没有分配重新分配的指针？

Question

也许这是一个愚蠢的问题，但我在这里卡了一段时间。

假设freq_tostring() 将词频freq 转换为字符串，然后freq_intostream() 将该字符串附加到流的末尾。

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

typedef struct {
    char *word; // null-terminated
    int freq;
} freq;

/**
 * Constructor
 */
void new_freq(freq *fq, const char *word, const int freq) {
    fq->word = (char *)malloc((strlen(word) + 1) * sizeof(char)); // +1 for null-terminator
    strcpy(fq->word, word);
    fq->freq = freq;
}

/**
 * Free memory
 */
void dispose_freq(void *fq) {
    freq *p = (freq *)fq;
    free(p->word);
    p->word = NULL;
}

/**
 * snprintf() will terminate the string with a null character, unless buf_size is zero.
 */
char *freq_tostring(freq *fq) {
    size_t wordlen = strlen(fq->word);
    char *buffer = (char *)malloc(wordlen + 16); // maximum integer has 10 digits
    snprintf(buffer, wordlen + 16, "[%s, %d]\n", fq->word, fq->freq);
    return buffer;
}

/**
 * Append the string of freq to the end of stream.
 */
void freq_intostream(void *elem, void *stream) {
    freq *fq = (freq *)elem;
    char *str = *(char **)stream;
    size_t strsize = strlen(str);
    // printf("Stream = \"%s\", length = %lu\n", str, strsize);
    char *word = freq_tostring(fq);
    size_t wordsize = strlen(word);
    // printf("Element = \"%s\"%lu\n", word, wordsize);
    char *temp = (char *)realloc(str, strsize + wordsize + 1);
    strcpy(temp + strsize, word);
    temp[strsize + wordsize] = '\0';
    // printf("After strcpy(): \"%s\"\n", temp);
    str = temp;
    free(word);
}


int main(void) {
    freq apple, banana, kiwi;
    new_freq(&apple, "apple", 3);
    new_freq(&banana, "banana", 2);
    new_freq(&kiwi, "kiwi", 5);

    char *buffer = (char *)malloc(1);
    buffer[0] = '\0';
    freq_intostream(&apple, &buffer);
    freq_intostream(&banana, &buffer);
    freq_intostream(&kiwi, &buffer);

    assert(strlen(buffer) == 33); 
    assert(strcmp(buffer, "[apple, 3]\n[banana, 2]\n[kiwi, 5]\n") == 0);

    dispose_freq(&apple);
    dispose_freq(&banana);
    dispose_freq(&kiwi);
    free(buffer);
}

奇怪的是，当我运行 10 次时，它给了我大约 9 个pointer being realloc'd was not allocated，但可能在 1~2 种情况下，一切正常。

如果我注释掉printf()，它表明在附加第三个元素kiwi 之前，流是空的，这可能是realloc 失败的原因。但我确定我将char * stream 的指针传递给freq_intostream() 函数，这肯定是char **。我不知道是什么问题，谁能帮忙？

Answer 1

当你想要j = 3; 时，你已经完成了相当于i = j; i = 3; 的操作。显然，这些不做同样的事情。仔细看看这个函数中的标记线：

/**
 * Append the string of freq to the end of stream.
 */
void freq_intostream(void *elem, void *stream) {
    freq *fq = (freq *)elem;
    char *str = *(char **)stream;
    size_t strsize = strlen(str);
    // printf("Stream = \"%s\", length = %lu\n", str, strsize);
    char *word = freq_tostring(fq);
    size_t wordsize = strlen(word);
    // printf("Element = \"%s\"%lu\n", word, wordsize);
    char *temp = (char *)realloc(str, strsize + wordsize + 1);
    strcpy(temp + strsize, word);
    temp[strsize + wordsize] = '\0';
    // printf("After strcpy(): \"%s\"\n", temp);
    str = temp; // OOPS!!
    free(word);
}

您更改了str 的值，但str 是该函数的局部变量，函数一结束，它的值就会被丢弃。

您想要：*(char**)stream = temp; 更改调用者传递给您的指针的值。

如果你去掉所有的演员，这段代码会简单得多。如果elem 是char ** 类型，您可以只使用*elem = temp;，代码会更容易理解。