fwrite with BMP file returning 0 and segmentation fault - fwrite

Question

I'm trying to read a simple bmp file then invert the color then save it back to the file system using fread and fwrite binary mode.
So I did
BMP_Image * img;
BMP_Header * hdr;
fread(hdr,sizeof(BMP_Header),1, fptr);
img->width = hdr->width;
img->height = hdr->height;
img->bytes_per_pixel = hdr->bits*BIT_TO_BYTE;
img->header = *hdr;
img->data_size = hdr->size;
to get the header information where BMP_Header is a struct
Then read the image data and put into array
fread(data, sizeof(char), img->data_size - 54, fptr);
Then I subtract each element in my image data by 255
int i = 0;
for(i = 0;i<img->data_size;i++){
data[i] = 255 - data[i];
}
Now when I want to write the file back to the file system, I couldn't get it to work properly. I know you'd have to do fwrite 2 times. First writing the header, then writing the image data.
I have so far:
fwrite(header, 1 ,dataSize, fptr_out);
which gives me sgmentation fault and 0 byte bmp file.
Here are the typedefs of my BMP_Header and BMP_image struct
typedef unsigned short int uint16_t;
typedef unsigned int uint32_t;
typedef int int32_t;
typedef struct {
uint16_t type; // Magic identifier
uint32_t size; // File size in bytes
uint16_t reserved1; // Not used
uint16_t reserved2; // Not used
uint32_t offset; // Offset to image data in bytes
uint32_t header_size; // Header size in bytes
int32_t width; // Width of the image
int32_t height; // Height of image
uint16_t planes; // Number of color planes
uint16_t bits; // Bits per pixel
uint32_t compression; // Compression type
uint32_t imagesize; // Image size in bytes
int32_t xresolution; // Pixels per meter
int32_t yresolution; // Pixels per meter
uint32_t ncolours; // Number of colors
uint32_t importantcolours; // Important colors
} BMP_Header;
typedef struct {
BMP_Header header;
int data_size;
int width;
int height;
int bytes_per_pixel; // This amount should be equals to number of bits/8
char *data;
} BMP_Image;