Write a Struct to Binary File in C [duplicate]

I am trying to write the contents of a struct into a binary file in C.

The Struct looks like:

typedef struct fileData fileData;
struct fileData {
    uint8_t data1;
    uint8_t data2;
    uint8_t data3;
}

I am trying to get these 3 data pieces to write to the header of the binary file (each as 1 byte).

fileData* fileWriting;
fileWriting->data1 = 10;
fileWriting->data2 = 20;
fileWriting->data3 = 30;

Code to write to file:

FILE * outputFile = fopen("test.bin","wb");
fwrite(&fileWriting->data1,1,1, outputFile);
fwrite(&fileWriting->data2,1,1, outputFile);
fwrite(&fileWriting->data3,1,1, outputFile);
fclose(outputFile);

However, this is giving errors (i.e. bus:10). What is wrong/is there a better way?