并发编程网 - ifeve.com

图1：预期的内存可见性。线程A设置x=6和y=7，线程B在其后执行z=x*y，我们期望获取z=42的结果。

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<div>/*——————————- mutex_01.c ——————————–*
On Linux, compile with:
cc -std=c99 -pthread mutex_01.c -o mutex_01

Check your system documentation how to enable C99 and POSIX threads on
other Un*x systems.

Copyright Loic Domaigne.
Licensed under the Apache License, Version 2.0.
*————————————————————————–*/

#define _POSIX_C_SOURCE 200112L // use IEEE 1003.1-2004

#include // sleep()
#include #include
#include // EXIT_SUCCESS
#include // strerror()
#include

/***************************************************************************/
/* our macro for errors checking */
/***************************************************************************/
#define COND_CHECK(func, cond, retv, errv) \
if ( (cond) ) \
{ \
fprintf(stderr, "\n[CHECK FAILED at %s:%d]\n| %s(…)=%d (%s)\n\n",\
__FILE__,__LINE__,func,retv,strerror(errv)); \
exit(EXIT_FAILURE); \
}

#define ErrnoCheck(func,cond,retv) COND_CHECK(func, cond, retv, errno)
#define PthreadCheck(func,rc) COND_CHECK(func,(rc!=0), rc, rc)

/*****************************************************************************/
/* real work starts here */
/*****************************************************************************/
/*
* Accordingly to the Intel Spec, the following situation
*
* thread A: thread B:
* mov [_x],1 mov [_y],1
* mov r1,[_y] mov r2,[_x]
*
* can lead to r1==r2==0.
*
* We use this fact to illustrate what bad surprise can happen, if we don’t
* use mutex to ensure appropriate memory visibility.
*
*/
volatile int Arun=0; // to mark if thread A runs
volatile int Brun=0; // dito for thread B

pthread_barrier_t barrier; // to synchronize start of thread A and B.

/*****************************************************************************/
/* threadA- wait at the barrier, set Arun to 1 and return Brun */
/*****************************************************************************/
void*
threadA(void* arg)
{
pthread_barrier_wait(&barrier);
Arun=1;
return (void*) Brun;
}

/*****************************************************************************/
/* threadB- wait at the barrier, set Brun to 1 and return Arun */
/*****************************************************************************/
void*
threadB(void* arg)
{
pthread_barrier_wait(&barrier);
Brun=1;
return (void*) Arun;
}

/*****************************************************************************/
/* main- main thread */
/*****************************************************************************/
/*
* Note: we don’t check the pthread_* function, because this program is very
* timing sensitive. Doing so remove the effect we want to show
*/
int
main()
{
pthread_t thrA, thrB;
void *Aval, *Bval;
int Astate, Bstate;

for (int count=0; ; count++)
{
// init
//
Arun = Brun = 0;
pthread_barrier_init(&barrier, NULL, 2);

// create thread A and B
//
pthread_create(&thrA, NULL, threadA, NULL);
pthread_create(&thrB, NULL, threadB, NULL);

// fetch returned value
//
pthread_join(thrA, &Aval);
pthread_join(thrB, &Bval);

// check result
//
Astate = (int) Aval; Bstate = (int) Bval;
if ( (Astate == 0) && (Bstate == 0) ) // should never happen
{
printf("%7u> Astate=%d, Bstate=%d (Arun=%d, Brun=%d)\n",
count, Astate, Bstate, Arun, Brun );
}

} // forever

// never reached
//
return EXIT_SUCCESS;
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