使用valgrind時無效讀取大小1

Question

使用valgrind時，我得到以下輸出：

==19923== Invalid read of size 1
==19923==    at 0x52CCCC0: vfprintf (vfprintf.c:1632)
==19923==    by 0x52F4772: vasprintf (vasprintf.c:59)
==19923==    by 0x52D3A56: asprintf (asprintf.c:35)
==19923==    by 0x400D77: addToList (pa1.c:124)
==19923==    by 0x4010AF: md5Hash (pa1.c:220)
==19923==    by 0x401138: newFile (pa1.c:244)
==19923==    by 0x401260: searchDirects (pa1.c:280)
==19923==    by 0x401451: main (pa1.c:339)
==19923==  Address 0x585b720 is 0 bytes inside a block of size 27 free'd
==19923==    at 0x4C2EDEB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19923==    by 0x401144: newFile (pa1.c:246)
==19923==    by 0x401260: searchDirects (pa1.c:280)
==19923==    by 0x401382: directoryCheck (pa1.c:312)
==19923==    by 0x4012CD: searchDirects (pa1.c:290)
==19923==    by 0x401451: main (pa1.c:339)
==19923==  Block was alloc'd at
==19923==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19923==    by 0x52F47D7: vasprintf (vasprintf.c:73)
==19923==    by 0x52D3A56: asprintf (asprintf.c:35)
==19923==    by 0x40112C: newFile (pa1.c:239)
==19923==    by 0x401260: searchDirects (pa1.c:280)
==19923==    by 0x401382: directoryCheck (pa1.c:312)
==19923==    by 0x4012CD: searchDirects (pa1.c:290)
==19923==    by 0x401451: main (pa1.c:339)

在這里，我將其范圍縮小為代碼中的問題：

else
{    
    currentList = headList;
    printf("This is where I will put the code to create the Linked list!\n");
    if(currentList != 0)
    {
        while(currentList->nextList != 0)
        {
            if(currentList->key = key)
            {
                asprintf(&buff, "%s %s", currentList->value, dirValue); // PROBLEM IS HERE
                printf("Here is the new string that holds the directory paths: %s\n", buff);
                currentList->value = buff;
                free(buff);
                printf("Here is the new string that holds the directory paths: %s\n", currentList->value);
                return;
            }
            currentList = currentList->nextList;
        }

        currentList->nextList = malloc(sizeof(struct List));
        printf("Adding a new node\n");
        //Reached the end of the Linked list and didn't find the
        //same key so create a new node.
        currentList = currentList->nextList;
        currentList->key = key;
        currentList->nextList = NULL;
        currentList->value = dirValue;
    }
}

我有很多錯誤，我覺得提供整個valgrind輸出會過大。 我得到的所有錯誤都返回到我使用asprintf（）的位置("addToList (pa1.c:124)" line in the report) 。 在代碼上方，我初始化了char *buff = NULL; ，並且currentList->value包含一個我想使用asprintf附加到的字符串。 對於我為什么會收到大量無效讀取大小1錯誤的任何猜測，將不勝感激。 另外，當我確實打印出asprintf（）的結果時，該字符串將按預期追加。

Answer 1

無效的讀取是在已釋放的內存塊的開頭； 您要在釋放內存后嘗試使用內存。

currentList->value = buff;
free(buff);
printf("Here is the new string that holds the directory paths: %s\n", currentList->value);
return;

您使用currentList->value = buff; 在printf() ，但是您剛剛釋放了buff 。 這意味着一場即將來臨的災難。 如果在return之后將在調用函數中使用currentList->value ，則會遇到重大問題。

您可以通過將printf()移到free() printf()前面來解決一個警告，但是代碼看起來將遇到重大問題。 刪除free(buff);可能更好free(buff); -盡管您必須找出實際釋放數據的位置。