asm keyword as identifier name in C? - c

Question

I've seen a project which uses the asm keyword as variable name and the code compiles fine with the provided Makefile on Linux:
int
main(int ac, char *av[])
{
struct TMap *tm;
FILE *inf, *hf;
char *f, *sep;
int c, asm; // <---
asm = Defasm;
// ...
}
Here is the entire Makefile:
BIN = qbe
V = #
OBJDIR = obj
SRC = main.c util.c parse.c cfg.c mem.c ssa.c alias.c load.c copy.c \
fold.c live.c spill.c rega.c gas.c
AMD64SRC = amd64/targ.c amd64/sysv.c amd64/isel.c amd64/emit.c
ARM64SRC = arm64/targ.c arm64/abi.c arm64/isel.c arm64/emit.c
SRCALL = $(SRC) $(AMD64SRC) $(ARM64SRC)
AMD64OBJ = $(AMD64SRC:%.c=$(OBJDIR)/%.o)
ARM64OBJ = $(ARM64SRC:%.c=$(OBJDIR)/%.o)
OBJ = $(SRC:%.c=$(OBJDIR)/%.o) $(AMD64OBJ) $(ARM64OBJ)
CFLAGS += -Wall -Wextra -std=c99 -g -pedantic
$(OBJDIR)/$(BIN): $(OBJ) $(OBJDIR)/timestamp
#test -z "$(V)" || echo "ld $#"
$(V)$(CC) $(LDFLAGS) $(OBJ) -o $#
$(OBJDIR)/%.o: %.c $(OBJDIR)/timestamp
#test -z "$(V)" || echo "cc $<"
$(V)$(CC) $(CFLAGS) -c $< -o $#
$(OBJDIR)/timestamp:
#mkdir -p $(OBJDIR)
#mkdir -p $(OBJDIR)/amd64
#mkdir -p $(OBJDIR)/arm64
#touch $#
$(OBJ): all.h ops.h
$(AMD64OBJ): amd64/all.h
$(ARM64OBJ): arm64/all.h
obj/main.o: config.h
config.h:
#case `uname` in \
*Darwin*) \
echo "#define Defasm Gasmacho"; \
echo "#define Deftgt T_amd64_sysv"; \
;; \
*) \
echo "#define Defasm Gaself"; \
case `uname -m` in \
*aarch64*) \
echo "$define Deftgt T_arm64"; \
;; \
*) \
echo "#define Deftgt T_amd64_sysv";\
;; \
esac \
;; \
esac > $#
install: $(OBJDIR)/$(BIN)
mkdir -p "$(DESTDIR)/$(PREFIX)/bin/"
cp $< "$(DESTDIR)/$(PREFIX)/bin/"
uninstall:
rm -f "$(DESTDIR)/$(PREFIX)/bin/$(BIN)"
clean:
rm -fr $(OBJDIR)
clean-gen: clean
rm -f config.h
check: $(OBJDIR)/$(BIN)
tools/test.sh all
check-arm64: $(OBJDIR)/$(BIN)
TARGET=arm64 tools/test.sh all
src:
#echo $(SRCALL)
80:
#for F in $(SRCALL); \
do \
awk "{ \
gsub(/\\t/, \" \"); \
if (length(\$$0) > $#) \
printf(\"$$F:%d: %s\\n\", NR, \$$0); \
}" < $$F; \
done
.PHONY: clean clean-gen check check-arm64 src 80 install uninstall
I'm wondering how to accomplish that the compiler will allow asm as variable name (ignoring the fact that it's bad style to do this). I tried porting the Makefile to a CMakeLists.txt but the compiler threw an error because of the invalid usage of asm. How to fix this without changing the code? I already forced the C99 standard as the Makefile did and used the same C compiler flags but that didn't help:
cmake_minimum_required(VERSION 3.10.2)
project(qbe C)
set(CMAKE_C_STANDARD 99)
set(CMAKE_C_FLAGS "-Wall -Wextra -std=c99 -g -pedantic")
include_directories(amd64)
add_executable(qbe
amd64/all.h
amd64/emit.c
amd64/isel.c
amd64/sysv.c
amd64/targ.c
alias.c
all.h
cfg.c
copy.c
fold.c
gas.c
live.c
load.c
main.c
mem.c
ops.h
parse.c
rega.c
spill.c
ssa.c
util.c)
I want to use CMake due to IDE support.

Answer 1

asm is not a keyword of standard C, nor a reserved identifier, though the standard recognizes it as a common extension. The standard furthermore remarks that
The inclusion of any extension that may cause a strictly conforming
program to become invalid renders an implementation nonconforming.
Examples of such extensions are new keywords, extra library functions
declared in standard headers, or predefined macros with names that do
not begin with an underscore.
This is in Annex J, which is non-normative, but it merely summarizes a conclusion that can be drawn from the normative text. Thus a conforming implementation will not reject code on the basis that it uses asm as an identifier.
I find that GCC, for example, accepts usage of asm as an identifier when any of the strict conformance modes it supports is used (-std=c89 or -std=c99 or -std=c11), but rejects it with an error by default or with GNU extensions specifically enabled (e.g. with -std=gnu11). If you are using a different compiler then you will need to consult its documentation for how to obtain conformance to the standard in this area, if indeed it is possible to do so.
Addendum
As for why your CMake attempt is failing, the CMAKE_C_STANDARD property does not do what you seem to think it does. Its tells CMake to choose, if possible, compiler options that provide the features of the specified version of the C standard, but it does not request strict conformance to that standard. The docs specifically postulate it causing the -std=gnu11 option to be used under some circumstances, and that or similar inclusive options would have an effect opposite to what you want.

Answer 2

asm is not a standard C keyword. A conforming C compiler may not refuse compiling a program that uses asm as an identifier.
Choosing a standard explicitly with -std=c99 will also remove asm keyword. You can continue to use __asm__ however, since all names beginning with two underscores are always reserved by the implementation for any use.
However, it is still very bad style, since C99, C11 and C17 revisions do mention it as a common extension.