cgi - 关于php线程安全的问题

        if (phys_pages == 0)
        {
            phys_pages = __sysconf (_SC_PHYS_PAGES);
            //__sysconf函数在sysdeps/posix/sysconf.c中
            //_SC_PHYS_PAGES对应到函数__get_phys_pages()
            //位于文件sysdeps/unix/sysv/linux/getsysstats.c中
            //通过phys_pages_info()打开/proc/meminfo来读取内存信息
            //(这就定位到了qsort打开文件的问题)

            if (phys_pages == -1)
                /* Error while determining the memory size.  So let's
                  assume there is enough memory.  Otherwise the
                  implementer should provide a complete implementation of
                  the `sysconf' function.  */
                phys_pages = (long int) (~0ul >> 1);

            /* The following determines that we will never use more than
              a quarter of the physical memory.  */
            phys_pages /= 4;

            pagesize = __sysconf (_SC_PAGESIZE);
        }
        //注意，上面这一段if会产生竞争，出现线程安全安全：
        //如果两个线程都调用qsort，当线程1获取了phys_pages之后，线程2
        //才到达if，线程2就会跳过这一段代码，直接执行下面的if语句——
        //而此时pagesize尚未初始化（=0），于是就会出现除零错误，导致
        //core dump

        /* Just a comment here.  We cannot compute (phys_pages * pagesize)
          and compare the needed amount of memory against this value.
          The problem is that some systems might have more physical
          memory then can be represented with a `size_t' value (when
          measured in bytes.  */

        /* If the memory requirements are too high don't allocate memory.  */
        //如果所需的内存页数大于总的可用内存，则不分配内存（防止swap降低性能）
        if (size / pagesize > (size_t) phys_pages)
        {
            //直接使用stdlib/qsort.c中的 _quicksort 进行排序
            _quicksort (b, n, s, cmp, arg);
            return;
        }