C語言中指針的使用

Question

當我運行此代碼時，它返回Segmentation Fault (core dumped) 。 我嘗試了很多東西，但我相信我使用的指針是錯誤的。 我究竟做錯了什么？

要求：每個變量都必須是一個指針。

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

int main() { 
    /* All the variables that needed to be defined */ 
    int *i = malloc(sizeof(int)); 
    int *number = malloc(sizeof(int)); 
    int *failing_count = malloc(sizeof(int)); 
    double *total = malloc(sizeof(double)); 
    double *grade = malloc(sizeof(double)); 

    failing_count = 0; 
    total = 0; 
    grade = 0; 

    /* User is asked to enter the number of grades that are received */ 
    printf("Enter number of grades:\n "); 
    scanf("%d", number);     

    /* For Loop is run */ 
    for (*i = 0; i < number; *i++) { 
        printf("Enter a grade:\n "); 
        scanf("%lf", grade); 

        if (*grade < 70) {
            failing_count++; 
        } 
        *total = *total + *grade; 
    }        

    /* Program prints the results */ 
    printf("Average grade is %lf\n", *total / *number); 
    printf("Number of failing grades is %d\n", failing_count); 
    return 0; 

    free(i); 
    free(number); 
    free(failing_count); 
    free(total); 
    free(grade); 
}

Answer 1

在要使用所指向的值而不是指針的任何地方使用* 。

改變：

        failing_count = 0; 
        total = 0; 
        grade = 0; 

到：

        *failing_count = 0; 
        *total = 0; 
        *grade = 0; 

改變：

        for(*i = 0; i<number; *i++){ 

到：

        for(*i = 0; *i<*number; (*i)++){ 

改變：

                        failing_count++; 

到：

                        (*failing_count)++; 

改變：

        printf("Number of failing grades is %d\n", failing_count); 

到：

        printf("Number of failing grades is %d\n", *failing_count); 

在：

        return 0; 

        free(i); 
        free(number); 
        free(failing_count); 
        free(total); 
        free(grade); 

return之后的語句永遠不會執行。 將return移到最后。

Answer 2

而不是failing_count = 0; 你可能想做*failing_count = 0; 在您的代碼中，所有變量都是指針或地址。 如果分配這些變量，則更改它們指向的地址。 為了改變這些變量指向的值，你應該使用 *.

int* i = malloc(sizeof(int)); 
int* number = malloc(sizeof(int)); 
int* failing_count = malloc(sizeof(int)); 
double* total = malloc(sizeof(double)); 
double* grade = malloc(sizeof(double)); 

*failing_count = 0; 
*total = 0; 
*grade = 0; 
...

Answer 3

您的代碼中缺少一些* ：

• 每個指針都必須取消引用，除非這樣傳遞給scanf()和free() 。
• 還要注意*i++由於優先規則而無法完成這項工作：您應該編寫(*i)++或簡單地*i += 1 。

這是一個修改后的版本：

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

int main() { 
    /* All the variables that needed to be defined */ 
    int *i = malloc(sizeof(int)); 
    int *number = malloc(sizeof(int)); 
    int *failing_count = malloc(sizeof(int)); 
    double *total = malloc(sizeof(double)); 
    double *grade = malloc(sizeof(double)); 

    *failing_count = 0; 
    *total = 0; 
    *grade = 0; 

    /* User is asked to enter the number of grades that are received */ 
    printf("Enter number of grades:\n "); 
    scanf("%d", number);     

    /* For Loop is run */ 
    for (*i = 0; *i < *number; *i += 1) { 
        printf("Enter a grade:\n "); 
        scanf("%lf", grade); 

        if (*grade < 70) {
            *failing_count += 1; 
        } 
        *total += *grade; 
    }        

    /* Program prints the results */ 
    printf("Average grade is %lf\n", *total / *number); 
    printf("Number of failing grades is %d\n", *failing_count); 

    free(i); 
    free(number); 
    free(failing_count); 
    free(total); 
    free(grade); 
    return 0; 
}

請注意，由於目標是使所有內容都成為指針，因此您也可以將函數轉換為指針並將參數取消引用到scanf()和free()太：

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

int main() { 
    /* All the variables that needed to be defined */ 
    int *i = (*malloc)(sizeof(*i)); 
    int *number = (*malloc)(sizeof(*number)); 
    int *failing_count = (*malloc)(sizeof(*failing_count)); 
    double *total = (*malloc)(sizeof(*total)); 
    double *grade = (*malloc)(sizeof(*grade)); 

    *failing_count = 0; 
    *total = 0; 
    *grade = 0; 

    /* User is asked to enter the number of grades that are received */ 
    (*printf)("Enter number of grades:\n "); 
    (*scanf)("%d", &*number);     

    /* For Loop is run */ 
    for (*i = 0; *i < *number; *i += 1) { 
        (*printf)("Enter a grade:\n "); 
        (*scanf)("%lf", &*grade); 

        if (*grade < 70) {
            *failing_count += 1; 
        } 
        *total += *grade; 
    }        

    /* Program prints the results */ 
    (*printf)("Average grade is %lf\n", *total / *number); 
    (*printf)("Number of failing grades is %d\n", *failing_count); 

    (*free)(&*i); 
    (*free)(&*number); 
    (*free)(&*failing_count); 
    (*free)(&*total); 
    (*free)(&*grade); 
    return 0; 
}

Answer 4

failing_count = 0; 
total = 0; 
grade = 0; 

您正在重新分配malloc提供給您的指針，這意味着您丟失了對已分配內存的唯一引用，並且當您稍后增加、取消引用並嘗試free指針時，您會出現未定義的行為。

您還應該始終檢查malloc的返回值是否為NULL以確保分配成功。

注意： free(NULL)是良性的。