关闭与数据库的连接时不会释放内存（C，postgres - libpq）答案

【问题标题】：Memory is not freed when connection to database is closed (C, postgres - libpq)关闭与数据库的连接时不会释放内存（C，postgres - libpq）
【发布时间】：2020-01-12 11:44:28
【问题描述】：

如果你创建的程序只有以下两行：

PGconn *conn = PQconnectdb("user=dbtest password=dbtest dbname=testdb host=localhost port=5432");
PQfinish(conn);

然后像这样通过 valgrind 运行它：

valgrind --leak-check=full --show-leak-kinds=all ./program

你看到了这个：

==25388== Memcheck, a memory error detector
==25388== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==25388== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==25388== Command: ./program
==25388== 
==25388== 
==25388== HEAP SUMMARY:
==25388==     in use at exit: 109 bytes in 2 blocks
==25388==   total heap usage: 6,948 allocs, 6,946 frees, 666,147 bytes allocated
==25388== 
==25388== 13 bytes in 1 blocks are still reachable in loss record 1 of 2
==25388==    at 0x483874F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==25388==    by 0x4CB0635: CRYPTO_strdup (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
==25388==    by 0x4BD8AFB: BIO_meth_new (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
==25388==    by 0x488BE8B: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x488D19C: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x487507F: PQconnectPoll (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x4875E3E: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x4878CD7: PQconnectdb (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x109158: main (in /workspace/test/src/program)
==25388== 
==25388== 96 bytes in 1 blocks are still reachable in loss record 2 of 2
==25388==    at 0x483874F: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==25388==    by 0x4CA70E8: CRYPTO_zalloc (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
==25388==    by 0x4BD8ADF: BIO_meth_new (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
==25388==    by 0x488BE8B: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x488D19C: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x487507F: PQconnectPoll (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x4875E3E: ??? (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x4878CD7: PQconnectdb (in /usr/lib/x86_64-linux-gnu/libpq.so.5.11)
==25388==    by 0x109158: main (in /workspace/test/src/program)
==25388== 
==25388== LEAK SUMMARY:
==25388==    definitely lost: 0 bytes in 0 blocks
==25388==    indirectly lost: 0 bytes in 0 blocks
==25388==      possibly lost: 0 bytes in 0 blocks
==25388==    still reachable: 109 bytes in 2 blocks
==25388==         suppressed: 0 bytes in 0 blocks
==25388== 
==25388== For counts of detected and suppressed errors, rerun with: -v
==25388== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

我注意到打开和关闭更多连接似乎不会影响丢失或仍可访问的字节数。

我对C不是很熟悉，所以不得不问：

为什么还有字节可以访问？当我关闭与数据库的连接时，我希望一切都被释放。

【问题讨论】：

这里没有丢失任何东西，still reachable 非常好。你应该只关心definitely lost 和indirectly lost。 possibly lost 通常也是虚惊一场。
@KirilKirov 可能没问题，但我想知道这个数字为什么大于零。据我所知，所有分配的东西都应该被释放。

标签： c postgresql libpq

【解决方案1】：

这些分配是在 libcrypto 的初始化期间执行的，postgresql 客户端使用它来通过 SSL/TLS 与服务器通信。初始化只进行一次；您打开多少个连接并不重要。（但是，如果连接使用不同的密码，可能会有额外的初始化。）

Libcrypto 确实提供了可用于释放在初始化期间分配的存储空间的接口。这些记录在OpenSSL documentation 中。因此，您可以尝试将它们添加到您的代码中。但是，老实说，除了使 valgrind 报告更清晰之外，这真的无关紧要。

【讨论】：