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;
}
如果没有对多线程做特殊处理的话，任何会带来副作用的函数内都可能导致这个问题。
给你举个栗子：glibc的qsort函数在多线程使用的时候也可能会core。为什么呢？因为有一段代码是这样的
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; }完整的代码阅读笔记可参考 glibc的qsort源码阅读笔记
顺便提一下，解决qsort问题的办法，是在主线程中先调用一次qsort，让它初始化了pagesize，这样只会就不会遇到竞争了。
Windows中运行PHP建议使用FastCGI方式，不会遇到线程安全问题。