那些遇到过的问题

valgrind错误和ucontext.为什么"使用8英寸的未初始化值"？

jac*_*ver 8 c linux valgrind ucontext

我一直试图理解为什么valgrind抱怨使用ucontexts的小型测试程序"使用8号未初始化的值".它基本上是一个创建"n_ucs"ucontexts并在"max_switch"时间内切换它们的程序.

我理解"警告:客户端切换堆栈？" (这基本上就是程序的全部内容),但我对所有"使用未初始化的8号值"都没有意义

我想得到一些帮助,了解Valgrind错误是否为误报,或者该程序是否存在根本错误.(我在一个使用相同机制的更大的程序中看到了很多它们,但是我把它提炼到最低限度以便在这里发布).

任何帮助表示赞赏.

谢谢,

插口

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <ucontext.h>

#define STACK_SIZE   (8*1024)

int n_ucs = 1;
int max_switchs = 10;
int n_switchs = 0;
int tid = 0;

ucontext_t *ucs;
static ucontext_t engine_uc;

static void func(int arg)
{
    while (n_switchs < max_switchs) {
        int c_tid = tid;
        int n_tid = (tid + 1) % n_ucs;
        n_switchs++;
        tid = n_tid;
        swapcontext(&ucs[c_tid], &ucs[n_tid]);
    }
}

int main(int argc, char **argv)
{
    if (argc > 1)
        n_ucs = atoi(argv[1]);
    if (argc > 2)
        max_switchs = atoi(argv[2]);

    ucs = malloc(sizeof(ucontext_t) * n_ucs);
    int i;
    for (i = 0; i < n_ucs; i++) {
        /* Create initial ucontext_t, including stack */
        getcontext(&ucs[i]);
        ucs[i].uc_stack.ss_sp = malloc(STACK_SIZE);
        ucs[i].uc_stack.ss_size = STACK_SIZE;
        ucs[i].uc_stack.ss_flags = 0;
        ucs[i].uc_link = &engine_uc;
        makecontext(&ucs[i], (void (*)())func, 1, i);
    }

    /* jump to the first uc */
    swapcontext(&engine_uc, &ucs[tid]);

    /* destroy stacks */
    for (i = 0; i < n_ucs; i++)
        free(ucs[i].uc_stack.ss_sp);
    free(ucs);
    return 0;
}
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用gcc main.c编译并运行./a.out 2 2

gcc -v

使用内置规格.COLLECT_GCC = gcc COLLECT_LTO_WRAPPER =/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper目标:x86_64-linux-gnu配置为:../ src /configure -v --with-pkgversion ='Ubuntu 4.8 .2-19ubuntu1' - with-bugurl = file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages = c,c ++,java,go,d,fortran,objc,obj -c ++ --prefix =/usr --program-suffix = -4.8 --enable-shared --enable-linker-build-id --libexecdir =/usr/lib --without-included-gettext --enable-threads = posix --with-gxx-include-dir =/usr/include/c ++/4.8 --libdir =/usr/lib --enable-nls --with-sysroot =/--enable-clocale = gnu --enable -libstdcxx-debug --enable-libstdcxx-time = yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable- java-awt = gtk --enable-gtk-cairo --with-java-home =/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with -jvm-root-dir =/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir =/usr/lib/jvm-exports/java-1.5.0- gcj-4.8-amd64 --with-arch-directory = amd64 --with-e cj-jar =/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32 = i686 --with-abi = m64 - -with-multilib-list = m32,m64,mx32 --with-tune = generic --enable-checking = release --build = x86_64-linux-gnu --host = x86_64-linux-gnu --target = x86_64- linux-gnu线程模型:posix gcc版本4.8.2(Ubuntu 4.8.2-19ubuntu1)

ldd --version

ldd(Ubuntu EGLIBC 2.19-0ubuntu6.3)2.19版权所有(C)2014 Free Software Foundation,Inc.这是免费软件; 查看复制条件的来源.没有保修; 甚至不适用于适销性或特定用途的适用性.由Roland McGrath和Ulrich Drepper撰写.

