c - pthread_mutex_lock() 处的段错误

Question

我有一个可以打印一些数据结构的代码。

void
print_wheel_timer(wheel_timer_t *wt){

 <code to print data structure>
}

Then i sandwiched the code in-between lock and unlock mutex calls. 

void
print_wheel_timer(wheel_timer_t *wt){
pthread_mutex_lock(&wt->global_lock);
<code to print data structure>
pthread_mutex_unlock(&wt->global_lock);
}

现在，出现了段错误。我已经禁用了代码中的所有线程，现在程序是单线程的，仍然是段错误。我使用的是 gcc，ubuntu 19.04。

gdb 显示 pthread_mutex_lock() 调用正在搞乱数据结构!这很奇怪!我将观察点放在由任意值写入的地址上，它显示对 pthread_mutex_lock() 的调用正在修改它。当我删除这些锁定/解锁调用时，没有看到段错误。

(gdb) watch *(glthread_t *)&(wt->slotlist[1].slots.right)
Hardware watchpoint 2: *(glthread_t *)&(wt->slotlist[1].slots.right)
(gdb) c
Continuing.
Printing Wheel Timer DS

Thread 1 "test.exe" hit Hardware watchpoint 2: *(glthread_t *)&(wt->slotlist[1].slots.right)

Old value = {left = 0x0, right = 0x0}
New value = {left = 0x1, right = 0x0}     <<< corruption !!
0x00007ffff7fa3934 in __GI___pthread_mutex_lock (mutex=0x5555555771d8)
at ../nptl/pthread_mutex_lock.c:80
80      ../nptl/pthread_mutex_lock.c: No such file or directory.
(gdb) bt
#0  0x00007ffff7fa3934 in __GI___pthread_mutex_lock (mutex=0x5555555771d8)
at ../nptl/pthread_mutex_lock.c:80
#1  0x000055555555f3a2 in print_wheel_timer (wt=0x555555577180)
at WheelTimer/WheelTimer.c:276

Code :
========
void
print_wheel_timer(wheel_timer_t *wt){

int i = 0, j = 0;
glthread_t *curr;
glthread_t *slot_list_head = NULL;
wheel_timer_elem_t *wt_elem = NULL;

printf("Printing Wheel Timer DS\n");
pthread_mutex_lock(&wt->global_lock);
printf("wt->current_clock_tic  = %d\n", wt->current_clock_tic);
printf("wt->clock_tic_interval = %d\n", wt->clock_tic_interval);
printf("wt->wheel_size         = %d\n", wt->wheel_size);
printf("wt->current_cycle_no   = %d\n", wt->current_cycle_no);
printf("wt->wheel_thread       = %p\n", &wt->wheel_thread);
printf("WT uptime              = %s\n", hrs_min_sec_format(WT_UPTIME(wt)));
printf("wt->no_of_wt_elem      = %u\n", wt->no_of_wt_elem);
printf("printing slots : \n");

for(; i < wt->wheel_size; i++){
    slot_list_head = WT_SLOTLIST_HEAD(wt, i);
    ITERATE_GLTHREAD_BEGIN(slot_list_head, curr){           << segfaulting here for i = 1
        wt_elem = glthread_to_wt_elem(curr);

如果某个神圣的灵魂想在他的机器上尝试一下。 请在这里下载代码: https://github.com/sachinites/tcpip_stack

After downloading :
switch to branch "Hellos"
git checkout Hellos
Compile :
make all
run
./test.exe
to reproduce , run the cmd : 
debug show node H1 timer 

现在它将以函数为核心:在WheelTimer/WheelTimer.c中实现的print_wheel_timer()

Answer 1

问题出在我的数据结构上，其中零长度数组是 C 结构的非最后一个成员，在读/写到此类数组时会损坏内存。可拉伸(stretch)数组应该是结构的最后一个成员。

typedef struct _wheel_timer_t {
    int current_clock_tic;
    int clock_tic_interval;
    int wheel_size;
    int current_cycle_no;
    pthread_t wheel_thread;
    slotlist_t slotlist[0];            << problem, make it last member
    slotlist_t reschd_list;
    unsigned int no_of_wt_elem;
    pthread_mutex_t global_lock;
} wheel_timer_t;