CODEWITHSUNDEEP
carrayspointersstructmalloc
Nadia Noormohamed
Nadia Noormohamed

Reputation: 19

How can I load a struct after allocating memory for it using malloc?

I am creating a struct using malloc:

structurePointer = (struct Person*)malloc(sizeof(struct Person));

struct Person is referring to the following type:

struct Person {
  char name;
  int age;
}

What I am currently doing to load the struct is:

strcpy(&(structurePointer -> name), names);

names is just a pointer to an array element that is someones name which I pass to the function containing the above code. Than to load the age:

structurePointer + 1 -> age = ages;

Although adding 1 feels wrong as adding 1 would be pointing to the start of the next 32 bits or 16 bits depending on the architecture? If this is the way to do it, I don't understand how the compiler knows the whereabouts of the starting address of the age variable of the struct by adding 1 as obviously the name variable is of type char so it will be of arbitrary size?

Thanks I need to create an array of pointers to the structs, so I assume each array element will have the start address of the name and than using this to print the struct out I can print the age out by probably adding 1?

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

#define HOW_MANY 7
//pointer declaration required?
char *names[HOW_MANY]= {"Simon", "Suzie", "Alfred", "Chip", "John",
                        "Tim", "Harriet"};
//pointer as it is  an array
int ages[HOW_MANY]= {22, 24, 106, 6, 18, 32, 24}; 

struct Person {
    char name;
    int age;
}; //struct-person

//elements in the array consists of the struct Person type
struct Person *people[7];

//passing the pointer to the struct Person array
static void insert(struct Person *people[], char *names, int ages)
{
    struct Person *structurePointer;
    structurePointer = (struct Person*)malloc(sizeof(30));

    int incrementVar = 0;

    strcpy(&(structurePointer-> name), names); //
    people[incrementVar]= structurePointer;
    structurePointer -> age = ages;    

    incrementVar++;
} //insert-method

int main(int argc, char **argv)
{
    struct Person *people[7]; 

    for (int i = 0; i <= HOW_MANY - 1; i++) {
        //passing name as a pointer as it has
        insert (people, names[i], ages[i]); 
    }

    for (int i = 0; i <= HOW_MANY - 1; i++) {
        printf("%d ", people[i]-> age);
        printf("%s", &(people[i] -> name));
        printf("\n");
    }

    return 0;
} //main

Upvotes: 0

Views: 501

Answers (2)

user3629249
user3629249

Reputation: 16540

regarding: 

structurePointer + 1 -> age = ages;

this is wrong.

What you really want to remember is that adding a value to a pointer increases the pointer by the number of bytes in the pointer type (in this case sizeof( struct Person ))

And remember that the -> operator has a higher precedence than the + operator, so the order of operations is not correct.

Suggest using: 

structurePointer->age = ages;

Upvotes: 0

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53006

Your very little code has a lot of issues,

  1. A char variable is not of string type. There is NO string type in and you need to read what a is.

  2. You call a unexsistent function strpy()

  3. You claim to be using an invalid syntax, because 1-> is going to be a compilation error for sure.

In my opinion, you need to study more. To access structure elements if you allocated space for it correctly you just need to use the -> operator, for example

struct Person {
    char name[100]; // Be careful with array bounds here
    int age;
}

struct Person *person = malloc(sizeof *person);
if (person != NULL) {
    strcpy(person->name, "your name");
    person->age = 33;
}

Upvotes: 2

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