为什么valgrind在调用`pcap_open_offline`时报告内存泄漏？

Question

我想弄清楚我是在白痴还是libpcap中确实存在内存泄漏。 我正在运行Ubuntu 17.10和libpcap 1.8.1-5ubuntu1。 这样一个成熟的库似乎不太可能出现泄漏。

我已经裁剪出所有内容来制作MVCE，因此，此代码除了演示泄漏之外并没有做任何事情：

#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（添加了重点）：

==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

在libpcap联机帮助页中 ，进行了稍微编辑：

pcap_fopen_offline()返回指向pcap_t的指针，该指针是用于读取数据包的句柄……要关闭句柄，请使用pcap_close() 。

free(fd)仅释放一个内存块，因为free()对pcap_t的内部pcap_t 。 为了正确处理分配的资源，您需要使用pcap_close(fd) ，如文档所示。