Question

I have two structs and I have an array of 30 pointer StudentType.
I have a problem with malloc(). When I compile it it’s ok. But when I try to debug it, it shows “Segmentation Fault” in Dev c++.

In Eclipse, it shows up anything on console. I think that my mistakes are on these lines of code:

students[0]=(StudentType *)malloc(sizeof(StudentType)*NumOfStudents);
(*students[NumOfStudents]).firstName=(char*)malloc(sizeof(char[30]));
(*students[NumOfStudents]).lastName=(char*)malloc(sizeof(char[30]));

That’s a part of my code.

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

typedef struct{
    float firstAssignment;
    float secondAssignment;
    float midterm;
    float final;
    float finalMark;
}StudentRecordType;

typedef struct{
    char *firstName;
    char *lastName;
    int idNumber;
    StudentRecordType marks;
}StudentType;
StudentType *students[30];
char firstName[30];
char lastName[30];

int ReadFromFile();
int PrintAll();
int NumOfStudents;
int i;

int main(void)
{
    ReadFromFile();
}

int ReadFromFile()
{
    FILE *fp;
    fp=fopen("project2-askhsh2.dat","r");
    if(fp==NULL)
    {
        printf("Error opening file.\n");
    }
    else
    {
        printf("Successful open of project2-askhsh2.dat\n");
    }
    fscanf(fp,"%d",&NumOfStudents);
    printf("%d\n",NumOfStudents);
    students[0]=(StudentType *)malloc(sizeof(StudentType)*NumOfStudents);
    (*students[NumOfStudents]).firstName=(char*)malloc(sizeof(char[30]));
    (*students[NumOfStudents]).lastName=(char*)malloc(sizeof(char[30]));

    for(i=0;i<NumOfStudents;i++)
    {
        (*students[i]).idNumber=i;
        fscanf(fp,"%s %s", (*students[i]).firstName,(*students[i]).lastName);
        fscanf(fp,"%f %f %f %f",(*students[i]).marks.firstAssignment,(*students[i]).marks.secondAssignment,(*students[i]).marks.midterm,(*students[i]).marks.final);
        printf("%s",(*students[i]).firstName);//, students[i].lastName);
    }

}

Answer 1

(*students[NumOfStudents]).firstName=(char*)malloc(sizeof(char[30]));

I think you meant:

students[some_index]->firstName = malloc(30);

With some_index something below NumOfStudents.

So your loop becomes:

for(i=0; i < NumOfStudents ; i++)
    {
        /* students[] is an array of pointers
        ** students[i] is a pointer
        */
        students[ i ] = malloc(sizeof *students[ i ]);
        students[ i ]->idNumber=i;

        students[ i ]->firstName = malloc(30);
        students[ i ]->lastName= malloc(30);

        fscanf(fp,"%s %s"
            , students[i]->firstName
            , students[i]->lastName
            );
        fscanf(fp,"%f %f %f %f"
            , &students[i]->marks.firstAssignment
            , &students[i]->marks.secondAssignment
            , &students[i]->marks.midterm
            , &students[i]->marks.final
            );
        printf("%s",students[i]->firstName);//, students[i].lastName);
    }

And using a global index named “i” can be considered bad style.