gists/c-programming/fwrite_le.c

/*
 * fwrite_le.c
 *
 * Author: Intel A80486DX2-66
 * License: Creative Commons Zero 1.0 Universal
 */

#include "fwrite_le.h"

int detect_endianness(void) {
	volatile uint32_t i = 0x01234567;
	uint8_t* bytes = (uint8_t*)(&i);
	if (bytes[0] == 0x01 &&
		bytes[1] == 0x23 &&
		bytes[2] == 0x45 &&
		bytes[3] == 0x67)
		return DETECTED_BIG_ENDIAN;
	else if (
		bytes[0] == 0x67 &&
		bytes[1] == 0x45 &&
		bytes[2] == 0x23 &&
		bytes[3] == 0x01)
		return DETECTED_LITTLE_ENDIAN;
	return UNSUPPORTED_ENDIANNESS;
}

void reorder_le_be(
#if FWRITE_LE_NO_MODIFICATION
	uint8_t* dest, uint8_t* src,
#else
	uint8_t* bytes,
#endif
	size_t count, size_t step) {
	for (size_t i = 0; i < count; i += step) {
		const size_t div_size = step / 2;
		for (size_t j = 0; j < div_size; j++) {
			const size_t old_pos = i + j, new_pos = i + step - j - 1;
#if FWRITE_LE_NO_MODIFICATION
			dest[old_pos] = src[new_pos];
			dest[new_pos] = src[old_pos];
#else
			uint8_t temp = bytes[old_pos];
			bytes[old_pos] = bytes[new_pos];
			bytes[new_pos] = temp;
#endif
		}
	}
}

size_t fwrite_le(void* ptr, size_t size, size_t count, FILE* stream) {
	/*
	 * warning: this function modifies `void* ptr` by default!
	 *          (if FWRITE_LE_NO_MODIFICATION in the header is 0)
	 */
	if (ptr == NULL)
		return 0;

	int endianness = detect_endianness();
	if (endianness == UNSUPPORTED_ENDIANNESS) {
		fprintf(stderr, "Unsupported endianness\n");
		exit(EXIT_FAILURE);
	} else if (endianness == DETECTED_LITTLE_ENDIAN)
		return fwrite(ptr, size, count, stream);

	// case: big-endian
	size_t bytes_count = size * count;
#if FWRITE_LE_NO_MODIFICATION
	uint8_t* bytes = calloc(bytes_count, sizeof(uint8_t));
	if (bytes == NULL) {
		perror("calloc");
		exit(EXIT_FAILURE);
	}
	memcpy(bytes, ptr, bytes_count);
#endif

	reorder_le_be(
#if FWRITE_LE_NO_MODIFICATION
		bytes,
#endif
		(uint8_t*) ptr, bytes_count, size);

#if FWRITE_LE_NO_MODIFICATION
	size_t res =
#else
	return
#endif
		fwrite(
#if FWRITE_LE_NO_MODIFICATION
			bytes,
#else
			(uint8_t*) ptr,
#endif
			size, count, stream);
#if FWRITE_LE_NO_MODIFICATION
	free(bytes);
	return res;
#endif
}