Array of structs - How to create them correctly - c

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)