Handout

2025-04-14 11:20:52 +02:00
commit 5a2e32aaeb
126 changed files with 16742 additions and 0 deletions
--- a/tools/build.mk
+++ b/tools/build.mk
@@ -0,0 +1,146 @@
+# Build the kernel
+
+# Top Level Folder all generated files will be placed in.
+ROOTBUILDDIR =  build
+# Folder the generated files will be placed in.
+BUILDDIR ?= $(ROOTBUILDDIR)
+# Build folder suffixes
+OPTTAG = opt
+NOOPTTAG = noopt
+DBGTAG = dbg
+VERBOSETAG = verbose
+
+# C++
+CXX ?= $(PREFIX)clang++
+# on macos the default compiler is called c++, switch to clang++
+ifeq ($(shell uname),Darwin)
+	ifeq ($(CXX),c++)
+		CXX := clang++
+	endif
+endif
+
+CXXFLAGS_ARCH = -m64
+CXXFLAGS_DEFAULT = -std=c++23 -ffreestanding -fno-pic -nodefaultlibs -nostdlib -nostdinc -I. -fno-rtti -fno-exceptions -Wno-write-strings -fno-stack-protector -mno-red-zone -g -gdwarf-2 -fno-use-cxa-atexit -no-pie -nostartfiles
+# Enable 16-byte compare-and-exchange instruction for debugging purposes (stack alignment)
+CXXFLAGS_DEFAULT += -mcx16
+CXXFLAGS_OPT = -O3 -fomit-frame-pointer
+CXXFLAGS_WARNING = -Wall -Wextra -Werror -Wno-error=unused-parameter -Wno-error=unused-variable -Wno-non-virtual-dtor
+CXXFLAGS_CLANG = -Wno-error=unused-private-field -Wno-implicit-exception-spec-mismatch -Wno-error=unused-const-variable -Wno-unused-command-line-argument -Wno-unused-const-variable -fno-strict-aliasing
+ifeq ($(shell uname),Darwin)
+	CXXFLAGS_CLANG += -target x86_64-pc-linux-gnu -fuse-ld=lld
+endif
+CXXFLAGS_GCC = -fno-tree-loop-distribute-patterns -Wstack-usage=4096 -Wno-error=stack-usage=
+CXXFLAGS_NOFPU = -mno-mmx -mno-sse -mgeneral-regs-only
+CXXFLAGS = $(CXXFLAGS_ARCH) $(CXXFLAGS_DEFAULT) $(CXXFLAGS_OPT) $(CXXFLAGS_NOFPU) $(CXXFLAGS_WARNING)
+# Compiler specific flags
+ifneq (,$(findstring clang,$(CXX)))
+	COMPILER := CLANG
+	CXXFLAGS += $(CXXFLAGS_CLANG)
+else ifneq (,$(findstring g++,$(CXX)))
+	COMPILER := GCC
+	# g++ 6 does not support general-regs-only flag
+	ifeq "$(shell expr `$(CXX) -dumpversion | cut -f1 -d.` \<= 6)" "1"
+		CXXFLAGS := $(filter-out -mgeneral-regs-only,$(CXXFLAGS))
+	endif
+	# Configures -Warray-bounds warning to allow accessing in the NULL page
+	# Necessary to read the Extended BIOS Data Area at 0x40e
+	ifeq "$(shell expr `$(CXX) -dumpversion | cut -f1 -d.` \>= 12)" "1"
+		CXXFLAGS += --param=min-pagesize=0x40e
+	endif
+	CXXFLAGS += $(CXXFLAGS_GCC)
+else
+	COMPILER :=
+endif
+
+# Assembly
+ASM = nasm
+ASMFLAGS = -f elf64
+
+# Additional build utilities
+# If llvm tools are available, use them
+ifneq (,$(shell which llvm-objcopy 2>/dev/null))
+	PREFIX = llvm-
+endif
+OBJCOPY = $(PREFIX)objcopy
+STRIP = $(PREFIX)strip
+AR = $(PREFIX)ar
+
+# Subdirectories with sources
+VPATH = $(sort $(dir $(CC_SOURCES) $(ASM_SOURCES)))
+
+# Lists of object files that are generated by compilation:
+# Note that the variables containing the input files are to be defined by
+# the Makefiles prior to including this common.mk.
