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【C】判断大端小端

作者:hunanldc / 发布于2013/10/24/ 486


	/*
	 * We don't handle PDP-endian or similar orders: if a short is big-endian,
	 * so must int and long be big-endian for us to generate the IS_BIG_ENDIAN
	 * #define and the IS_LITTLE_ENDIAN #undef.
	 */
	 {
	 int big_endian = 0, little_endian = 0, ntests = 0;

	 if (sizeof(short) == 2) {
	 /* force |volatile| here to get rid of any compiler optimisations
	 * (var in register etc.) which may be appiled to |auto| vars -
	 * even those in |union|s...
	 * (|static| is used to get the same functionality for compilers
	 * which do not honor |volatile|...).
	 */
	 volatile static union {
	 short i;
	 char c[2];
	 } u;

	 u.i = 0x0102;
	 big_endian += (u.c[0] == 0x01 && u.c[1] == 0x02);
	 little_endian += (u.c[0] == 0x02 && u.c[1] == 0x01);
	 ntests++;
	 }

	 if (sizeof(int) == 4) {
	 /* force |volatile| here ... */
	 volatile static union {
	 int i;
	 char c[4];
	 } u;

	 u.i = 0x01020304;
	 big_endian += (u.c[0] == 0x01 && u.c[1] == 0x02 &&
	 u.c[2] == 0x03 && u.c[3] == 0x04);
	 little_endian += (u.c[0] == 0x04 && u.c[1] == 0x03 &&
	 u.c[2] == 0x02 && u.c[3] == 0x01);
	 ntests++;
	 }

	 if (sizeof(long) == 8) {
	 /* force |volatile| here ... */
	 volatile static union {
	 long i;
	 char c[8];
	 } u;

	 /*
	 * Write this as portably as possible: avoid 0x0102030405060708L
	 * and <<= 32.
	 */
	 u.i = 0x01020304;
	 u.i <<= 16, u.i <<= 16;
	 u.i |= 0x05060708;
	 big_endian += (u.c[0] == 0x01 && u.c[1] == 0x02 &&
	 u.c[2] == 0x03 && u.c[3] == 0x04 &&
	 u.c[4] == 0x05 && u.c[5] == 0x06 &&
	 u.c[6] == 0x07 && u.c[7] == 0x08);
	 little_endian += (u.c[0] == 0x08 && u.c[1] == 0x07 &&
	 u.c[2] == 0x06 && u.c[3] == 0x05 &&
	 u.c[4] == 0x04 && u.c[5] == 0x03 &&
	 u.c[6] == 0x02 && u.c[7] == 0x01);
	 ntests++;
	 }

	 if (big_endian && big_endian == ntests) {
	 printf("#undef IS_LITTLE_ENDIAN\n");
	 printf("#define IS_BIG_ENDIAN 1\n\n");
	 } else if (little_endian && little_endian == ntests) {
	 printf("#define IS_LITTLE_ENDIAN 1\n");
	 printf("#undef IS_BIG_ENDIAN\n\n");
	 } else {
	 fprintf(stderr, "%s: unknown byte order"
	 "(big_endian=%d, little_endian=%d, ntests=%d)!\n",
	 argv[0], big_endian, little_endian, ntests);
	 return EXIT_FAILURE;
	 }
	 }

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