gcc unsigned to signed upcast 테스트

귀찮으니 날로먹는 코딩으로 테스트

$ cat 1.c #include <stdio.h> void main() {         unsigned char a = 0xFF;         char b =a;         short c = a;         short d = (char)a;         short e = (int)a;         int f = a;         int g = (int)a;         int h = (int)(char)a;         printf("a %d\n",a);         printf("a %d\n",(int)a);         printf("a %d\n",(char)a);         printf("b %d\n",b);         printf("c %d\n",c);         printf("d %d\n",d);         printf("e %d\n",e);         printf("f %d\n",f);         printf("g %d\n",g);         printf("h %d\n",h); }

 

컴파일러 버전과 아키텍쳐, 그리고 결과인데... 머냐..?!?!

$ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none:hsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu 9.3.0-17ubuntu1~20.04' --with-bugurl=file:///usr/share/doc/gcc-9/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,gm2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-9 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-9-HskZEa/gcc-9-9.3.0/debian/tmp-nvptx/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)

$ arm-linux-gnueabihf-gcc -v Using built-in specs. COLLECT_GCC=arm-linux-gnueabihf-gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc-cross/arm-linux-gnueabihf/7/lto-wrapper Target: arm-linux-gnueabihf Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 7.5.0-3ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libitm --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --with-target-system-zlib --enable-multiarch --enable-multilib --disable-sjlj-exceptions --with-arch=armv7-a --with-fpu=vfpv3-d16 --with-float=hard --with-mode=thumb --disable-werror --enable-multilib --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=arm-linux-gnueabihf --program-prefix=arm-linux-gnueabihf- --includedir=/usr/arm-linux-gnueabihf/include Thread model: posix gcc version 7.5.0 (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04)

$ ./1 a 255 a 255 a -1 b -1 c 255 d -1 e 255 f 255 g 255 h -1

# /mnt/1 a 255 a 255 a 255 b 255 c 255 d 255 e 255 f 255 g 255

 

라즈베리 파이 4에서 시도. arm 아키텍쳐용 컴파일러의 특성인가?

$ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/aarch64-linux-gnu/8/lto-wrapper Target: aarch64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 8.3.0-6' --with-bugurl=file:///usr/share/doc/gcc-8/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-8 --program-prefix=aarch64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --disable-libphobos --enable-multiarch --enable-fix-cortex-a53-843419 --disable-werror --enable-checking=release --build=aarch64-linux-gnu --host=aarch64-linux-gnu --target=aarch64-linux-gnu Thread model: posix gcc version 8.3.0 (Debian 8.3.0-6)

$ ./1 a 255 a 255 a 255 b 255 c 255 d 255 e 255 f 255 g 255 h 255

 

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두개중에 하나만 주면 되는진 모르겠지만, 둘 다 주거나 -fsigned-char 만 주어도 결과가 -1로 나오긴 한다.

원문으로 보니 --signed-chars는 RVCT 컴파일러를 위한 옵션인 듯.

The ANSI C standard specifies a range for both signed (at least -127 to +127) and unsigned (at least 0 to 255) chars. Simple chars are not specifically defined and it is compiler dependent whether they are signed or unsigned. Although the ARM architecture has the LDRSB instruction, that loads a signed byte into a 32-bit register with sign extension, the earliest versions of the architecture did not. It made sense at the time for the compiler to treat simple chars as unsigned, whereas on the x86 simple chars are, by default, treated as signed. One workaround for users of GCC is to use the -fsigned-char command line switch or --signed-chars for RVCT, that forces all chars to become signed, but a better practice is to write portable code by declaring char variables appropriately. Unsigned char must be used for accessing memory as a block of bytes or for small unsigned integers. Signed char must be used for small signed integers and simple char must be used only for ASCII characters and strings. In fact, on an ARM core, it is usually better to use ints rather than chars, even for small values, for performance reasons. You can read more on this in Optimizing Code to Run on ARM Processors.

[링크 : https://developer.arm.com/.../Miscellaneous-C-porting-issues/unsigned-char-and-signed-char]

 

LDRSB (Thumb*) - Load Register Signed Byte

[링크 : http://qcd.phys.cmu.edu/QCDcluster/intel/vtune/reference/LDRSB_(Thumb).htm]

 

LDRB - Load Register Byte

[링크 : http://qcd.phys.cmu.edu/QCDcluster/intel/vtune/reference/INST_LDRB.htm]

 

char -> signed char: -fsigned-char == -fno-unsigned-char char -> unsigned char: -funsigned-char == -fno-signed-char

[링크 : https://jooojub.github.io/gcc-options-fsigned-char/]

 

-fsigned-char Let the type char be signed, like signed char. Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. -funsigned-char Let the type char be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default. Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

[링크 : https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html]

 

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2021.07.09

으잉? singed char로 하면 되긴 한다. char가 signed 아니었어?!