+CC_OBJECTS = $(addprefix $(BUILDDIR)/,$(CC_SOURCES:.cc=.o))
+DEP_FILES = $(addprefix $(BUILDDIR)/,$(CC_SOURCES:.cc=.d) $(addsuffix .d,$(ASM_SOURCES)))
+ASM_OBJECTS = $(addprefix $(BUILDDIR)/,$(addsuffix .o,$(ASM_SOURCES)))
+
+# Dependency files
+$(BUILDDIR)/%.d : %.cc $(MAKEFILE_LIST)
+	@echo "DEP		$<"
+	@mkdir -p $(@D)
+	$(VERBOSE) $(CXX) $(CXXFLAGS) -MM -MT $(BUILDDIR)/$*.o -MF $@ $<
+
+$(BUILDDIR)/%.asm.d : %.asm $(MAKEFILE_LIST)
+	@echo "DEP		$<"
+	@mkdir -p $(@D)
+	$(VERBOSE) $(ASM) $(ASMFLAGS) -M -MT $(BUILDDIR)/$*.asm.o -MF $@ $<
+
+# Object files
+$(BUILDDIR)/%.o : %.cc $(MAKEFILE_LIST)
+	@echo "CXX		$<"
+	@mkdir -p $(@D)
+	$(VERBOSE) $(CXX) -c $(CXXFLAGS) -o $@ $<
+
+$(BUILDDIR)/%.asm.o : %.asm $(MAKEFILE_LIST)
+	@echo "ASM		$<"
+	@mkdir -p $(@D)
+	$(VERBOSE) $(ASM) $(ASMFLAGS) -o $@ $<
+
+# The standard target 'clean' removes the whole generated system, the object files, and the dependency files.
+clean::
+	rm -rf "$(BUILDDIR)"
+
+# Target issuing a nested call to make generating a fully optimized systems without assertions.
+%-$(OPTTAG):
+	$(VERBOSE) $(MAKE) BUILDDIR="$(BUILDDIR)/$(OPTTAG)" ISODIR="$(ISODIR)/$(OPTTAG)" CXXFLAGS_OPT="-Ofast -fomit-frame-pointer -flto -march=westmere -DNDEBUG" $*
+
+# Target issuing a nested call to make generating a non-optimized system.
+%-$(NOOPTTAG):
+	$(VERBOSE) $(MAKE) BUILDDIR="$(BUILDDIR)/$(NOOPTTAG)" ISODIR="$(ISODIR)/$(NOOPTTAG)" CXXFLAGS_OPT="-O0" $*
+
+# Target issuing a nested call to make generating a system optimized for debugging.
+%-$(DBGTAG):
+	$(VERBOSE) $(MAKE) BUILDDIR="$(BUILDDIR)/$(DBGTAG)" ISODIR="$(ISODIR)/$(DBGTAG)" CXXFLAGS_OPT="-Og -fno-omit-frame-pointer" $*
+
+# Target issuing a nested call to make generating a system with verbose output.
+%-$(VERBOSETAG):
+	$(VERBOSE) $(MAKE) BUILDDIR="$(BUILDDIR)/$(VERBOSETAG)" ISODIR="$(ISODIR)/$(VERBOSETAG)" CXXFLAGS_OPT="-DVERBOSE" $*
+
+# Documentation
+help::
+	@$(echo) "" \
+		"All targets exist in different flavours in addition to \e[2;3m<name>\e[0m:\n" \
+		"\e[2;3m<name>\e[0;3m-noopt\e[0m, \e[2;3m<name>\e[0;3m-opt\e[0m, \e[2;3m<name>\e[0;3m-dbg\e[0m, and \e[2;3m<name>\e[0;3m-verbose\e[0m.\n" \
+		"Targets suffixed with \e[3m-noopt\e[0m are compiled without optimizations,\n" \
+		"\e[3m-opt\e[0m targets produce a highly optimized binary, while\n" \
+		"\e[3m-dbg\e[0m targets only use optimizations not hindering debugging.\n" \
+		"Targets suffixed with \e[3m-verbose\e[0m generate binaries including\n" \
+		"verbose output (via \e[3mDBG_VERBOSE\e[0m), making such targets useful for debugging.\n" \
+		"To get a verbose make output, clear VERBOSE, e.g. \e[3mmake VERBOSE=\e[0m.\n" \
+		"The following targets are available (each target can be suffixed by \e[3m-noopt\e[0m\n" \
+		"and \e[3m-verbose\e[0m):\n\n" \
+		"	\e[3mall\e[0m      Builds $(PROJECT), generating an ELF binary\n\n"
+
+# Print warnings, if appropriate
+ifeq (,$(COMPILER))
+$(warning Unknown (and potentially unsupported) compiler "$(CXX)"!)
