Binary Files

C differentiates between two types of file I/O: Text Files (which handle data as sequences of characters, often with conversions for newlines) and Binary Files (which handle data as raw bytes, with no translation or formatting).

Binary files are used to store data exactly as it is represented in the computer’s memory, making them faster and more compact for storing numerical data or complex structures.

1. Binary File Modes

To open a file for binary I/O, you append a b to the standard file mode string:

Example:

FILE *bin_fp = fopen("records.bin", "wb"); // Open for binary writing

if (bin_fp == NULL) {
    // Handle error...
}

2. Binary I/O Functions (fread and fwrite)

Binary operations use two functions that read and write blocks of raw bytes, rather than formatted characters. They are ideal for reading/writing entire arrays or structures in one operation.

Syntax for both:

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream);
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

3. Example: Writing a Structure to a Binary File

Writing a structure directly is the most common use case for binary I/O.

// Assume this struct is defined globally
struct Record {
    int id;
    float value;
};

// ... in main() or a function ...
struct Record rec = {101, 3.14f};
FILE *fp = fopen("records.bin", "wb");

// Write one element (rec) whose size is sizeof(struct Record)
fwrite(&rec, sizeof(struct Record), 1, fp);

fclose(fp);

4. Example: Reading a Structure from a Binary File

Reading the data back involves using fread() to place the raw bytes back into a structure variable.

struct Record rec_read;
FILE *fp = fopen("records.bin", "rb");

// Read one element of size sizeof(struct Record) into the address of rec_read
size_t records_read = fread(&rec_read, sizeof(struct Record), 1, fp);

if (records_read == 1) {
    printf("Read ID: %d, Value: %.2f\n", rec_read.id, rec_read.value);
}

fclose(fp);

5. File Positioning (fseek, ftell, rewind)

These functions allow random access within a file, letting you jump to any record or position rather than reading sequentially.