如何将unsigned char数组转换为unsigned long long？

假设我有8个unsigned char，我想转换为unsigned long long。

例如，如果所有char都等于0xFF，则无符号long long将等于0xFFFFFFFFFFFFFFFF。

使用C或C ++最有效的方法是什么？

Answer 1

例如：

 unsigned char buffer[8] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF }; unsigned long long target; memcpy(&target, buffer, sizeof target); 

请注意，如果不是buffer所有元素都具有相同的值，则结果将取决于字节顺序（little-endian与big-endian）。

这也假设unsigned long long恰好是8个字节。 这是非常普遍的，但并不能保证。 （它也不能保证一个字节正好是8位;它可以更多。 unsigned char定义为1字节。）

Answer 2

您可以直接分配位，而不是memcpy

  unsigned char buffer[8] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff }; unsigned long long l = 0; for (int i = 0; i < 8; ++i) { l = l | ((unsigned long long)buffer[i] << (8 * i)); } 

我相信这对endianness是免疫的。

Answer 3

抱歉我最初的愚蠢回答，真的应该更仔细地阅读这个问题。 希望这个更有帮助。 转换的方式取决于字节数组中长long表示的字节顺序是否与您正在处理的体系结构的字节顺序相匹配。

C ++：

 #include  #include  using namespace std; // Assuming ca is at least 8 bytes, the size of long long, interpret the // first 8 bytes as long long. // differentEndian == true means the endianness of the machine is // different from the representation in ca. long long getLongLong( unsigned char * ca, bool differentEndian ) { long long retVal; if (differentEndian) { for (int i = 0; i < 4; i++) { unsigned char _tmpCh = ca[i]; ca[i] = ca[7-i]; ca[7-i] = _tmpCh; } } retVal = *reinterpret_cast(ca); return retVal; } int main() { unsigned char cArray[] = {0xff, 0x1, 0x70, 0x2, 0x61, 0x3, 0x52, 0x4}; unsigned long long ll = getLongLong( cArray, false ); cout << "Result for same endian: " << hex << ll << " or " << dec << ll << endl; ll = getLongLong( cArray, true ); cout << "Result for different endian: " << hex << ll << " or " << dec << ll << endl; return 0; } 

C：

 #include  #include  // Assuming ca is at least 8 bytes, the size of long long, interpret the // first 8 bytes as long long. // differentEndian != 0 means the endianness of the machine is // different from the representation in ca. long long getLongLong( unsigned char * ca, int differentEndian ) { long long retVal; if (differentEndian) { int i; for (i = 0; i < 4; i++) { unsigned char _tmpCh = ca[i]; ca[i] = ca[7-i]; ca[7-i] = _tmpCh; } } memcpy( &retVal, ca, sizeof(long long)); return retVal; } int main() { unsigned char cArray[] = {0xff, 0x1, 0x70, 0x2, 0x61, 0x3, 0x52, 0x4}; unsigned long long ll = getLongLong( cArray, 0 ); printf("Result for same endian: %llx or %llu\n", ll, ll); ll = getLongLong( cArray, 1 ); printf("Result for different endian: %llx or %llu\n", ll, ll); return 0; } 

两个版本的输出是：

 Result for same endian: 4520361027001ff or 311315039429591551 Result for different endian: ff01700261035204 or 1837509111016818739