Unterschied signiertes / unsigned char [duplicate]

Ich weiß also, dass der Unterschied zwischen a signed int und unsigned int ist, dass ein Bit verwendet wird, um anzuzeigen, ob die Zahl positiv oder negativ ist, aber wie gilt dies für a char? Wie kann ein Charakter positiv oder negativ sein?

Es gibt keinen dedizierten “Zeichentyp” in der C-Sprache. char ist ein Integer-Typ, dasselbe (in dieser Hinsicht) wie int, short und andere Integer-Typen. char ist zufällig der kleinste ganzzahlige Typ. Wie jeder andere ganzzahlige Typ kann er also vorzeichenbehaftet oder vorzeichenlos sein.

Es stimmt, dass (wie der Name schon sagt) char dient hauptsächlich zur Darstellung von Zeichen. Aber Zeichen in C werden durch ihre ganzzahligen “Codes” dargestellt, also ist es nichts Ungewöhnliches, dass ein ganzzahliger Typ char wird verwendet, um diesen Zweck zu erfüllen.

Der einzige allgemeine Unterschied zwischen char und andere Integer-Typen ist das einfach char ist nicht gleichbedeutend mit signed charwährend bei anderen Integer-Typen die signed Modifikator ist optional/impliziert.

I slightly disagree with the above. The unsigned char simply means: Use the most significant bit instead of treating it as a bit flag for +/- sign when performing arithmetic operations.

It makes significance if you use char as a number for instance:

typedef char BYTE1;
typedef unsigned char BYTE2;

BYTE1 a;
BYTE2 b;

For variable a, only 7 bits are available and its range is (-127 to 127) = (+/-)2^7 -1.
For variable b all 8 bits are available and the range is 0 to 255 (2^8 -1).

If you use char as character, “unsigned” is completely ignored by the compiler just as comments are removed from your program.

There are three char types: (plain) char, signed char and unsigned char. Any char is usually an 8-bit integer* and in that sense, a signed and unsigned char have a useful meaning (generally equivalent to uint8_t and int8_t). When used as a character in the sense of text, use a char (also referred to as a plain char). This is typically a signed char but can be implemented either way by the compiler.

* Technically, a char can be any size as long as sizeof(char) is 1, but it is usually an 8-bit integer.

Representation is the same, the meaning is different. e.g, 0xFF, it both represented as “FF”. When it is treated as “char”, it is negative number -1; but it is 255 as unsigned. When it comes to bit shifting, it is a big difference since the sign bit is not shifted. e.g, if you shift 255 right 1 bit, it will get 127; shifting “-1” right will be no effect.

A signed char is a signed value which is typically smaller than, and is guaranteed not to be bigger than, a short. An unsigned char is an unsigned value which is typically smaller than, and is guaranteed not to be bigger than, a short. A type char without a signed or unsigned qualifier may behave as either a signed or unsigned char; this is usually implementation-defined, but there are a couple of cases where it is not:

1. If, in the target platform’s character set, any of the characters required by standard C would map to a code higher than the maximum `signed char`, then `char` must be unsigned.
2. If `char` and `short` are the same size, then `char` must be signed.

Part of the reason there are two dialects of “C” (those where char is signed, and those where it is unsigned) is that there are some implementations where char must be unsigned, and others where it must be signed.

The same way — e.g. if you have an 8-bit char, 7 bits can be used for magnitude and 1 for sign. So an unsigned char might range from 0 to 255, whilst a signed char might range from -128 to 127 (for example).

This because a char is stored at all effects as a 8-bit number. Speaking about a negative or positive char doesn’t make sense if you consider it an ASCII code (which can be just signed*) but makes sense if you use that char to store a number, which could be in range 0-255 or in -128..127 according to the 2-complement representation.

*: it can be also unsigned, it actually depends on the implementation I think, in that case you will have access to extended ASCII charset provided by the encoding used