Easiest way to allocate "blank" block of data to .dat file

Question

Looking for a quick way to allocate a block of data to be managed from disk. I'm allocating a block of 50 structs, and while most of the memory allocates fine, when I read it all back I get junk messages returned in some of the fields that should be blank. I assume this is me allocating the space incorrectly somehow that allows some junk from memory to leak in there.

if ((fpBin = fopen(BINARYFILE, "w+b")) == NULL)
{
    printf("Could not open binary file %s.\n", BINARYFILE);
    return;
}
fwrite(fpBin, sizeof(struct student), 50, fpBin); //Write entire hash table to disk

struct definition

typedef struct student
{
    char firstName[20]; //name
    char lastName[20];
    double amount;  //amount owed
    char stuID[5];   //4 digit code
}student;

Is how I was taught, yet I'm still getting some junk in my data instead of it being a clean slate. So question: How do I set all fields to blank?

Answer:

student tempStu[50] = {0};
fwrite(tempStu, sizeof(struct student), BUCKETSIZE, fpBin); //Write entire hash table to disk

Answer 1

fwrite(fpBin, sizeof(struct student), 50, fpBin);

You're writing your file pointer, not your student structs, to disk. That first fpBin should instead be a pointer to your data. That data can be an array of 50 student structs initialized to 0, perhaps with calloc or by defining it at file scope, but it has to be somewhere. Instead, you are writing 50*sizeof(struct student) bytes from your fpBin pointer, which is undefined behavior -- you'll either crash with an access violation or you'll write junk to disk. That junk is what you're getting when you read it back.

Also, using a constant like 50 is bad practice ... it should be a variable (or manifest constant) that holds the number of students that you're writing out.

BTW, on Linux and other POSIX systems, you could allocate a block of zeroes on disk just by writing the last byte (or in some other way making the file that large).