Why does my output gives a garbage value? - c

Question

I am learning structure and function and I'm trying to pass a pointer to structure as an argument. My code is to input student name,age and marks. This is my program
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
struct student {
char name[15];
int age;
double marks;
};
void display();
int main(){
struct student *s1;
s1 =(struct student *) malloc(sizeof(struct student));
for(int i=0; i<2; i++)
{
printf("\nEnter details of student %d\n\n", i+1);
printf("Enter name: ");
scanf("%s", s1->name);
printf("Enter age: ");
scanf("%d", &s1->age);
printf("Enter marks: ");
scanf("%f", &s1->marks);
}
display(&s1);
}
void display(struct student *s1){
printf("\nName\tAge\tMarks\n");
for(int i = 0; i<2;i++ )
{
printf("Name: %s",s1->name);
printf("\nAge: %d",s1->age);
printf("\nMarks: %.2lf",s1->marks);
}
}
the code runs but it gave a wrong outputs and garbage value. What did i do wrong?

Answer 1

There are several syntax error. I have included documentation in the code below.
Also, you are overwriting the content of s1 everytime when you are prompting user to input. Test it with two different data for example (name: "alex", age: 19, mark: 100.0 and name: "jason", age: 22, mark: 100.0) your program will output only the later.
To solve this, I would suggest you to allocate your student pointer before looping so that you have two distinct memory for each student and you wont overwrite them in this case.
Basically, each student struct is treated as an array and instead of s1->name you will dereference them as s1[i].name
display function declaration
void display(struct student *s1);
main function
int main()
{
int i;
struct student *s1;
s1 =(struct student *) malloc(sizeof(struct student) * 2); /* 2 = number of students */
for(i=0; i<2; i++)
{
printf("\nEnter details of student %d\n\n", i+1);
printf("Enter name: ");
scanf("%s", s1[i].name);
printf("Enter age: ");
scanf("%d", &s1[i].age);
printf("Enter marks: ");
scanf("%lf", &s1[i].marks); /* Instead of %f, it should be %lf */
}
display(s1); /* Instead of &, it should be s1 */
/* Free heap memory */
for(i=0; i<2; i++)
{
free(s1);
s1 = NULL;
}
return 0;
}
Display function, you need some modification in your display function as well
void display(struct student *s1)
{
int i;
printf("\nName\tAge\tMarks\n");
for(i = 0; i<2;i++ )
{
printf("%s\t",s1[i].name);
printf("%d\t",s1[i].age);
printf("%.2f\t",s1[i].marks); /* Instead of %.2lf, it should be %.2f */
printf("\n");
}
}
Struct for student
struct student
{
char name[15];
int age;
double marks;
};