如何在C中声明指向struct的指针数组，然后使用它？

Question

My aim is do declare an array of pointer to my person struct and then use it to modify the name and the age of the persons.I am pretty lost in my code so any help would be appreciated.Thanks!!!

#include <stdio.h>

#define HOW_MANY 7

char *names[HOW_MANY]= {"p1", "p2", "p3", "p4", "p5", "p6", "p7"};

int ages[HOW_MANY]= {2, 1, 16, 8, 10, 3, 4};

struct person
{
    char person_name[30];
    int person_age;
}*array[10];

static void insert(struct person array[], char *name, int age) 
{
    static int nextfreeplace = 0;
    array[nextfreeplace] = (struct person*) malloc(sizeof(struct person));
    &array[nextfreeplace]->person_name = name;
    &array[nextfreeplace]->person_age = age;
    nextfreeplace++;  
}

int main(int argc, char **argv) 
{ 
    struct person *array[10];
    int index;
    int personNumber = 1;

    for (index = 0; index < HOW_MANY; index++) 
    {
        insert (array[index], names[index], ages[index]);
    }


    for (index = 0; index < HOW_MANY; index++)
    {
        printf("\nPerson number %d has name %s and age %d \n", personNumber,
             array[index]->person_name, array[index]->person_age);
        personNumber++;
    }

    return 0;
}

The errors I get:

arrays.c: In function ‘insert’:
arrays.c:20: warning: implicit declaration of function ‘malloc’
arrays.c:20: warning: incompatible implicit declaration of built-in 
function   ‘malloc’
arrays.c:20: error: incompatible types when assigning to type 
‘struct   person’ from type ‘struct person *’
arrays.c:21: error: invalid type argument of ‘->’ (have ‘struct person’)
arrays.c:22: error: invalid type argument of ‘->’ (have ‘struct person’)
make: *** [arrays] Error 1

Answer 1

You have successfully declared an array of pointers to your struct here

  struct person *array[10];

You then need to allocate memory such that each pointer in this array actually points to an instance of your struct

Currently you pass

array[index]

to insert as first argument but array[index] is of type person* but your insert function expects person [] .

You try to access array[nextfreeplace] within your insert function, but as you are passing a person* to insert this will attempt to read a memory address that is potentially not valid because

array[nextfreeplace] = (struct person*) malloc(sizeof(struct person));

is identical to

*(array + nextfreeplace) = (struct person*) malloc(sizeof(struct person));

and so whenever nextfreeplace is nonzero you are effectively attempting to allocate dynamic memory and store a pointer to it at an address that you should not be.

This leads to potential undefined behaviour. You cannot treat the pointers inside array as if there are arrays themselves (unless you allocate sufficient memory and treat them as such - which is not the case in your code). You certainly store the return value of malloc to an address at some offset of this pointer. As your code stands each array[index] is not an array that you can freely read from and write to and should not be treated as such without expecting problems.

You might consider changing insert to

void insert(struct person** p, char *name, int age) 
{
    *p = malloc(sizeof(struct person));
} 

and then changing your loop in main to

for (index = 0; index < HOW_MANY; index++) 
{
    insert (&array[index], names[index], ages[index]);
}