为什么在一个简单的 OCaml 程序中 valgrind 会出现内存泄漏？

Question

let main =
  print_endline "Hello world"

这是一个简单的OCaml程序^

当我用ocamlc编译，然后在valgrind中运行这个程序：

==12457== Memcheck, a memory error detector
==12457== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==12457== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==12457== Command: ./a.out
==12457== 
Hello world
==12457== 
==12457== HEAP SUMMARY:
==12457==     in use at exit: 7,391,113 bytes in 30 blocks
==12457==   total heap usage: 56 allocs, 26 frees, 7,470,447 bytes allocated
==12457== 
==12457== 256 bytes in 1 blocks are definitely lost in loss record 15 of 27
==12457==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==12457==    by 0x11D6BC: caml_stat_alloc (in /usr/bin/ocamlrun)
==12457==    by 0x13627C: caml_executable_name (in /usr/bin/ocamlrun)
==12457==    by 0x13C5C4: caml_main (in /usr/bin/ocamlrun)
==12457==    by 0x11CDB1: main (in /usr/bin/ocamlrun)
==12457== 
==12457== 3,936,288 bytes in 1 blocks are possibly lost in loss record 27 of 27
==12457==    at 0x483B7F3: malloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==12457==    by 0x11D601: caml_alloc_for_heap (in /usr/bin/ocamlrun)
==12457==    by 0x13EF42: caml_init_major_heap (in /usr/bin/ocamlrun)
==12457==    by 0x12EFF5: caml_init_gc (in /usr/bin/ocamlrun)
==12457==    by 0x13C43D: caml_main (in /usr/bin/ocamlrun)
==12457==    by 0x11CDB1: main (in /usr/bin/ocamlrun)
==12457== 
==12457== LEAK SUMMARY:
==12457==    definitely lost: 256 bytes in 1 blocks
==12457==    indirectly lost: 0 bytes in 0 blocks
==12457==      possibly lost: 3,936,288 bytes in 1 blocks
==12457==    still reachable: 3,454,569 bytes in 28 blocks
==12457==         suppressed: 0 bytes in 0 blocks
==12457== Reachable blocks (those to which a pointer was found) are not shown.
==12457== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==12457== 
==12457== For lists of detected and suppressed errors, rerun with: -s
==12457== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

当我用ocamlopt编译，然后在valgrind中运行这个程序：

==12562== Memcheck, a memory error detector
==12562== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==12562== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==12562== Command: ./a.out
==12562== 
Hello world
==12562== 
==12562== HEAP SUMMARY:
==12562==     in use at exit: 7,080,897 bytes in 27 blocks
==12562==   total heap usage: 27 allocs, 0 frees, 7,080,897 bytes allocated
==12562== 
==12562== LEAK SUMMARY:
==12562==    definitely lost: 0 bytes in 0 blocks
==12562==    indirectly lost: 0 bytes in 0 blocks
==12562==      possibly lost: 3,936,288 bytes in 1 blocks
==12562==    still reachable: 3,144,609 bytes in 26 blocks
==12562==         suppressed: 0 bytes in 0 blocks
==12562== Rerun with --leak-check=full to see details of leaked memory
==12562== 
==12562== For lists of detected and suppressed errors, rerun with: -s
==12562== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

为什么用ocaml编译的程序会出现内存泄漏？这些只是虚假的内存泄漏吗？我需要担心这个吗？

Answer 1

我相信 OCaml GC 比不知道 OCaml 工作原理的外部工具更明智地了解哪些内存是可访问的。此类工具需要做出很容易出错的非常笼统的假设。

几乎没有泄漏报告，只是“可能的”泄漏（无论这意味着什么）。您可以想象 OCaml 字节码运行时在启动时分配少量全局存储。在我看来，称之为“内存泄漏”是不正确的。在退出时释放内存是没有意义的，因为这只是不必要的工作。

本机运行时（使用 ocamlopt 编译）没有显示泄漏。所以似乎没有什么需要解释的。

所以，总而言之，我不会担心这个。与其说是错误，不如说是内存检查器输出中的耸人听闻的术语。如果这些是 C 程序，则可能有更多理由担心。

（但是，在退出之前释放全局内存仍然没有意义。因此，许多 C 程序会通过这种分析报告类似的虚假“泄漏”。）

Answer 2

字节码解释器正在“泄漏”一个包含可执行文件名称的 256 字节字符串。由于该名称在程序的整个生命周期内都拥有，这并不重要。

而且由于“泄漏”存在于字节码解释器中，因此本机编译的程序不会发生这种情况。

更普遍地，使用 valgrind 对于垃圾收集语言（编译器开发之外）并不是真正有用，因为垃圾收集器会吞噬所有内存。最好使用了解 GC 的内存分析器，例如 statmemprof。