realloc()内存泄漏

Question

我有一个向字符串添加字符的函数：

void AddChToString(char **str,char ch){
    int len=(*str)?strlen(*str):0;
    (*str)=realloc(*str, len+2);
    (*str)[len]=ch;
    (*str)[len+1]='\0';
}

Instruments (在mac上)和Valgrind指示行：(*str)=realloc(*str，len+2)是内存泄漏。这是realloc的实现问题吗？还是我用错了？

下面是Valgrind的输出：

==39230== 6 bytes in 1 blocks are definitely lost in loss record 1 of 7
==39230==    at 0x100018B2D: realloc (vg_replace_malloc.c:525)
==39230==    by 0x100002259: AddChToString (in ./OpenOtter)
==39230==    by 0x10000477B: QueryMapFromString (in ./OpenOtter)
==39230==    by 0x100684CD2: ???
==39230==    by 0x100001FB0: RequestHandler (in ./OpenOtter)
==39230==    by 0x100065535: _pthread_start (in /usr/lib/libSystem.B.dylib)
==39230==    by 0x1000653E8: thread_start (in /usr/lib/libSystem.B.dylib)
==39230== 
==39230== 9 bytes in 1 blocks are definitely lost in loss record 2 of 7
==39230==    at 0x100018B2D: realloc (vg_replace_malloc.c:525)
==39230==    by 0x100002259: AddChToString (in ./OpenOtter)
==39230==    by 0x10000298E: ParseHTTPRequest (in ./OpenOtter)
==39230==    by 0x100004151: OpenRoutesFile (in ./OpenOtter)
==39230==    by 0x10000142B: main (in ./OpenOtter)
==39230== 
==39230== 45 bytes in 5 blocks are definitely lost in loss record 3 of 7
==39230==    at 0x100018B2D: realloc (vg_replace_malloc.c:525)
==39230==    by 0x100002259: AddChToString (in ./OpenOtter)
==39230==    by 0x10000298E: ParseHTTPRequest (in ./OpenOtter)
==39230==    by 0x100001EB4: RequestHandler (in ./OpenOtter)
==39230==    by 0x100065535: _pthread_start (in /usr/lib/libSystem.B.dylib)
==39230==    by 0x1000653E8: thread_start (in /usr/lib/libSystem.B.dylib)
==39230== 
==39230== LEAK SUMMARY:
==39230==    definitely lost: 60 bytes in 7 blocks
==39230==    indirectly lost: 0 bytes in 0 blocks
==39230==      possibly lost: 0 bytes in 0 blocks
==39230==    still reachable: 1,440 bytes in 4 blocks
==39230==         suppressed: 0 bytes in 0 blocks

谢谢。

Answer 1

调用realloc本身不会泄漏内存。你应该确保重新分配的字符串的内存在不再需要时释放。

Answer 2

您的工具是否表明存在实际泄漏或潜在泄漏？

如果realloc()失败，像你这样使用realloc()会泄漏内存。在这种情况下，它将返回NULL，但不会释放原始块。因此，您将丢失指向该块的指针，并且无法释放它(除非该指针存储在其他地方)。

但这应该是很少见的(例如，当你耗尽内存时)。

如果这就是你的工具所抱怨的，你应该能够用下面这样的东西来修复泄漏警告：

void AddChToString(char **str,char ch){
    int len=(*str)?strlen(*str):0;
    char* tmp = realloc(*str, len+2);

    if (!tmp) {
        // whatever error handling is appropriate
    }

    (*str)=tmp;
    (*str)[len]=ch;
    (*str)[len+1]='\0';
}

Answer 3

我不知道关于realloc的实现问题，但这段代码中肯定存在内存泄漏的机会:来自realloc手册页：

如果realloc()失败，原始块保持不变；它不会被释放或移动。

而且，由于realloc在失败时返回NULL，因此如果失败，您将丢失指向已经分配的内存块的唯一指针，因此存在内存泄漏。

要避免该问题，您应该执行以下操作：

char * temp=realloc(*str, len+2);
if(temp==NULL)
{
    /* handle the error in some way */
}
else
    *str=temp;