如何在C语言中使用指针

Question

当我运行这段代码时，它返回了Segmentation Fault (core dumped)。我尝试了很多方法，但我相信我在使用指针方面出了问题。我做错了什么？

要求：每个变量都必须是指针。

#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); 
}

- gcop

---

请检查total = 0;。你可能需要使用*total = 0;。 - chux - Reinstate Monica

称呼指针为 i 是在自找麻烦 :) - chqrlie

Answer 1

在需要使用指针所指的值而非指针本身的地方，全部使用 *。

修改为：

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

to:

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

变更：

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

to:

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

变更:

                        failing_count++; 

致：

                        (*failing_count)++; 

变更：

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

to:

        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之后的语句将不会执行。将return移到最后。

- Eric Postpischil

Answer 2

与其使用failing_count = 0;，你可能想要使用*failing_count = 0; 在你的代码中，所有的变量都是指针或地址。如果你给这些变量赋值，你会改变它们所指向的地址。 为了改变这些变量所指向的值，你应该使用星号(*)。

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; 
...

- Stephane Ronse

Answer 3

你的代码中缺少一些 *：

• 每个指针都必须被解引用，除非作为参数传递给scanf()和free()。
• 还要注意，*i++不能完成任务，因为优先级规则：应该写成(*i)++或者简单地写成*i += 1。

这是修改后的版本：

#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; 
}

请注意，由于目标是使所有内容都成为指针，因此您也可以将函数转换为指针，并对 scanf() 和 free() 的参数进行解引用：

#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; 
}

- chqrlie

Answer 4

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

你正在重新分配由malloc给你的指针，这意味着你失去了对已分配内存的唯一引用，当你稍后增加、解除引用和尝试free指针时，会出现未定义的行为。

你还应该始终检查malloc的返回值不是NULL，以确保分配成功。

注意：free(NULL)是无害的。

- Govind Parmar