如何在Xcode中启用MALLOC_PROTECT_BEFORE？

Question

在Xcode中打开一些调试选项后，我得到了这个输出：

GuardMalloc[Roadcast-4010]: free: magic is 0x0000090b, not 0xdeadbeef.
GuardMalloc[Roadcast-4010]: free: header magic value at 0x43f49bf0, for block 0x43f49c00-0x43f50000, has been trashed by a buffer underrun.
GuardMalloc[Roadcast-4010]: Try running with MALLOC_PROTECT_BEFORE to catch this error immediately as it happens.

如何开启MALLOC_PROTECT_BEFORE？

更新：

Mac Developer Library > Guard_Malloc记录了MALLOC_PROTECT_BEFORE所做的事情：

  

libgmalloc的行为可以通过几个附加更改   环境变量：

     

MALLOC_PROTECT_BEFORE

     

如果设置了此标志，则libgmalloc会更加努力地检测缓冲区   欠载运行。具体来说，libgmalloc放置已分配的开始   缓冲区在虚拟内存页面的开头，然后保护页面   之前。缓冲区欠载然后导致错误。没有的行为   这个变量集是将缓冲区的末尾放在   分配的最后一页，并保护页面。

Answer 1

要在Xcode中启用MALLOC_PROTECT_BEFORE，请在Xcode菜单中转到

产品＆gt;方案＆gt;编辑方案......

然后在弹出的页面中，转到Arguments并在Environment Variables下，添加MALLOC_PROTECT_BEFORE并为其赋值1。您可以在屏幕截图1中看到这一点：

要实际使用它，现在点击诊断并勾选“启用Guard Malloc”，如Scrrenshot 2所示：

然后点击“确定”即可完成。

要测试检测缓冲区欠载现在是否有效，请使用以下代码：

    int* allocated = malloc(1024);

    printf("If this is the last line printed, then MALLOC_PROTECT_BEFORE is enabled and buffer underruns are detected.\n");

    int value = allocated[-5]; // fails if MALLOC_PROTECT_BEFORE is set AND activated

    printf("The value read from unallocated memory space is %i.\n", value);
    printf("If this line is printed, then MALLOC_PROTECT_BEFORE is NOT enabled and buffer underruns can occur unnoticed.\n");

    free(allocated);
    allocated = NULL;