Student Struct Troubles

Question

I am learning structs and I am still little confused about them and what they do. The code I have is my attempt of it and I keep getting segmentation faults. My goal for the main is to ask the user to see how many students they want to add, and each information for name and score while calling a function. I also want to print the data array off to the user.

• For the loadStudentData() function, I want to store the newName and NewScore into the newStudent Structure.

• For printStudentData() I want to print data for a single student

• Then for the printStudentArray() I want to call printStudentData() function on each member for the students array.

My code is:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define STUDENT_NAME_LENGTH 20

typedef struct{
char* name;
int score;
}Student;

void loadStudentData(Student *newStudent,char* newName,int newScore);
int printStudentData(Student student);
void printStudentArray(Student* students, int numStudents);


/*
 *
 */
int main(int argc, char** argv) {
int numberOfStudents;
Student *newStudent;
char* newName;
int newScore;
int student;


Student students[numberOfStudents];

printf("how many students: ");
scanf("%d", &numberOfStudents);


    loadStudentData(newStudent, newName, newScore);

    printStudentArray(students, numberOfStudents);


  return (EXIT_SUCCESS);
}

void loadStudentData( Student *newStudent, char* newName, int newScore){
 int i;
    char *studentName = (char *) malloc(STUDENT_NAME_LENGTH *  sizeof(char));
    scanf("%s", studentName);
    newStudent->name = studentName;

    int nScore;
    scanf("%d", &nScore);
    newStudent[i].score = newScore;
  }
 int printStudentData(Student student){

int i;

    printf("Student name\t%s\n",) ;
    printf("Student score\t%d\n",);

     }



 void printStudentArray(Student* students, int numStudents){
 int i;
for (i = 0; i < numStudents; i++) {
    printf("Student name\t%s\n", students[i].name);

    printf("Student score\t%d\n", students[i].score);
   } 
   }

Answer 1

• Point 1: Student students[numberOfStudents]; is invalid, as numberOfStudents is uninitialized.

• Point 2: In your code, you're calling printStudentArray() before populating student itself.

  This function tries to access the uninitialized memory at name variable which causes undefined behaviour.

• Point 3: In your loadStudentData() function, you're using i uninitialized . Again UB.

That said,

1. Always check for success of malloc() before using the retuned pointer.
2. Please do not cast the return value of malloc() and family in C.

Answer 2

You call printStudentArray before you actually initialize the array of structures. That means all the data in the array will be indeterminate and using it in any way except to initialize it will lead to undefined behavior (which is a common cause of crashes).

More specifically, the problem is probably when you try to print the name, as the name pointer could point anywhere.

Answer 3

You seem to not allocate the structs you are creating. Be aware that when you declare a variable such as Student *newStudent; you first have to allocate it. Because thus far, you have only declared a variable that holds a pointer to a Student struct, but this pointer points to nowhere. So use the malloc function everywhere you expect a valid pointer:

newStudent = malloc(sizeof(newStudent));