NULL, slowWorkerThr, NULL)) exit(0); // slow worker
     void *res = NULL;
     pthread_join(slowWorkerThrID, &res);
     pthread_join(bigWorkerThrID, &res);
Here is my compiling command and environment:
# $ uname -a
# SunOS mscorail 5.8 Generic_117350-45 sun4u sparc SUNW,Sun-Fire-V210
(SunFire V210 (2 x 1336 MHz) with 4 Go of memory)
# $ CC -V
# CC: Sun C++ 5.8 2005/10/13
# $ CC -mt main.cpp -o main

Answer 1

The way you are programming the locks is incorrect. You cannot depend on
the "unlock" statement to cause the other thread to run. The most EFFICIENT way for the OS to run a threaded program, is to keep running the same
thread until it must block for some reason. If you want two threads to
hand off back and forth to share a resource, you have to add explicit
variables or extra synchronization to directly cause the threads take
turns the way you want.
--chris