+endif
+
+# Include dependency files (generated via gcc flag -MM)
+ifneq ($(MAKECMDGOALS),clean)
+-include $(DEP_FILES)
+endif
+
+# Phony targets
+.PHONY: clean help
--- a/tools/common.mk
+++ b/tools/common.mk
@@ -0,0 +1,33 @@
+# Common include Makefile
+
+# Hide commands
+VERBOSE = @
+# Prefix for toolchain binaries
+PREFIX ?=
+# Project name
+PROJECT ?= "MPStuBS"
+
+help::
+	@$(echo) "\n" \
+		"\e[1mMAKEFILE for the teaching operating system $(PROJECT)\e[0m\n" \
+		"--------------------------------------------------\n\n" \
+		"Executing '\e[4mmake\e[0m' will compile the operating system from source.\n"
+
+# Get current directory path
+CURRENT_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
+
+# Include Makefile scripts
+include $(CURRENT_DIR)/build.mk
+include $(CURRENT_DIR)/qemu.mk
+include $(CURRENT_DIR)/image.mk
+include $(CURRENT_DIR)/linter.mk
+include $(CURRENT_DIR)/remote.mk
+
+# Disable buitlin rules
+MAKEFLAGS += --no-builtin-rules
+
+# Disable buitlin rules
+MAKEFLAGS += --no-print-directory
+
+# Disable buitlin suffixes
+.SUFFIXES:
--- a/tools/cpplint.py
+++ b/tools/cpplint.py
--- a/tools/image.mk
+++ b/tools/image.mk
@@ -0,0 +1,96 @@
+# Generates a bootable ISO image that can be transferred to external media, such as CDs or USB sticks.
+# This will install, in addition to your kernel, the bootloader GRUB (https://www.gnu.org/software/grub/).
+#
+# The target 'gemu-iso' is used to test the image generated with 'iso'.
+#
+# Assuming that a USB mass-storage devices is connected as, for instance /dev/sdc, the target 'usb-sdc'
+# can be used to make your device bootable (requires root access, substitute sdc with the matching device).
+# Alternatively, you can burn the .iso file directly to CD.
+
+DD = dd
+XORRISO = xorriso
+MKISO = grub-mkrescue
+
+ISODIR = $(BUILDDIR)-iso
+ISOGRUBCFG = boot/grub/grub.cfg
+ISOKERNEL = boot/kernel
+ISOINITRD = initrd
+GRUBTITLE = $(shell id -un)s $(PROJECT)
+GRUBTIMEOUT = 2
+GRUBBIN = /usr/lib/grub/i386-pc
+
+# Default ISO target
+iso: $(ISOFILE)
+
+# Create Grub config
+$(ISODIR)/$(ISOGRUBCFG):
+	@echo "GEN		$@"
+	@mkdir -p $(dir $@)
+	@$(echo) "set timeout=$(GRUBTIMEOUT)\nset default=0\n\nmenuentry \"$(GRUBTITLE)\" {\n\tmultiboot /$(ISOKERNEL)\n\tmodule /$(ISOINITRD)\n\tboot\n}" > $@
+
+# Strip debug symbols from kernel binary
+$(ISODIR)/$(ISOKERNEL): all
+	@echo "STRIP		$@"
+	@mkdir -p $(dir $@)
+	$(VERBOSE) $(STRIP) --strip-debug --strip-unneeded -p -o $@ $(KERNEL)
+
+# copy inital ramdisk
+$(ISODIR)/$(ISOINITRD): all
+	@echo "CPY		$@"
+	@mkdir -p $(dir $@)
+	@if [ -s $(INITRD) ] ; then cp -a $(INITRD) $@ ; else touch $@ ; fi
