为什么调用 `pcap_open_offline` 时 valgrind 会报告内存泄漏？

Question

我正试图弄清楚我是否有一个白痴时刻，或者 libpcap 中是否真的存在内存泄漏。我正在运行 Ubuntu 17.10 和 libpcap 1.8.1-5ubuntu1。这么成熟的库似乎不太可能出现泄漏。

我已经删除了所有内容来制作一个 MVCE，因此，除了演示泄漏之外，这段代码实际上并没有做任何事情：

#include <stdio.h>
#include <stdlib.h>
#include <pcap.h>

int main(int argc, char **argv)
{
        char errbuf[PCAP_ERRBUF_SIZE];

        pcap_t *fd = pcap_open_offline(argv[1], errbuf);

        if (!fd) {
                printf("error: %s\n", errbuf);
        }

        free(fd); fd = 0;

        return 0;
}

valgrind 报告（已添加重点）：

==6871==
==6871== HEAP SUMMARY:
==6871==     in use at exit: 262,696 bytes in 2 blocks
==6871==   total heap usage: 4 allocs, 2 frees, 267,432 bytes allocated
==6871==
==6871== Searching for pointers to 2 not-freed blocks
==6871== Checked 73,072 bytes
==6871==
==6871== 262,144 bytes in 1 blocks are definitely lost in loss record 2 of 2
==6871==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6871==    by 0x4E5B89F: ??? (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.8.1)
==6871==    by 0x4E5AE5C: pcap_fopen_offline_with_tstamp_precision (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.8.1)
==6871==    by 0x4E5B05D: pcap_open_offline_with_tstamp_precision (in /usr/lib/x86_64-linux-gnu/libpcap.so.1.8.1)
==6871==    by 0x1087A0: main (test.c:9)
==6871==
==6871== LEAK SUMMARY:
==6871==    definitely lost: 262,144 bytes in 1 blocks
==6871==    indirectly lost: 0 bytes in 0 blocks
==6871==      possibly lost: 0 bytes in 0 blocks
==6871==    still reachable: 552 bytes in 1 blocks
==6871==         suppressed: 0 bytes in 0 blocks
==6871== Reachable blocks (those to which a pointer was found) are not shown.
==6871== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==6871==
==6871== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==6871== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Answer 1

来自libpcap manpage， 稍作修改：

pcap_fopen_offline() return[s] 指向pcap_t 的指针，这是用于读取数据包的句柄……要关闭句柄，请使用pcap_close()。

free(fd) 只释放一块内存，因为free() 对pcap_t 的内部结构一无所知。如文档所示，为了正确处理分配的资源，您需要改用pcap_close(fd)。