C语言 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。现在我有一些开箱即用的示例代码，如下所示 "学习C The Hard Way">

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是struct Person类型，因此据我所知，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 (( 整数时出现段错误。

#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:%sn",who->name);
    printf("What food do you eat?n");
    scanf("%29s",who->food);
    fprintf(file,"Food:%sn",who->food);
    printf("How old are you?n");
    scanf("%d",who->age);
    fprintf(file,"Age:%dn",who->age);
    printf("Whats your height?n");
    scanf("%f",who->height);
    fprintf(file,"Height:%fn",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;
}

• 那么导致问题的区别是什么？
• 我也需要单独分配成员吗？
• 是因为变量已经在示例代码中设置了吗？

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，您复制它是不确定的值，供scanf视为地址。

一个简单的本地修复：

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

当我们讨论这个主题时，您为字符串分配的内存比您可能预期的要多得多。 sizeof(who->name)是指针的大小，它不小于它指向的char的大小。更重要的是，保证sizeof(char) == 1.因此，字符串的分配可以完全简化：

who->name = malloc(STRINGLENGTH);