+
+# Pack to ISO
+$(ISOFILE): $(ISODIR)/$(ISOKERNEL) $(ISODIR)/$(ISOINITRD) $(ISODIR)/$(ISOGRUBCFG)
+	@echo "ISO		$@"
+	@which $(XORRISO) >/dev/null || echo "Xorriso cannot be found - if building the ISO fails, this may be the reason!" >&2
+	$(VERBOSE) $(MKISO) -d $(GRUBBIN) -o $@ $(ISODIR)
+
+# Run ISO in QEMU/KVM
+%-iso: $(ISOFILE)
+	@${MAKE} -s QEMUKERNEL="-cdrom $<" $*
+
+# Copy ISO to USB device
+usb: $(ISOFILE)
+ifeq (,$(USBDEV))
+	@echo "The environment variable USBDEV must contain the path to the USB mass-storage device:" >&2
+	@lsblk -o TYPE,KNAME,SIZE,MODEL -a -p | grep "^disk" | cut -b 6-
+	@exit 1
+else
+	$(VERBOSE) $(DD) if=$< of=$(USBDEV) bs=4M status=progress && sync
+endif
+
+# Shorthand to copy ISO to a specific USB device
+usb-%:
+	@$(MAKE) USBDEV=/dev/$* usb
+
+# Burn ISO to CD
+cd: $(ISOFILE)
+ifeq (,$(CDRWDEV))
+	@echo "The environment variable CDRWDEV must contain the path to the CD/DVD writer" >&2
+	@exit 1
+else
+	$(VERBOSE) $(XORRISO) -as cdrecord -v dev=$(CDRWDEV) -dao $<
+endif
+
+# Shorthand to nurn ISO to specific CD device
+cd-%:
+	@$(MAKE) CDRWDEV=/dev/$* cd
+
+# The standard target 'clean' removes the whole generated system, the object files, and the dependency files.
+clean::
+	@echo "RM		$(ISODIR)"
+	$(VERBOSE) rm -rf "$(ISODIR)" "$(ISODIR)$(OPTTAG)" "$(ISODIR)$(NOOPTTAG)" "$(ISODIR)$(DBGTAG)" "$(ISODIR)$(VERBOSETAG)"
+
+# Documentation
+help::
+	@$(echo) "Bootable Images\n" \
+		"	\e[3miso\e[0m      Generates a bootable system image (File: $(ISOFILE))\n\n" \
+		"	\e[3m*-iso\e[0m    Simulate the system by booting from the virtual CD drive. (e.g. qemu-iso)\n\n" \
+		"	\e[3musb\e[0m      Generates a bootable USB mass-storage device; the environment\n" \
+		"	         variable \e[4mUSBDEV\e[0m should point to the USB device\n\n" \
+		"	\e[3mcd\e[0m       Generates a bootable CD; the environment variable \e[4mCDRWDEV\e[0m\n" \
+		"	         should point to the CD writer\n\n"
+
+# Phony targets
+.PHONY: iso cd usb help
--- a/tools/linter.mk
+++ b/tools/linter.mk
@@ -0,0 +1,30 @@
+# Perform static code checks
+
+TIDY ?= clang-tidy
+CPPLINT ?= /usr/bin/env python "$(CURRENT_DIR)/cpplint.py"
+
+# Check sources with Clang Tidy
+tidy::
+ifeq (,$(CC_SOURCES))
+	@echo "(nothing to tidy)"
+else
+	$(VERBOSE) $(TIDY) --format-style=google -header-filter=.* -warnings-as-errors="readability*" -checks="readability*,google-readability-casting,google-explicit-constructor,bugprone*,-bugprone-narrowing-conversions,-bugprone-reserved-identifier,-readability-else-after-return,-readability-magic-numbers" $(filter-out utils/png.cc,$(CC_SOURCES)) -- $(CXXFLAGS_ARCH) $(CXXFLAGS_DEFAULT) $(CXXFLAGS_OPT)
