Array of structs - How to create them correctly

Question

I'm learning about structs and memory allocation.

One of the things i found out by studying was that there is not a way to see if a block of memory was allocated correctly.

I think it's working properly, but i was getting ready to read about linked lists and got all confused again.

I ask this, because on linked lists, i have seen:

typedef LIST* Lint;

int main(){
    Lint *lint;
}

and by defining the type of class like this on my code:

typedef CLASS* t;

int main(){
    t class; // WITHOUT THE * BEFORE class
}

it worked! I thought it would be ok, to give the definition like this. To me makes sense, the class is now a pointer to a type (type ie. CLASS);

So, am i on the right track or this working was pure luck?

The code structure sucks, but i'm still learning :)

typedef struct student {
    char    name[50];
    int     grade;
} TURMA;

typedef CLASS* t;

void showS(struct student i){
    printf("Name of Student: %s \n Grade of Student: %d\n", i.name, i.grade);
}

void insert(struct student *i,int k){
    char n[100]; int q=0;
    printf("Define name of student %d\n", k+1); scanf("%s", n);
    printf("Define the grade of %s\n", n); scanf("%d", &q);
    strcpy(i->name,n);
    i->grade = q;
}


int main()
{
    t class;
    int total,plus,i;
    printf("Define number of total students: \n"); scanf("%d", &total);
    class = (struct student*) malloc(total*(sizeof(CLASS)));

    for(i=0; i<total; i++){
        insert(&class[i], i);
    }
    printf("\n\nThis is the complete class: \n");
    for(i=0; i<total; i++){
        showS(class[i]);
    }

    printf("(Total size after malloc: %lu) Add more students (Int)\n",sizeof(*turma)); scanf("%d", &plus);
    class = realloc(class, plus*sizeof(CLASS));
    free(class);

    printf("(Total size after malloc: %lu)", sizeof(*class));
    return 0;
}

Answer 1

It is really hard to tell exactly what you are trying to accomplish. I looks like you are trying to create a singly-linked-list out of struct student with a typedef of struct student to TURMA. That's about where the useful information ends. Attempting to declare typedef CLASS* t; makes no sense. The compiler doesn't know what CLASS is. Guessing what you were attempting, the following is what flows from your code.

note: I added one function flush_stdin to empty stdin so you wouldn't have pesky newline characters left in the buffer. I also changed the scanf format string to prevent leaving newlines following your reading of character strings. Take a look below, and I'll look at your latest additions.

Also note in your code, you cannot free (class); then expect to take sizeof(*class)) -- that is undefined behavior.

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

#define MAXN 100

typedef struct student {
    char    name[50];
    int     grade;
} TURMA;

typedef TURMA* t;

void flush_stdin () 
{
    int c = 0;
    while ((c = getchar()) != '\n' && c != EOF);
}

void showS (TURMA i)
{
    printf("Name of Student: %s \n Grade of Student: %d\n", i.name, i.grade);
}

void insert (t i, int k)
{
    char n[MAXN] = {0}; int q=0;

    printf("Define name of student %d\n", k+1); scanf ("%[^\n]%*c", n);
    printf("Define the grade of %s\n", n); scanf("%d", &q); flush_stdin();

    strcpy(i->name,n);
    i->grade = q;
}


int main()
{
    t class;
    int total,plus,i;

    printf("Define number of total students: \n"); scanf("%d", &total); flush_stdin();
    class = malloc (total * sizeof *class);
    if (!class) {
        fprintf (stderr, "error: virtual memory exhausted.\n");
        exit (EXIT_FAILURE);
    }

    for (i=0; i<total; i++){
        insert (&class[i], i);
    }

    printf("\n\nThis is the complete class: \n");
    for (i=0; i<total; i++){
        showS (class[i]);
    }

    printf("(Total size after malloc: %lu) Add more students (Int)\n", sizeof *class * total); 
    scanf("%d", &plus); flush_stdin();

    TURMA *turma = realloc (class, plus * sizeof *turma);
    if (!turma) {
        fprintf (stderr, "error: virtual memory exhausted.\n");
        exit (EXIT_FAILURE);
    }

    free (class);

    printf("(Total size after realloc: %lu)\n", sizeof *turma * (total + plus));

    free (turma);

    return 0;
}

Output

$ ./bin/llclass
Define number of total students:
2
Define name of student 1
John James
Define the grade of John James
8
Define name of student 2
Jill James
Define the grade of Jill James
9


This is the complete class:
Name of Student: John James
 Grade of Student: 8
Name of Student: Jill James
 Grade of Student: 9
(Total size after malloc: 112) Add more students (Int)
2
(Total size after realloc: 224)