从指向 C 中结构的指针数组中获取信息

Question

So i'm writing this code for an assignment for school, and what I have to do is have two functions as they're written, with createMonster returning a dynamically allocated monster with data, and readMonsters returning an array of pointers to the monsters made in createMonster . I've been having trouble understanding how to get the functions and structs to cooperate, and this is what I have so far:

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

typedef struct monster {
    int id;
    char *name;
    char *element;
    int population;
} monster;

monster* createMonster(char *name, char *element, int population)
{
    monster *amonster = (monster*)malloc(sizeof(monster));
    amonster->name = (char*)malloc(sizeof(char)*sizeof(name));
    amonster->element = (char*)malloc(sizeof(char)*sizeof(element));
    amonster->population = (int)malloc(sizeof(int)*sizeof(population));
    amonster->name = name;
    amonster->element = element;
    amonster->population = population;
    return amonster;
}

monster** readMonsters(FILE* infile, int *monsterCount)
{
    char n[15], e[15];
    int p;
    monster **a_array = malloc(*monsterCount * sizeof(struct a*));
    for (int i = 0; i < *monsterCount; i++) {
        
        a_array[i] = malloc(sizeof(monster));
        
        fscanf(infile,"%s %s %d",n, e, &p);
        printf("%s %s %d\n", n, e, p);
        a_array[i] = createMonster(n,e,p);
       
    }
    monster ***m = &a_array;
    return *m;
}

int main(){
    int monsterCount;
    char name[15];
    
    
    FILE *fp = fopen ( "in.txt", "r" );
    fscanf(fp,"%d %s",&monsterCount,name);
    //printf("test %d\n",monsterCount);
    monster **mptr = readMonsters(fp,&monsterCount);
    
    printf("%s %s %d\n", (mptr)[3]->name,(mptr)[3]->element,(mptr)[3]->population);
    
    fclose(fp);
    return 0;
}

With the input file being:

8 monsters
StAugustine Grass 12
Zoysia Grass 8
WholeWheat Bread 6
MultiGrain Bread 10
Rye Bread 10
Cinnamon Spice 5
Pepper Spice 10
Pumpkin Spice 30

However, when I run it, I can see it works as far as making them the first time, but when I try and go back to access the data it returns garbage for the strings, with this being my output:

StAugustine Grass 12
Zoysia Grass 8
WholeWheat Bread 6
MultiGrain Bread 10
Rye Bread 10
Cinnamon Spice 5
Pepper Spice 10
Pumpkin Spice 30
`7 w├?]╨@ 10
   w├?]╨@ 10

I've tried rearranging the functions, pointers, etc. and have tried many different versions of these functions trying to follow guides online, but each time it either doesn't work at all or returns garbage. I'm looking for any help in understanding how to get this working or how it could be organized better since I will readily admit my experience with C is moderate at best.

Answer 1

The code you posted has the following issues:

The line

amonster->name = (char*)malloc(sizeof(char)*sizeof(name));

does not make sense. sizeof(name) is the size of the pointer, which is 32-bit or 64-bit, depending on your platform. However, you probably need to allocate more than that. You need to allocate strlen(name) + 1 bytes, because that is the length of the string that is passed to the function (including the null terminating character). The same also applies to the next line:

amonster->element = (char*)malloc(sizeof(char)*sizeof(element));

Also, the line

amonster->population = (int)malloc(sizeof(int)*sizeof(population));

does not make sense, because amonster->population is not a pointer. Why are you trying to store a pointer to dynamically allocated memory in it? You shouldn't need dynamic memory allocation here, because amonster->population is not a string, but a fixed-length variables, for which you have already allocated space, because it is part of the struct . Therefore, you can delete this line. All you need is the line amonster->population = population; , which you already have.

Additionally, the line

amonster->name = name;

does not do what you want. It does not copy the string, it only copies the pointer, ie the memory address. That way, you are copying a pointer which will be a dangling pointer by the time you return to the function main . In order to copy the actual string, you must write strcpy( amonster->name, name ); . The same applies for the following line:

amonster->element = element;

The line

a_array[i] = malloc(sizeof(monster));

is unnecessary and only creates a memory leak . You have already allocated space for all structs in the function createMonster .

The lines

monster ***m = &a_array;
return *m;

are unnecessarily cumbersome and can be simplified to the following:

return a_array;

After applying all of these fixes, your code should look like this:

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

typedef struct monster {
    int id;
    char *name;
    char *element;
    int population;
} monster;

monster* createMonster(char *name, char *element, int population)
{
    monster *amonster = (monster*)malloc(sizeof(monster));
    amonster->name = (char*)malloc(sizeof(char)*(strlen(name)+1)); //fixed
    amonster->element = (char*)malloc(sizeof(char)*(strlen(element)+1)); //fixed
    //amonster->population = (int)malloc(sizeof(int)*sizeof(population)); //redundant
    strcpy( amonster->name, name ); //fixed
    strcpy( amonster->element, element ); //fixed
    amonster->population = population;
    return amonster;
}

monster** readMonsters(FILE* infile, int *monsterCount)
{
    char n[15], e[15];
    int p;
    monster **a_array = malloc(*monsterCount * sizeof(struct a*)); //what is struct a???
    for (int i = 0; i < *monsterCount; i++) {
        
        //a_array[i] = malloc(sizeof(monster)); //redundant
        
        fscanf(infile,"%s %s %d",n, e, &p);
        printf("%s %s %d\n", n, e, p);
        a_array[i] = createMonster(n,e,p);
       
    }
    //monster ***m = &a_array; //removed
    //return *m; //removed
    return a_array;
}

int main(){
    int monsterCount;
    char name[15];
    
    FILE *fp = fopen ( "in.txt", "r" );
    fscanf(fp,"%d %s",&monsterCount,name);
    //printf("test %d\n",monsterCount);
    monster **mptr = readMonsters(fp,&monsterCount);
    
    printf("%s %s %d\n", (mptr)[3]->name,(mptr)[3]->element,(mptr)[3]->population);
    
    fclose(fp);
    return 0;
}

As demonstrated here , your program now provides the correct output.