Binary I/O

There are different types of files. Instead of working with text files (one byte stores one char through ANSI standard mapping), we work directly with 0's and 1's. We call anything that isn't a text file a "binary file".

• Storing binary can be much more efficient than text (for some types of data). For example, numbers stored in ANSI uses one byte per decimal digit. Instead of storing 0-255, one byte is used to store only 0-9.
• Large data files like images, audios, and video files are typically stored in binary format.
  • If you want to store a lot of 0-255 values (images). In text, storing 255 takes 3 bytes. If we use binary, it only takes 1 byte (8 bits). This reduces size by a factor of three.

Reading and Writing to Binary File

Tell C to open a file as a binary file using the "b" mode.

FILE *fp = fopen("data.dat", "rb");

This will open the file in binary read mode.

Use fread for reading and fwrite for writing.

• Works for arrays, structs, and arrays of structs.
• Useful for reading/writing large amounts of data in one operation.
• It copies bits from disk to memory (fread) or memory to disk (fwrite).
• Binary files are less portable than text because some types are different sizes on some architectures.

fread and fwrite

fread and fwrite take a pointer to a block of memory, an element size, a number of elements, and a file handle

fread reads size_of_el * num_els bytes of memory from the file beginning at the fursor location fp, and stores them starting at pointer location where_to:

int items_read = fread(where_to, size_of_el, num_els, fp);

fwrite does the opposite, copying data from memory to the specified file:

int items_written = fwrite(where_from, size_of_el, num_els, fp);

Example

// bin_io.c:
#include <stdio.h>

int main() {

	const int SIZE = 100;  
	int arr_write[SIZE];  
	for (int i = 0; i < 100; i++) {
		arr_write[i] = i * 10; 
	}
	FILE *fp = fopen("data.dat", "wb");
	if (!fp) {
		printf("Error opening data.dat\n");
		return 1;
	}
	// writes an array of integers
	fwrite(arr_write, sizeof(arr_write[0]), SIZE, fp);
	fclose(fp);
	
	int arr_read[SIZE];  
	fp = fopen("data.dat","rb");
	if (!fp) {
		printf("Error opening data.dat\n");
		return 1;
	}
	// reads an array of integer
	int num_of_ints = fread(arr_read, sizeof(arr_read[0]), SIZE, fp);
	if(num_of_ints != SIZE) { 
		printf("problem reading data.dat\n");
		return 1;
	}  
	if (feof(fp)) {
		printf("error: unexpected eof\n");
		return 1; 
	}
	if (ferror(fp)) {  
		printf("error reading data.dat\n");
	}  
	for (int i = 0; i < 100; i++) {
		printf("arr_read[%d] = %d\n", i, arr_read[i]);
	}
	fclose(fp); 
}