未初始化的值是由堆栈分配创建的 - valgrind [英] Uninitialised value was created by a stack allocation - valgrind

问题描述

我使用 valgrind 通过选项 track-origins=yes 调试我的代码并遇到了这个错误.

I used valgrind to debug my code with the option track-origins=yes and came across this error.

$ valgrind --track-origins=yes ./frgtnlng < in > out
==7098== 
==7098== Conditional jump or move depends on uninitialised value(s)
==7098==    at 0x4C2F1BC: strcmp (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==7098==    by 0x400857: main (frgtnlng.c:24)
==7098==  Uninitialised value was created by a stack allocation
==7098==    at 0x40064C: main (frgtnlng.c:9)
==7098== 
==7098== Conditional jump or move depends on uninitialised value(s)
==7098==    at 0x40085A: main (frgtnlng.c:24)
==7098==  Uninitialised value was created by a stack allocation
==7098==    at 0x40064C: main (frgtnlng.c:9)

第 9 行是:

scanf("%d", &t);

我不明白这会如何导致问题.

I don't understand how this could cause the problem.

frgtnlng.c:

frgtnlng.c:

#include <stdio.h>
#include <string.h>

int main(void)
{
    int t, n, k, l, i, j, z, out[100];
    char f[5][100], m[5][50][50];

    scanf("%d", &t);
    while (t--) {
        for (i = 0; i < 100; i++)
            out[i] = 0;
        scanf("%d%d", &n, &k);
        for (i = 0; i < n; i++)
            scanf("%s", f[i]);
        for (i = 0; i < k; i++) {
            scanf("%d", &l);
            for (j = 0; j < l; j++)
                scanf("%s", m[i][j]);
        }
        for (i = 0; i < k; i++)
            for (j = 0; j < l; j++)
                for (z = 0; z < n; z++) {
                    if (strcmp(m[i][j], f[z]) == 0)
                        out[z] = 1;
                }
        for (i = 0; i < n; i++) {
            if (out[i])
                printf("YES ");
            else
                printf("NO ");
        }
        printf("\n");
    }
    return 0;
}

在:

2
3 2
piygu ezyfo rzotm
1 piygu
6 tefwz tefwz piygu ezyfo tefwz piygu
4 1
kssdy tjzhy ljzym kegqz
4 kegqz kegqz kegqz vxvyj

推荐答案

valgrind 的行号关闭:它应该报告分配行号的 7，而不是 9.然而，错误行 24 是正确的 - 问题在这里:

valgrind's line number is off: it should have reported 7, not 9, for the allocation line number. The error line 24, however, is correct - the problem is here:

if (strcmp(m[i][j], f[z]) == 0)

问题是 j 从 0 到 l-1 循环，包括在内，但 l 是它在最后 读取二维数组的循环的迭代，即 4.这就是为什么每次遇到数组中少于 4 个条目的行时，它从数组的未初始化部分读取.

The issue is that j loops from 0 to l-1, inclusive, but l is whatever it has been set to in the last iteration of the loop that reads the 2D array, i.e. 4. That's why each time it comes to a line in the array with fewer than 4 entries it reads from uninitialized portion of the array.

解决方法是通过将 l 设为数组 l[5] 并使用 l[i] 来分别存储各个行的长度在两个循环中:

The fix is to store lengths of individual rows separately by making l an array l[5], and using l[i] in both loops:

for (i = 0; i < k; i++) {
    scanf("%d", &l[i]);
    for (j = 0; j < l[i]; j++)
        scanf("%s", m[i][j]);
}
for (i = 0; i < k; i++)
    for (j = 0; j < l[i]; j++)
        for (z = 0; z < n; z++) {
            if (strcmp(m[i][j], f[z]) == 0)
                out[z] = 1;
        }

这篇关于未初始化的值是由堆栈分配创建的 - valgrind的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！