再谈野指针问题Free Heap block xxx modified at xxx after it was freed
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Free Heap block XXXXXX modified at XXXXXX after it was freed
向原创致敬 ↑
相信不少朋友碰到过以上问题,而往往苦于无法定位到错误的代码位置,其实出现该问题一般就是使用了野指针导致的.
那么何为野指针呢?
野指针与NULL指针不同.所谓野指针是指指针所指的内存已经回收,而继续使用该指针,导致未定义行为.
例如:
- char *p = new char[512];
- *p = '/0';
- delete[] p;
- *p = 'a'; // 此处出现了野指针
所以为了避免此类问题。往往采用删除后立即清空指针变量的方法
- delete []p;
- p = NULL; // 进行置NULL
- *p = 'a'; // 这里调试器将给出提示,成功定位错误位置
这样看似很好解决野指针的问题,其实不然,因为要知道内存删除与指针变量使用往往是脱节的。
例如:
- char *p = new char[512];
- char *q = p;
- *p = '/0';
- delete[] p;
- p = NULL;
- *q = 'a'; // q与内存删除代码往往不在同一地点,没办法指定q=NULL;所以照样出现了野指针。
那么此类问题如何定位呢?
这里就要利用文章开头写的那个提示信息了。
Free Heap block d49418 modified at d49574 after it was freed
这句话的意思是,释放了内存开始为0xd49418的堆块后,堆块所在内存的0xd49574位置被更改了。
这样就好办了,我们只需要查找到0xd49418所在位置属于分配给了哪个对象,然后再定位0xd49574所处的成员变量
然后查找所有使用该变量并进行了变量内容的修改的代码就可以定位到错误位置了。再根据情况修改代码,避免内存释放后仍然进行修改即可。
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