Valgrind 报告一个非常简单的 C 程序的错误

Question

我正在向Learn C The Hard Way 学习 C 语言。我在 exercise 6 上，虽然我可以让它工作，但 valgrind 报告了很多错误。

这是从文件 ex6.c 中剥离出来的最小程序：

#include <stdio.h>

int main(int argc, char *argv[])
{
    char initial = 'A';
    float power = 2.345f;

    printf("Character is %c.\n", initial);
    printf("You have %f levels of power.\n", power);

    return 0;
}

Makefile 的内容就是CFLAGS=-Wall -g。

我使用$ make ex6 编译程序（没有编译器警告或错误）。使用$ ./ex6 执行会产生预期的输出。

当我使用$ valgrind ./ex6 运行程序时，我遇到了无法解决的错误。这是完整的输出：

==69691== Memcheck, a memory error detector
==69691== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==69691== Using Valgrind-3.11.0.SVN and LibVEX; rerun with -h for copyright info
==69691== Command: ./ex6
==69691==
--69691-- UNKNOWN mach_msg unhandled MACH_SEND_TRAILER option
--69691-- UNKNOWN mach_msg unhandled MACH_SEND_TRAILER option (repeated 2 times)
--69691-- UNKNOWN mach_msg unhandled MACH_SEND_TRAILER option (repeated 4 times)
==69691== Conditional jump or move depends on uninitialised value(s)
==69691==    at 0x1003FBC3F: _platform_memchr$VARIANT$Haswell (in /usr/lib/system/libsystem_platform.dylib)
==69691==    by 0x1001EFBB6: __sfvwrite (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001FA005: __vfprintf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x10021F9CE: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x10021FCA0: __xvprintf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001F5B91: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001F39F7: printf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x100000F1B: main (ex6.c:8)
==69691==
Character is A.
==69691== Invalid read of size 32
==69691==    at 0x1003FBC1D: _platform_memchr$VARIANT$Haswell (in /usr/lib/system/libsystem_platform.dylib)
==69691==    by 0x1001EFBB6: __sfvwrite (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001FA005: __vfprintf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x10021F9CE: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x10021FCA0: __xvprintf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001F5B91: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x1001F39F7: printf (in /usr/lib/system/libsystem_c.dylib)
==69691==    by 0x100000F31: main (ex6.c:9)
==69691==  Address 0x100809680 is 32 bytes before a block of size 32 in arena "client"
==69691==
You have 2.345000 levels of power.
==69691==
==69691== HEAP SUMMARY:
==69691==     in use at exit: 39,365 bytes in 429 blocks
==69691==   total heap usage: 510 allocs, 81 frees, 45,509 bytes allocated
==69691==
==69691== LEAK SUMMARY:
==69691==    definitely lost: 16 bytes in 1 blocks
==69691==    indirectly lost: 0 bytes in 0 blocks
==69691==      possibly lost: 13,090 bytes in 117 blocks
==69691==    still reachable: 26,259 bytes in 311 blocks
==69691==         suppressed: 0 bytes in 0 blocks
==69691== Rerun with --leak-check=full to see details of leaked memory
==69691==
==69691== For counts of detected and suppressed errors, rerun with: -v
==69691== Use --track-origins=yes to see where uninitialised values come from
==69691== ERROR SUMMARY: 5 errors from 2 contexts (suppressed: 0 from 0)

我在 OS X 优胜美地。 Valgrind 通过brew 使用此命令$ brew install valgrind --HEAD 安装。

那么，有谁知道这里的问题是什么？如何修复 valgrind 错误？

Answer 1

如果您通过 Valgrind 运行的程序正是您在问题中发布的程序，那么它显然没有任何内存泄漏。事实上，你自己甚至都不使用 malloc/free！

在我看来，这些是 Valgrind 在 OS X 上检测到的虚假错误/误报（仅限！），类似于 what happened to myself some time ago。

如果您可以访问其他操作系统，例如一台 Linux 机器，尝试在该系统上使用 Valgrind 分析程序。

编辑：我自己没有尝试过，因为我现在无法使用 Mac，但你应该尝试一下 M Oehm 建议：try to use a supressions file as mentioned in this other SO question.

Answer 2

此问题已针对 Darwin 14.3.0 (Mac OS X 10.10.2) 修复，使用 Xcode6.2 的 Valgrind r14960 with VEX r3124 和 Xcode 6.3 的 Valgrind r15088。

如果您使用的是 Macports（在撰写本文时），sudo port install valgrind-devel 会给您Valgrind r14960 with VEX r3093。

这是我要安装的构建脚本Valgrind r14960 with VEX r3124：

#! /usr/bin/env bash

mkdir -p buildvalgrind
cd buildvalgrind
svn co svn://svn.valgrind.org/valgrind/trunk/@14960 valgrind
cd valgrind
./autogen.sh
./configure --prefix=/usr/local
make && sudo make install

# check that we have our valgrind installed
/usr/local/bin/valgrind --version 

（参考：http://calvinx.com/2015/04/10/valgrind-on-mac-os-x-10-10-yosemite/）

我的 macports 安装的 valgrind 位于 /opt/local/bin/valgrind。

如果我现在运行

/opt/local/bin/valgrind --leak-check=yes --suppressions=`pwd`/objc.supp ./ex6

我将得到与您上面描述的完全相同的错误。 （在这里使用我的objc.supp 文件https://gist.github.com/calvinchengx/0b1d45f67be9fdca205b）

但如果我跑了

/usr/local/bin/valgrind --leak-check=yes --suppressions=`pwd`/objc.supp ./ex6

一切都按预期工作，我没有出现系统级内存泄漏错误。

Answer 3

从this 主题来看，我认为 valgrind 不能保证在您的平台上给出正确的结果。如果可以，请在其他平台上尝试此代码。

罪魁祸首要么在于 valgrid 本身，要么在于您系统的 printf 实现，这两者对您来说都是不切实际的。

Rerun with --leak-check=full to see details of leaked memory. 这应该会给你一些关于你正在经历的泄漏的更多信息。如果没有任何帮助，您可以创建一个suppression file 以阻止显示错误。