03-20-2012
Threads are independent, remember. You can't make any assumptions about timing unless you enforce them yourself.
Your mutex looks correct, and will stop them from accessing it simultaneously. Nothing stops main() from rolling along and printing the global value before your thread changes it, though!
In fact, it's quite likely. Threads often don't start instantly, it might wait until the next context switch to launch them, and the next context switch could easily be the exit() system call.
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LEARN ABOUT OSF1
sched_yield
sched_yield(3) Library Functions Manual sched_yield(3)
NAME
sched_yield - Signals scheduler a willingness to yield to another thread.
LIBRARY
DECthreads POSIX 1003.1c Library (libpthread.so)
SYNOPSIS
#include <pthread.h>
void sched_yield(void);
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
IEEE Std 1003.1c-1995, POSIX System Application Program Interface
PARAMETERS
None
DESCRIPTION
This routine forces the running thread to relinquish the processor until it again becomes the head of its thread list. This routine noti-
fies the thread scheduler that the current thread is willing to release its processor to other threads of equivalent or greater scheduling
precedence. (A thread generally will release its processor to a thread of a greater scheduling precedence without calling this routine.)
If no other threads of equivalent or greater scheduling precedence are ready to execute, the thread continues.
This routine can allow knowledge of the details of an application to be used to improve its performance. If a thread does not call
sched_yield, other threads may be given the opportunity to run at arbitrary points (possibly even when the interrupted thread holds a
required resource). By making strategic calls to sched_yield, other threads can be given the opportunity to run when the resources are
free. This can sometimes improve performance by reducing contention for the resource.
As a general guideline, consider calling this routine after a thread has released a resource (such as a mutex) that is heavily contended
for by other threads. This can be especially important if the program is running on a uniprocessor machine, or if the thread acquires and
releases the resource inside a tight loop.
Use this routine carefully and sparingly, because misuse can cause unnecessary context switching which will increase overhead and degrade
performance. For example, it is counter-productive for a thread to yield while it holds a resource that the threads to which it is yielding
will need. Likewise, it is pointless to yield unless there is likely to be another thread that is ready to run.
RETURN VALUES
None
RELATED INFORMATION
Functions: pthread_attr_setschedparam(3), pthread_setschedparam(3)
Manuals: Guide to DECthreads and Programmer's Guide
delim off
sched_yield(3)