+endif
+
+# Check sources with cpplint
+lint::
+	@if $(CPPLINT) --quiet --recursive . ; then \
+		echo "Congratuations, coding style obeyed!" ; \
+	else \
+		echo "Coding style violated -- see CPPLINT.cfg for details" ; \
+		exit 1 ; \
+	fi
+
+# Documentation
+help::
+	@$(echo) "Static Analysis and Linter\n" \
+		"	\e[3mlint\e[0m     Checks the coding style using \e[4mCPPLINT\e[0m\n\n" \
+		"	\e[3mtidy\e[0m     Uses \e[4mClang Tidy\e[0m for a static code analysis\n\n"
+
+# Phony targets
+.PHONY: tidy lint help
--- a/tools/qemu.mk
+++ b/tools/qemu.mk
@@ -0,0 +1,111 @@
+# Targets for running and debugging in Qemu/KVM
+
+QEMUCPUS ?= 4
+INITRD ?= /dev/null
+
+# Switch to curses if no graphical output is available
+ifeq ($(DISPLAY),)
+	export QEMUDISPLAY    ?= curses
+else
+# Macos display
+	ifeq ($(shell uname),Darwin)
+		export QEMUDISPLAY    ?= cocoa
+	else
+		export QEMUDISPLAY    ?= gtk
+	endif
+endif
+
+export QEMUEXTRAFLAGS ?=
+
+# Architecture Specific flags
+QEMUFLAGS += -k en-us -machine pcspk-audiodev=pa -d guest_errors -m 2048
+# Macos sound
+ifeq ($(shell uname),Darwin)
+	QEMUFLAGS += -audiodev coreaudio,id=pa
+else
+	QEMUFLAGS += -audiodev pa,id=pa
+endif
+DBGKERNEL ?= $(KERNEL64)
+DBGARCH ?= i386:x86-64
+QEMU ?= qemu-system-x86_64
+QEMUKERNEL ?= -kernel $(KERNEL) -initrd $(INITRD)
+KVMFLAGS = -enable-kvm -cpu host
+QEMUDBGFLAGS = -no-shutdown -no-reboot
+
+GDB = $(PREFIX)gdb
+ifneq ($(XDG_RUNTIME_DIR),)
+	# We should prefer using domain sockets in a private directory
+	GDBPORT ?= $(XDG_RUNTIME_DIR)/stubs-gdb.sock
+else
+	# but fall back to PID + 1024, which should be an unprivileged unique port on single- and multiuser systems
+	GDBPORT ?= :$(shell echo $$(($$(id -u) + 1024)))
+endif
+ifneq ($(findstring /,$(GDBPORT)),)
+	QEMUGDB = -chardev socket,path=${GDBPORT},server=on,wait=off,id=gdb0 -gdb chardev:gdb0
+else
+	QEMUGDB = -gdb tcp:${GDBPORT}
+endif
+# (gdb itself supports either :port or a path as target)
+
+qemu: all
+	@echo "QEMU		${KERNEL}"
+	${VERBOSE} $(QEMU) $(QEMUKERNEL) $(QEMUGDB) -display ${QEMUDISPLAY} -smp $(QEMUCPUS) $(QEMUFLAGS) ${QEMUEXTRAFLAGS}
+
+# Runs StuBS in Qemu with with hardware accelerations (KVM support) enabled
+# The started emulator provides several virtual CPUs that execute in parallel.
+kvm: all
+	@echo "KVM		${KERNEL}"
+	${VERBOSE} ${QEMU}  $(QEMUKERNEL) $(QEMUGDB) -display ${QEMUDISPLAY} -smp $(QEMUCPUS) $(QEMUFLAGS) ${QEMUEXTRAFLAGS} $(KVMFLAGS)
+
+# Execute Qemu with activated GDB stub and directly connect GDB to the spawned Qemu.