valgrind --track-origins=yes ./a.out 2 2
==21949== Memcheck, a memory error detector
==21949== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==21949== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==21949== Command: ./a.out 2 2
==21949==
==21949== Warning: client switching stacks?  SP change: 0xffefffdd8 --> 0x51ff7b8
==21949==          to suppress, use: --max-stackframe=68616717856 or greater
==21949== Use of uninitialised value of size 8
==21949==    at 0x400738: func (main.c:25)
==21949==    by 0x4E58EC4: (below main) (libc-start.c:287)
==21949==  Uninitialised value was created by a stack allocation
==21949==    at 0x4E7E445: swapcontext (swapcontext.S:92)
==21949==
==21949== Conditional jump or move depends on uninitialised value(s)
==21949==    at 0x4E807A7: __start_context (__start_context.S:37)
==21949==    by 0x4E58EC4: (below main) (libc-start.c:287)
==21949==  Uninitialised value was created by a stack allocation
==21949==    at 0x4E7E445: swapcontext (swapcontext.S:92)
==21949==
==21949== Syscall param rt_sigprocmask(set) contains uninitialised byte(s)
==21949==    at 0x4E7E0EC: setcontext (setcontext.S:47)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==  Uninitialised value was created by a stack allocation
==21949==    at 0x4E7E445: swapcontext (swapcontext.S:92)
==21949==
==21949== Use of uninitialised value of size 8
==21949==    at 0x4E7E0F5: setcontext (setcontext.S:54)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==  Uninitialised value was created by a stack allocation
==21949==    at 0x4E7E445: swapcontext (swapcontext.S:92)
==21949==
==21949== Use of uninitialised value of size 8
==21949==    at 0x4E7E0FE: setcontext (setcontext.S:56)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==    by 0x4E807AD: __start_context (__start_context.S:39)
==21949==  Uninitialised value was created by a stack allocation
==21949==    at 0x4E7E445: swapcontext (swapcontext.S:92)
==21949==
==21949== Warning: client switching stacks?  SP change: 0x51ff7c0 --> 0xffefffde0
==21949==          to suppress, use: --max-stackframe=68616717856 or greater
==21949==
==21949== HEAP SUMMARY:
==21949==     in use at exit: 0 bytes in 0 blocks
==21949==   total heap usage: 3 allocs, 3 frees, 18,256 bytes allocated
==21949==
==21949== All heap blocks were freed -- no leaks are possible
==21949==
==21949== For counts of detected and suppressed errors, rerun with: -v
==21949== ERROR SUMMARY: 5 errors from 5 contexts (suppressed: 0 from 0)
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Nal*_*war 1

我仍然不明白为什么 valgrind 会准确地显示这些未初始化的错误，但我会尽力解释我到目前为止所理解的内容；

在通过 valgrind 运行和分析程序时，根据 swapcontext(3) 和 getcontext(3) 手册页的信息，我认为它无法检测到某些上下文交换（无法看到 swapcontext 的堆栈指针从 tid 0 更改为tid 1 以及从 tid 1 回到 tid 0 的交换上下文）

如下所示：谁的堆栈[调用次数]：函数调用

所以，我认为函数调用跟踪是这样的：

主要：swapcontext（主要，tid 0）->

main[tid 0 的第一个函数调用]:func() ->

tid 0:swapcontext(tid 0, tid 1) -> {堆栈 => tiod 0 }

tid 1:func() ->

swapcontext(tid 1, tid 0) -> {堆栈 => tiod 1 }

tid 0[第二次调用]: func() ->

立即返回，因为 n_switchs = 2 ->

pop tid 0[第 2 次调用]：来自 tid 1 堆栈的 func() 堆栈帧 -> {根据 valgrind 的第 1 次未初始化访问}

tid 0[第二次调用]：func() 完成 -> 检查 uc_link；找到设置在那里的engine_uc（主上下文）->

从这里开始，事情对我来说变得不清楚，但以下似乎是可能的痕迹：

重置 sigprocmask -> {2nd 未初始化访问} setcontext()s 返回主上下文 -> {3rd 未初始化访问？} { Stack => main }

返回时，[tid 0's 1st call] 的堆栈帧从 main 的堆栈中弹出 ->

main [tid 0's 1st call]:func() 也完成，因为 n_switchs = 2 -> check uc_link; 再次找到engine_uc -> 重置sigprocmask -> {不是未初始化的访问？}

返回时， main:swapcontext() 的堆栈帧从 main 的堆栈中弹出 ->

setcontext() 返回主上下文 -> {4th 未初始化访问？} { Stack => main }

我们回到 main()，释放东西并退出

一些参考资料：

https://www.gnu.org/software/libc/manual/html_node/System-V-contexts.html http://www.cs.uwm.edu/classes/cs315/Bacon/Lecture/HTML/ch10s07.html

注意：我知道这不是一个完整的答案，但我不想在评论部分发布这么长的解释；因此发布在这里。

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