c - valgrind错误条件跳转

Question

我正在编写一个加载输入直到输入特定单词的程序，在本例中是单词“konec”。虽然我的程序似乎工作正常，但我无法解决这个 Valgrind 错误

==16573== Memcheck, a memory error detector   
==16573== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.  
==16573== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info  
==16573== Command: ./s_main_o  
==16573==   
==16573== Conditional jump or move depends on uninitialised value(s)  
==16573==    at 0x4C2A020: strcmp (mc_replace_strmem.c:711)  
==16573==    by 0x4008D7: main (main.c:41)  
==16573==  Uninitialised value was created by a heap allocation  
==16573==    at 0x4C28CCE: realloc (vg_replace_malloc.c:632)  
==16573==    by 0x40089C: main (main.c:38)  
==16573==   
==16573== Conditional jump or move depends on uninitialised value(s)  
==16573==    at 0x4C2A024: strcmp (mc_replace_strmem.c:711)  
==16573==    by 0x4008D7: main (main.c:41)  
==16573==  Uninitialised value was created by a heap allocation  
==16573==    at 0x4C28CCE: realloc (vg_replace_malloc.c:632)  
==16573==    by 0x40089C: main (main.c:38)  
==16573==   
==16573==   
==16573== HEAP SUMMARY:  
==16573==     in use at exit: 0 bytes in 0 blocks  
==16573==   total heap usage: 8 allocs, 8 frees, 1,125 bytes allocated  
==16573==   
==16573== All heap blocks were freed -- no leaks are possible  
==16573==   
==16573== For counts of detected and suppressed errors, rerun with: -v  
==16573== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 4 from 4)  

这里是部分使用的代码

int main() {

    int numberOfWords, i;
    char** words;
    char* word;
    int* rarity;
    char* konec = "konec";
    int amount = 0;
    double percentage;
    words = malloc(10 * sizeof (char*));
    rarity = calloc(256, sizeof (int));
    numberOfWords = 0;
    words[0] = 0;
    int working = 1;

    while (working == 1) {
        int length = 0;
        word = calloc((length + 1),sizeof (char));
        char c;
        while ((c = getchar()) != EOF) {
            if (c == ' ' || c == '\n') {
                break;
            }
            length++;
            word = realloc(word, length + 1);
            word[length - 1] = c;
        }
        if (strcmp(word, konec) == 0) {
            working = 0;
            free(word);
            break;
        }
    }
}

我发现很多主题都在讨论同一个问题，但我还是找不到解决方案。感谢您的回答。

Answer 1

问题是您没有在重新分配时添加空终止符:

word = realloc(word, length + 1);
word[length - 1] = c;

此时，word 字符串未终止，因此 strcmp 可能会在搜索空终止符时结束。例如，当您键入“ko”时，strcmp 将确定字符 0 和 1 相同，并尝试检查 word[2] - 您的程序未设置的位置。

添加这一行来解决问题:

word[length] = '\0';

当与 konec 比较不成功时，您还应该将代码添加到 free word。

注意:您没有正确使用 realloc:您不应将其分配回 word，而应将其分配给 temp ，并检查它是否为 NULL。否则，当 realloc 失败时，您将无法释放先前分配的字。