c - What causes a char to be signed or unsigned when using gcc?

Question

What causes if a char in C (using gcc) is signed or unsigned? I know that the standard doesn't dictate one over the other and that I can check CHAR_MIN and CHAR_MAX from limits.h but I want to know what triggers one over the other when using gcc

If I read limits.h from libgcc-6 I see that there is a macro __CHAR_UNSIGNED__ which defines a "default" char signed or unsigned but I'm unsure if this is set by the compiler at (his) built time.

I tried to list GCCs predefined makros with

$ gcc -dM -E -x c /dev/null | grep -i CHAR
#define __UINT_LEAST8_TYPE__ unsigned char
#define __CHAR_BIT__ 8
#define __WCHAR_MAX__ 0x7fffffff
#define __GCC_ATOMIC_CHAR_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __SCHAR_MAX__ 0x7f
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __UINT8_TYPE__ unsigned char
#define __INT8_TYPE__ signed char
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __CHAR16_TYPE__ short unsigned int
#define __INT_LEAST8_TYPE__ signed char
#define __WCHAR_TYPE__ int
#define __GCC_ATOMIC_CHAR16_T_LOCK_FREE 2
#define __SIZEOF_WCHAR_T__ 4
#define __INT_FAST8_TYPE__ signed char
#define __CHAR32_TYPE__ unsigned int
#define __UINT_FAST8_TYPE__ unsigned char

but wasn't able to find __CHAR_UNSIGNED__

Background: I've some code which I compile on two different machines:

Desktop PC:

• Debian GNU/Linux 9.1 (stretch)
• gcc version 6.3.0 20170516 (Debian 6.3.0-18)
• Intel(R) Core(TM) i3-4150
• libgcc-6-dev: 6.3.0-18
• char is signed

Raspberry Pi3:

• Raspbian GNU/Linux 9.1 (stretch)
• gcc version 6.3.0 20170516 (Raspbian 6.3.0-18+rpi1)
• ARMv7 Processor rev 4 (v7l)
• libgcc-6-dev: 6.3.0-18+rpi
• char is unsigned

So the only obvious difference is the CPU architecture...

Answer 1

According to the C11 standard (read n1570), char can be signed or unsigned (so you actually have two flavors of C). What exactly it is is implementation specific.

Some processors and instruction set architectures or application binary interfaces favor a signed character (byte) type (e.g. because it maps nicely to some machine code instruction), other favor an unsigned one.

gcc has even some -fsigned-char or -funsigned-char option which you should almost never use (because changing it breaks some corner cases in calling conventions and ABIs) unless you recompile everything, including your C standard library.

You could use feature_test_macros(7) and <endian.h> (see endian(3)) or autoconf on Linux to detect what your system has.

In most cases, you should write portable C code, which does not depend upon those things. And you can find cross-platform libraries (e.g. glib) to help you in that.

BTW gcc -dM -E -x c /dev/null also gives __BYTE_ORDER__ etc, and if you want an unsigned 8 bit byte you should use <stdint.h> and its uint8_t (more portable and more readable). And standard limits.h defines CHAR_MIN and SCHAR_MIN and CHAR_MAX and SCHAR_MAX (you could compare them for equality to detect signed chars implementations), etc...

BTW, you should care about character encoding, but most systems today use UTF-8 everywhere. Libraries like libunistring are helpful. See also this and remember that practically speaking an Unicode character encoded in UTF-8 can span several bytes (i.e. char-s).