What is -(-128) for signed single byte char in C?

Question

My little program:

#include <stdio.h>

int main() {
    signed char c = -128;
    c = -c;
    printf("%d", c);
    return 0;
}

print:

-128

Is minus (-) operator portable across CPU?

score 9 · Accepted Answer · answered Jul 04 '13 at 12:10

9

The operand of the unary minus first undergoes standard promitions, so it is of type int, which can represent the value -128. The result of the operation is the value 128, also of type int. The conversion from int to signed char, being a narrowing of signed types, is implementation-defined.

(Your implementation seems to do a simple wrap-around: 125, 126, 127, -128, -127, ...)

answered Jul 04 '13 at 12:10

Kerrek SB

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Does ANSI C convert primitive types to **int** before performing arithmetical operations? This is true for Java (as states Java lang spec), but is this true for C? – gavenkoa Jul 04 '13 at 12:20
@gavenkoa May be your answer: [C 2011 (n1570) 6.3.1.8 (“Usual arithmetic conversions”) 1 states that the integer promotions are performed before considering whether the types are the same:](http://stackoverflow.com/questions/12841278/char-and-the-usual-arithmetic-conversion-rules) – Grijesh Chauhan Jul 04 '13 at 12:26
@gavenkoa: Loosely speaking, everything that's "smaller" than an `int` is promoted to `int` during arithmetic operations. The result is then assigned back to the left-hand side. – Kerrek SB Jul 04 '13 at 12:27
3

@gavenkoa: Types that have a lower conversion rank than `int` are promoted to `int`, unless `int` cannot represent all the values of the original type, in which case they are promoted to `unsigned int`. – caf Jul 04 '13 at 12:28

Grijesh Chauhan · Answer 2 · 2013-07-04T13:28:57.633

Note: -128 in 2's complement is 1000 0000 (in one byte) and 128 is also 1000 0000 . If you do char c = 128 and print it it will be -128 because of the following reason:

A char variable = 128 value stores in memory as follows.

MSB
+----+----+----+---+---+---+---+---+
|  1 |  0 | 0  | 0 | 0 | 0 | 0 | 0 |   
+----+----+----+---+---+---+---+---+
   7    6   5    4   3   2   1   0

Now,

this value will be interpreted as negative value because MSB is 1,

to print magnitude of this -ve number 2's complement needed, that is also 128 in one byte so output is: -128

2's complement:

  1000 0000

  0111 1111  1's complement 
+ 0000 0001 
 -----------
  1000 0000  2's complement   

 Magnitude = 128 
 So in one byte 128 == -128

score 1 · Answer 3 · answered Jul 04 '13 at 12:09

1

because a byte(char) can't hold 128

-128 = 0x80

what neg do is reverse it and plus 1

-(-128) = (~0x80) + 1 = 0x7F + 1 = 0x80

daha, you got 0x80 again

answered Jul 04 '13 at 12:09

Zang MingJie

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What is -(-128) for signed single byte char in C?

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