为结构及其成员分配内存

Question

我只是进入结构并为它们分配内存。现在我有一些示例代码，它们可以像下面的“Learn C The Hard Way”一样开箱即用

struct Person {
    char *name;
    int age;
    int height;
    int weight;
};

struct Person *Person_create(char *name, int age, int height,
        int weight)
{
    struct Person *who = malloc(sizeof(struct Person));
    assert(who != NULL);

    who->name = strdup(name);
    who->age = age;
    who->height = height;
    who->weight = weight;

    return who;
}

这就是我的理解。在函数*Person_create 中，指针*who 接收大小为struct Person 的内存块的地址。 Struct Person 有 4 个成员，一个指向字符串的指针和三个整数。由于指针*who 的类型为struct Person，据我所知，它应该知道它具有这些成员。

现在我尝试用一​​些自己的代码创建类似的东西。不幸的是，我在尝试为即将到来的变量 int age 扫描 f() 整数时遇到了段错误。

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

#define STRINGLENGTH 30
struct Person {
    char *name;
    char *food;
    int age;
    float height;
};

struct Person *createPerson(FILE *file){
    struct Person *who = malloc(sizeof(struct Person));
    assert(who != NULL);

    who->name = malloc(sizeof(who->name)*STRINGLENGTH);
    who->food = malloc(sizeof(who->food)*STRINGLENGTH);

    printf("What is the name of the person?\n");
    scanf("%29s",who->name);
    fprintf(file,"Name:%s\n",who->name);

    printf("What food do you eat?\n");
    scanf("%29s",who->food);
    fprintf(file,"Food:%s\n",who->food);

    printf("How old are you?\n");
    scanf("%d",who->age);
    fprintf(file,"Age:%d\n",who->age);

    printf("Whats your height?\n");
    scanf("%f",who->height);
    fprintf(file,"Height:%f\n",who->height);

    return who;
}

void freePerson(struct Person *who){
    free(who->name);
    free(who->food);
    free(who);
}

int main(int argc, char *argv[]){

    FILE *file;

    if((file = fopen("person.txt","a")) == NULL){
        perror(NULL);
        return EXIT_FAILURE;
    }

    printf("Creating a person...\n");

    struct Person *newPerson = createPerson(file);

    freePerson(newPerson);

    fclose(file);

    return EXIT_SUCCESS;
}

• 那么导致问题的差异是什么？
• 我也需要单独 malloc 成员吗？
• 是不是因为示例代码中已经设置了变量？

Answer 1

您会收到段错误，因为您没有传递年龄字段的地址。相反，您将 scanf 的不确定值复制为地址。

一个简单的本地修复：

scanf("%d",&(who->age));
fprintf(file,"Age:%d\n", who->age);

当我们讨论这个主题时，您为字符串分配的内存超出了您的预期。 sizeof(who->name) 是指针的大小，不小于它所指向的char 的大小。更重要的是，保证sizeof(char) == 1。所以字符串的分配可以完全简化：

who->name = malloc(STRINGLENGTH);