c - 打印双指针字符数组时出现运行时错误

Question

我正在努力创建一个系统，用于将学生姓名和分数输入数组并将相同的信息打印到屏幕上，不幸的是，我一直收到奇怪的输出。

我使用调试器单步执行我的程序，它显示一切运行顺利，直到我到达打印学生信息的函数，双指针 char 数组已经弄乱了值。

这是我运行程序时看到的图像。 ( http://s28.postimg.org/nv29feawt/Error.png )

注意:虽然我知道有更好、更简单的方法可以做到这一点，但我需要使用动态分配的内存和数组来完成这项任务。

int main(void)
{
    char **firstNames;
    char **lastNames;
    float *scores;

    int recordsLength;
    printf("Please indicate the number of records you want to enter: ");
    scanf("%d", &recordsLength);
    printf("\n\n");

    firstNames = (char **)malloc(recordsLength * sizeof(char *));
    lastNames = (char **)malloc(recordsLength * sizeof(char *));
    scores = (float *)malloc(recordsLength * sizeof(float));

    int i = 0;
    while(i < recordsLength)
    {
        createNewEntry(i, firstNames, lastNames, scores);
        i++;
    }

    printEntry(0, firstNames, lastNames, scores);
    free(firstNames);
    free(lastNames);
    free(scores);
    return 0;
}

void clearScreen()
{
#ifdef _WIN32 
    system("cls");
#elif _unix_ 
    system("clear");
#endif
}

void printEntry(int entryID, char *firstNames[], char *lastNames[], float scores[])
{
    clearScreen();
    printf("|-------------------------------------------------------------------------|\n");
    printf("|                           Student Entry                                 |\n");
    printf("|-------------------------------------------------------------------------|\n|\n");
    printf("|   First Name: %s   Last Name: %s   Score: %.1f\n|\n|\n|\n", firstNames[entryID], lastNames[entryID], scores[entryID]);
    printf("|-------------------------------------------------------------------------|\n");
    printf("|                                                                         |\n");
    printf("|-------------------------------------------------------------------------|\n\n");
}

void createNewEntry(int index, char *firstNames[], char *lastNames[], float scores[])
{
    printf("Please input the records of the new student.\n\n\n");
    char first[20];
    char last[20];
    float score = 100.0f;

    printf("Please enter the student's first name: ");
    scanf("%s", &first);
    printf("\n\n");

    printf("Please enter the student's last name: ");
    scanf("%s", &last);
    printf("\n\n");

    printf("Please enter the student's score: ");
    scanf("%f", &score);
    printf("\n\n");

    firstNames[index] = (char *)malloc((strlen(first)) * sizeof(char));
    firstNames[index] = first;

    lastNames[index] = (char *)malloc((strlen(last)) * sizeof(char));
    lastNames[index] = last;
    printf("first name: %s", firstNames[index]);
    printf("last name: %s", lastNames[index]);
    scores[index] = score;
}

Answer 1

firstNames[index] = (char *)malloc((strlen(first)) * sizeof(char));
firstNames[index] = first;  /* You are missing your allocated memory block and assigning local */

上面几行是不正确的。您不能使用分配 = 运算符分配 c-strings。你应该为此使用 strcpy。

您正在将本地数组分配给 firstnames，该数组在函数结束后没有生命。这会调用一个未定义的行为。 (您看到的是奇怪的字符，但情况可能更糟)。

应重写为(姓氏也类似)

firstNames[index] = malloc((strlen(first) + 1) * sizeof(char)); /* +1 for \0 */
if(firstNames[index] == NULL) {
  /* Malloc failed, error handling */
  ...
}
/* else */
strcpy(firstNames[index], first);  /* Use strcpy to copy contents */

Live example here

在释放 firstNames 和 lastNames 之前，您应该循环释放 firstNames 和 lastNames 的所有成员。