Read binary file into struct and also problems with endianness

• You must allocate the structure and read data into the structure instead of reading into an pointer directly. If you are going to read only one structure, you won't need to declare pointers for the structure.
• printf("%x ", test->magic); invokes undefined behavior because pointer (automatically converted from the array) is passed to where unsigned int is required.

In this case, the observed behavior is because:

Firstly, fread(&test,sizeof(test),1,fp); read the first few bytes from the file as pointer value.

Then, printf("%02x", test->magic); printed the first 4-byte integer from the file because test->magic is (converted to) the pointer to the array placed at the top of the structure, and the address of the array is same as the address of the structure itself, so the address read from the file is printed. One more lucky is that where to read 4-byte integer and address (pointer) from as function arguments are the same.

Finally, you didn't get any output from printf("%x ", test->version); because the address read from the file is unfortunately in region that is not readable and trying to read there caused Segmentation Fault.

Fixed code:

using namespace std;

#include <stdint.h>
#include <string.h>
#include <iostream>
#include <fstream>
#include "exampleh.h"

struct teststruct test; // allocate structure directly instead of pointer

int main() {
    FILE *fp = fopen("C:\image.dd", "rb");     // open file in binary mode

    if (fp == NULL) {
        fprintf(stderr, "Can't read file");
        return 0;
    }
    fread(&test,sizeof(test),1,fp); // now structure is read instead of pointer
    for (int i = 0; i < 6; i++) {
        printf("%02x", (unsigned char)test.magic[i]); // print using proper combination of format and data
    }
    printf(" ");
    printf("%x ", test.version); // also use . instead of ->

    fclose(fp);

    return 0;
}

struct teststruct *test; points to NULL, as it is defined in the global namespace. You do not allocate memory for this pointer, so test->version is UB.

fread(&test,sizeof(test),1,fp); is also wrong, this will read a pointer, not the content of the struct.

An easy fix is to change test to be a struct teststruct and not a pointer to it.

using namespace std;

#include <stdint.h>
#include <string.h>
#include <iostream>
#include <fstream>
#include "exampleh.h"

struct teststruct test; //not a pointer anymore

int main() {
    FILE *fp = fopen("C:\image.dd", "rb");     // open file in binary mode

    if (fp == NULL) {
        fprintf(stderr, "Can't read file");
        return 0;
    }
    fread(&test,sizeof(test),1,fp);
    //printf("%x ", test.magic);   //this works, but in the wrong order because of little/big endian
    printf("%x ", test.version);   //no output at all

    fclose(fp);

    return 0;
}