c - "Pointer being freed was not allocated"发生在 mac 但不是在 windows 7

Question

我正在做一本书的练习，将句子中的单词改成 pig latin。代码在window 7下运行正常，但是当我在mac上编译时，错误就出来了。

经过一些测试，错误来自那里。我不明白这个问题的原因。我为所有指针使用动态内存，并且还添加了空指针检查。

while (walker != NULL && *walker != NULL){
    free(**walker); 
    free(*walker);
    free(walker); 

    walker++;
}

完整源代码:

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

#define inputSize 81
void getSentence(char sentence [], int size);
int countWord(char sentence[]);
char ***parseSentence(char sentence[], int *count);
char *translate(char *world);
char *translateSentence(char ***words, int count);

int main(void){
    /* Local definition*/
    char sentence[inputSize];
    int wordsCnt;
    char ***head;
    char *result;

    getSentence(sentence, inputSize);
    head = parseSentence(sentence, &wordsCnt);

    result = translateSentence(head, wordsCnt);
    printf("\nFinish the translation: \n");
    printf("%s", result);


    return 0;
}

void getSentence(char sentence [81], int size){
    char *input = (char *)malloc(size);
    int length;

    printf("Input the sentence to big latin : ");
    fflush(stdout);
    fgets(input, size, stdin);

    // do not copy the return character at inedx of length - 1
    // add back delimater 
    length = strlen(input);
    strncpy(sentence, input, length-1);
    sentence[length-1]='\0';

    free(input);
}

int countWord(char sentence[]){
    int count=0;

    /*Copy string for counting */
    int length = strlen(sentence);
    char *temp = (char *)malloc(length+1);
    strcpy(temp, sentence);

    /* Counting */
    char *pToken = strtok(temp, " ");
    char *last = NULL;
    assert(pToken == temp);
    while (pToken){
        count++;

        pToken = strtok(NULL, " ");
    }

    free(temp);
    return count;
}
char ***parseSentence(char sentence[], int *count){
    // parse the sentence into string tokens
    // save string tokens as a array
    // and assign the first one element to the head
    char *pToken;
    char ***words;
    char *pW;

    int noWords = countWord(sentence);
    *count = noWords;

    /* Initiaze array */
    int i;
    words = (char ***)calloc(noWords+1, sizeof(char **));
    for (i = 0; i< noWords; i++){
        words[i] = (char **)malloc(sizeof(char *));
    }

    /* Parse string */
    // first element
    pToken = strtok(sentence, " ");

    if (pToken){
        pW = (char *)malloc(strlen(pToken)+1);
        strcpy(pW, pToken);
        **words = pW;
        /***words = pToken;*/

        // other elements
        for (i=1; i<noWords; i++){
            pToken = strtok(NULL, " ");
            pW = (char *)malloc(strlen(pToken)+1);
            strcpy(pW, pToken);
            **(words + i) = pW;
            /***(words + i) = pToken;*/
        }
    }

    /* Loop control */
    words[noWords] = NULL;


    return words;
}

/* Translate a world into big latin */
char *translate(char *word){
    int length = strlen(word);
    char *bigLatin = (char *)malloc(length+3);

    /* translate the word into pig latin */
    static char *vowel = "AEIOUaeiou";
    char *matchLetter;
    matchLetter = strchr(vowel, *word);
    // consonant
    if (matchLetter == NULL){
        // copy the letter except the head
        // length = lenght of string without delimiter
        // cat the head and add ay
        // this will copy the delimater,
        strncpy(bigLatin, word+1, length);
        strncat(bigLatin, word, 1);
        strcat(bigLatin, "ay");
    }
    // vowel
    else {
        // just append "ay"
        strcpy(bigLatin, word);
        strcat(bigLatin, "ay");
    }


    return bigLatin;
}

char *translateSentence(char ***words, int count){
    char *bigLatinSentence;
    int length = 0;
    char *bigLatinWord;

    /* calculate the sum of the length of the words */
    char ***walker = words;
    while (*walker){
        length += strlen(**walker);
        walker++;
    }

    /* allocate space for return string */
    // one space between 2 words
    // numbers of space required = 
    // length of words
    // + (no. of words * of a spaces (1) -1 ) 
    // + delimater
    // + (no. of words * ay (2) )
    int lengthOfResult = length + count + (count * 2);
    bigLatinSentence = (char *)malloc(lengthOfResult);
    // trick to initialize the first memory 
    strcpy(bigLatinSentence, "");

    /* Translate each word */
    int i;
    char *w;
    for (i=0; i<count; i++){
        w = translate(**(words + i));
        strcat(bigLatinSentence, w);
        strcat(bigLatinSentence, " ");
        assert(w != **(words + i));
        free(w);
    }


    /* free memory of big latin words */
    walker = words;
    while (walker != NULL && *walker != NULL){
        free(**walker); 
        free(*walker);
        free(walker); 

        walker++;
    }

    return bigLatinSentence;
}

Answer 1

你的代码不必要地复杂，因为你已经设置了这样的东西:

• n : 字数
• words : 指向可以容纳 n+1 的已分配内存char **按顺序值
• words[i] ( 0 <= i && i < n ): 指向可以容纳一个 char * 的已分配内存按顺序
• words[n] : NULL
• words[i][0] : 指向为单词分配的内存(和以前一样，0 <= i < n)

因为每个words[i]指向顺序的东西，有一个words[i][j]对于一些有效的整数 j ... 但是 j 的允许值始终为 0，因为只有一个 char * malloc() 在那里。所以你可以完全消除这种间接级别，只需要 char **words .

但这不是问题所在。释放循环以 walker 开始与 words 相同, 所以它首先尝试释放 words[0][0] (这很好并且有效)，然后尝试释放 words[0] (这很好并且有效)，然后尝试释放 words (这很好并且有效，但意味着您不能再访问任何其他 words[i] 以获得 i 的任何值——即“存储泄漏”)。然后它增加 walker , 使其或多或少等同于 &words[1] ;但是words已经free() d.

而不是使用 walker在这里，我将使用带有一些整数的循环 i :

for (i = 0; words[i] != NULL; i++) {
    free(words[i][0]);
    free(words[i]);
}
free(words);

我还建议删除所有 malloc() 上的强制转换和 calloc()返回值。如果您在执行此操作后收到编译器警告，它们通常意味着以下两种情况之一:

• 你忘了#include <stdlib.h> , 或
• 您正在对您的 C 代码调用 C++ 编译器。

后者有时有效，但却是痛苦的根源:好的 C 代码是坏的 C++ 代码，好的 C++ 代码不是 C 代码。 :-)

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编辑:PS:我错过了差一 lengthOfResult @David RF 捕获了。