如何在Xcode中启用MALLOC_PROTECT_BEFORE？

Question

在Xcode中打开了一些调试选项后，我得到了以下输出：

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：

在Mac Developer Library > Guard_Malloc中，文档详细说明了MALLOC_PROTECT_BEFORE的作用：

libgmalloc的行为可以通过多个额外的环境变量进行更改：

MALLOC_PROTECT_BEFORE

如果设置了此标志，则libgmalloc将更努力地检测缓冲区下溢。具体来说，libgmalloc将分配的缓冲区的开始放置在虚拟内存页面的开头，然后在前面保护该页面。缓冲区下溢会导致错误。如果没有设置此变量，则行为是将缓冲区的末尾放置在分配的最后一页的末尾，然后保护其后面的页面。

- Daniel S.

Answer 1

为了在Xcode中启用MALLOC_PROTECT_BEFORE，您需要在Xcode菜单中转到Product > Scheme > Edit Scheme...，然后在弹出的页面中，转至Arguments并在Environment Variables下添加MALLOC_PROTECT_BEFORE，并将其值设置为1。您可以在屏幕截图1中看到这一点： 为了确保实际使用该选项，请现在点击诊断，并标记“启用Guard Malloc”，如屏幕截图2所示： 然后点击OK即可完成。

为了测试是否现在可以检测到缓冲区下溢，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;

- Daniel S.