将unsigned char转换为signed int

Question

Consider the following program

void main(){
  char t = 179;
  printf("%d ",t);
}

Output is -77. But binary representation of 179 is

10110011

So, shouldn't output be -51, considering 1st bit is singed bit. Binary representation of -77 is

11001101

It seems bit order is reversed. What's going on? Please somebody advice.

Answer 1

You claimed that the binary representation of -77 is 11001101 , but the reversal is yours. The binary representation of -77 is 10110011 .

Binary 10110011 unsigned is decimal 179 .

Binary 10110011 signed is decimal -77 .

You assigned the out-of-range value 179 to a signed char . It might theoretically be Undefined Behaviour, but apart from throwing an error, it would be a very poor compiler that placed anything but that 8-bit value in the signed char .

But when printed, it's interpreted as a negative number because b7 is set.

Answer 2

Looks like char is a signed type on your system. The valid range for a char would be [-128, 127]

By using

char t = 179;

the compiler uses the 2's complement of 179 (which is most likely -77) and assigns that value to t .

Answer 3

To convert between a positive and a negative number in 2's complement you invert all the bits and then you add 1.

10110011

01001100 (invert all bits)

01001101 (add one)

That is 77 decimal

Answer 4

Char could be signed or unsigned: that's up to your compiler.

If signed, it could be 2s or 1 complement.

Furthermore, it could be larger than 8 bits, although sizeof char is defined to be 1.

So it's inadvisable to rely on a specific representation.

Answer 5

Please note, that on many systems char is signed. Hence, when you assign 179 (which is of type int ) to a char this value is outside of char range, hence it is unspecified behaviour.

6.3.1.3 Signed and unsigned integers:

When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged. Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more unsigned integer than the maximum value that can be represented in the new type until the value is in the range of the new conversion to type. Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

If you change the type to unsigned char your program will perform correctly.

Also note that char , signed char and unsigned char are 3 distinct types, unlike int and signed int . The signedness of char is implementation defined.