Cannot read struct from binary file

Question

I am trying to read the contents of a binary file into a struct, but each time I print out the contents of my struct, I get a pretty strange output. Below is the struct I am trying to set:

struct student {
    char name[32];
    unsigned int age;
    SEX sex;
    float gpa;
    struct course *courses;
    struct student *next;
}

With the course struct defined as:

struct course {
    char grade;
    unsigned int number;
    struct course *next;
}

And SEX defined as:

typedef enum _SEX {MALE = 'M', FEMALE = 'F', OTHER = 'O'} SEX;

Right now, my function is as follows:

void read_bin(char *filename){
    FILE *file;
    struct student myStudent;
    file = fopen(filename, "rb");
    if(file == NULL){
        printf("Unable to open file!");
        return;
    }
    fread(&myStudent, sizeof(struct student), 1, file);
    printf("\nName: %s, Age: %d", myStudent.name, myStudent.age);
    fclose(file);
}

But I keep on getting this strange output:

Age: 0

With name not even showing up and age set to an incorrect number. I thought this might be due to padding, so I tried using

fread(&myStudent.name, sizeof myStudent.name, 1, file);
fread(&myStudent.age, sizeof myStudent.age, 1, file);

So that it reads each individual element and pads as needed. However, I get the same output. When I ran hexdump to see what exactly I was dealing with, this is what came out:

f0 0d 03 00 00 00 46 72  65 64 00 00 00 00 00 00  |......Fred......|
00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00 00 00 00 00 00 1d 00  00 00 4d 00 00 00 cd cc  |..........M.....|
4c 40 01 00 00 00 44 02  00 00 41 4a 6f 65 00 00  |L@....D...AJoe..|
00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00 00 00 00 00 00 00 00  00 00 00 16 00 00 00 4d  |...............M|
00 00 00 33 33 73 40 03  00 00 00 6e 00 00 00 42  |...33s@....n...B|
dc 00 00 00 41 54 01 00  00 41 53 61 72 61 68 00  |....AT...ASarah.|
00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00 00 00 00 00 00 00 00  00 00 16 00 00 00 46 00  |..............F.|
00 00 00 00 40 40 03 00  00 00 78 00 00 00 42 dc  |....@@....x...B.|
00 00 00 41 4a 01 00 00  43                       |...AJ...C|

Any help would be greatly appreciated -- I've been stuck on this function for awhile.

Answer 1

Depending on the filetype the first few bytes can be a fileheader, here information can be stored like version numbers, length of the data etc.

As for the pointers, that won't work like others pointed out since pointers are adresses, not actual relevant data.

You want to fseek to sizeof(fileheader), before you start reading the file.

Also be wary that structs are padded in C and that the order in which you declared the elements in the struct might not be the order the compiler decided for it.