+gdb: all
+	${VERBOSE} $(GDB) "$(DBGKERNEL)" \
+		-ex "set arch $(DBGARCH)" \
+		-ex "target remote | exec $(QEMU) -gdb stdio $(QEMUKERNEL) -smp $(QEMUCPUS) -S $(QEMUFLAGS) $(DBGFLAGS)"
+
+################################################################
+# Rekursive Targets: Setzen einzelner QEMU Paramter
+################################################################
+
+################################################################
+# Debugging mit GDB
+# Sinnvoll anwendbar mit den Targets: qemu, kvm, *-curses, *-serial
+%-gdb:
+	${VERBOSE} ${MAKE} QEMUEXTRAFLAGS="${QEMUEXTRAFLAGS} -S" $*
+
+# um sich mit GDB auf dem Standardport zu verbinden
+connect-gdb:
+	${VERBOSE} gdb -ex "target remote ${GDBPORT}" $(DBGKERNEL)
+
+################################################################
+# Eine -display Variante erzwingen
+%-curses:
+	${VERBOSE} ${MAKE} QEMUDISPLAY="curses" QEMUSERIAL="vc:80Cx24C" $*
+
+%-x11:
+	${VERBOSE} ${MAKE} QEMUDISPLAY=gtk $*
+
+################################################################
+# Serial-Ausgabe
+QEMUSERIAL ?= vc:80Cx24C
+QEMUFLAGS += -serial $(QEMUSERIAL)
+%-serial:
+	@echo "HINT: C-a x: Terminate QEMU; C-a c: Open QEMU console"
+	${VERBOSE} ${MAKE} QEMUSERIAL=mon:stdio $*
+
+# Help for Qemu targets
+help::
+	@$(echo) "System Emulation\n" \
+		"	\e[3mqemu\e[0m     Starts $(PROJECT) in QEMU\n" \
+		"	         Due to the internal design of QEMU, some things (especially\n" \
+		"	         race conditions) might behave different compared to hardware!\n\n" \
+		"	\e[3mkvm\e[0m      Starts $(PROJECT) in KVM, a hardware-accelerated virtual machine\n\n" \
+		"	\e[3m*-serial\e[0m Redirect the serial console to stdout (e.g., qemu-serial, kvm-serial)\n\n" \
+		"	\e[3m*-curses\e[0m   Use QEMU's curses interface (e.g., qemu-curses).\n"\
+		"	\e[3m*-x11\e[0m      Use QEMU's GTK interface (e.g., qemu-x11).\n\n"\
+		"	\e[3m*-gdb\e[0m    Start Simulator with internal GDB stub and wait for a GDB\n" \
+		"	         to attach. (e.g., qemu-gdb)\n" \
+		"	\e[3mconnect-gdb\e[0m    Connect to waiting GDB. Execute in a second terminal!\n\n"\
+
+# Phony targets
+.PHONY: qemu kvm help
--- a/tools/remote.mk
+++ b/tools/remote.mk
@@ -0,0 +1,38 @@
+# Test your system on real hardware
+
+NETBOOT_LOCAL="/ibr/adm/user-boot/"
+NETBOOT_HOST="x1.ibr.cs.tu-bs.de"
+
+# The boot menu shows pairs of `vmlinuz-*` + `initrd-*.img` with owning user and timestamp.
+# We just need to choose a name that doesn't overlap with another user's.
+netboot: all
+	$(VERBOSE) \
+	if [ ! "$$DEPLOY_LOCAL" ] && [ -e "${NETBOOT_LOCAL}" ] ; then DEPLOY_LOCAL=1 ; fi ; \
+	if [ "$$DEPLOY_LOCAL" = 1 ] ; then \
+		user=$$USER ; \
+	else \
+		user=$$( ssh -G ${NETBOOT_HOST} | grep -oPe '^user \K.*' ) ; \
+	fi ; \
+	\
+	mkdir -p $(BUILDDIR)/netboot ; \
+	ln -fT ${KERNEL} $(BUILDDIR)/netboot/vmlinuz-$$user ; \
+	initrd="$(INITRD)" ; if [ -s "$$initrd" ] ; then \
+		ln -fT $$initrd $(BUILDDIR)/netboot/initrd-$$user.img ; \
+	else \
+		echo "(none)" > $(BUILDDIR)/netboot/initrd-$$user.img ; \
+	fi ; \
+	\
+	if [ "$$DEPLOY_LOCAL" = 1 ] ; then \
+		echo "CP to locally as $$user" ; \
+		cp $(BUILDDIR)/netboot/vmlinuz-$$user $(BUILDDIR)/netboot/initrd-$$user.img ${NETBOOT_LOCAL} ; \
+	else \
+		echo "SCP to ${NETBOOT_HOST} as $$user" ; \
+		scp $(BUILDDIR)/netboot/vmlinuz-$$user $(BUILDDIR)/netboot/initrd-$$user.img ${NETBOOT_HOST}:${NETBOOT_LOCAL} ; \
+	fi
+
+# Documentation
+help::
+	@$(echo) "Netboot\n" \
+	"	\e[3mnetboot\e[0m  Copies $(PROJECT) to the network share, allowing the test infrastructure\n" \
+	"	         to boot your system"
+