Advanced File Operations & Random Access

Basic file I/O runs sequentially: you read or write byte-by-byte from the beginning to the end. Advanced file operations allow you to jump to specific locations inside a file and read/write raw binary data blocks.

1. Binary block Read and Write

Instead of formatted text, binary I/O processes raw bytes directly from memory, which is faster and produces smaller files: - fwrite(buffer, element_size, count, file): Writes a block of elements to file. - fread(buffer, element_size, count, file): Reads a block of elements from file.

#include <stdio.h>

struct Point {
    int x, y;
};

int main() {
    struct Point p1 = {10, 20};

    // Open in binary write mode
    FILE *file = fopen("point.bin", "wb");
    if (file != NULL) {
        fwrite(&p1, sizeof(struct Point), 1, file);
        fclose(file);
    }
    return 0;
}

2. Random Access: Jumping in Files

C maintains a "file position indicator" (cursor). You can query or move this cursor using three standard functions: - fseek(file, offset, origin): Moves the file cursor. - origin can be: - SEEK_SET: Beginning of file. - SEEK_CUR: Current cursor location. - SEEK_END: End of file. - ftell(file): Returns the current byte offset of the file cursor. - rewind(file): Resets the cursor back to the beginning of the file.

Example: Reading arbitrary records

If we want to read the second point record inside a binary file without reading the first one:

#include <stdio.h>
#include <stdlib.h>

struct Point {
    int x, y;
};

int main() {
    FILE *file = fopen("point.bin", "rb");
    if (file == NULL) return 1;

    struct Point second_point;

    // Seek to the start of the 2nd record (skip 1 sizeof(struct Point) bytes)
    fseek(file, sizeof(struct Point), SEEK_SET);

    // Query position indicator
    long position = ftell(file);
    printf("Reading from byte offset: %ld\n", position);

    fread(&second_point, sizeof(struct Point), 1, file);
    fclose(file);

    return 0;
}