clang --target=avr cannot find avr-libc

[airtower-luna]

I'm trying to compile a small (blinking LED) example with clang instead of avr-gcc on Debian, with all parts from the respective Debian packages (avr-libc, gcc-avr, clang-19). clang fails to find avr-libc when linking:

clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include   -c -o main.o main.c
main.c:18:1: warning: unused function 'wdr' [-Wunused-function]
   18 | wdr(void)
      | ^~~
1 warning generated.
clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include -Wl,-Map=main.map -v -o main.elf main.o
Debian clang version 19.1.7 (10.1)
Target: avr
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: warning: no avr-libc installation can be found on the system, cannot link standard libraries [-Wavr-rtlib-linking-quirks]
clang: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked [-Wavr-rtlib-linking-quirks]
 "/usr/bin/avr-ld" -o main.elf --gc-sections --defsym=__DATA_REGION_ORIGIN__=0x800060 -Map=main.map main.o -mavr25

Looking at

llvm-project/clang/lib/Driver/ToolChains/AVR.cpp

Lines 455 to 474 in b31199b

	llvm::Optional<std::string> AVRToolChain::findAVRLibcInstallation() const {
	// Search avr-libc installation according to avr-gcc installation.
	std::string GCCParent(GCCInstallation.getParentLibPath());
	std::string Path(GCCParent + "/avr");
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	Path = GCCParent + "/../avr";
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	// Search avr-libc installation from possible locations, and return the first
	// one that exists, if there is no avr-gcc installed.
	for (StringRef PossiblePath : PossibleAVRLibcLocations) {
	std::string Path = getDriver().SysRoot + PossiblePath.str();
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	}
	return llvm::None;
	}

I suspect part of the issue may be that the Debian package install avr-libc in /usr/lib/avr/, not within /usr/lib/gcc/avr/14.2.0/, but I don't understand the code well enough to be certain yet.

The comment #50567 (comment) indicates I'm not the only one hitting this problem.

[llvmbot]

@llvm/issue-subscribers-libc

Author: Airtower (airtower-luna)

I'm trying to compile a small (blinking LED) example with `clang` instead of `avr-gcc` on Debian, with all parts from the respective Debian packages (`avr-libc`, `gcc-avr`, `clang-19`). `clang` fails to find avr-libc when linking:

clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include   -c -o main.o main.c
main.c:18:1: warning: unused function 'wdr' [-Wunused-function]
   18 | wdr(void)
      | ^~~
1 warning generated.
clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include -Wl,-Map=main.map -v -o main.elf main.o
Debian clang version 19.1.7 (10.1)
Target: avr
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: warning: no avr-libc installation can be found on the system, cannot link standard libraries [-Wavr-rtlib-linking-quirks]
clang: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked [-Wavr-rtlib-linking-quirks]
 "/usr/bin/avr-ld" -o main.elf --gc-sections --defsym=__DATA_REGION_ORIGIN__=0x800060 -Map=main.map main.o -mavr25

Looking at

llvm-project/clang/lib/Driver/ToolChains/AVR.cpp

Lines 455 to 474 in b31199b

	llvm::Optional<std::string> AVRToolChain::findAVRLibcInstallation() const {
	// Search avr-libc installation according to avr-gcc installation.
	std::string GCCParent(GCCInstallation.getParentLibPath());
	std::string Path(GCCParent + "/avr");
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	Path = GCCParent + "/../avr";
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	// Search avr-libc installation from possible locations, and return the first
	// one that exists, if there is no avr-gcc installed.
	for (StringRef PossiblePath : PossibleAVRLibcLocations) {
	std::string Path = getDriver().SysRoot + PossiblePath.str();
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	}
	return llvm::None;
	}

I suspect part of the issue may be that the Debian package install avr-libc in /usr/lib/avr/, not within /usr/lib/gcc/avr/14.2.0/, but I don't understand the code well enough to be certain yet.

[frederick-vs-ja]

Removing libc as this doesn't seem related to LLVM libc.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Airtower (airtower-luna)

I'm trying to compile a small (blinking LED) example with `clang` instead of `avr-gcc` on Debian, with all parts from the respective Debian packages (`avr-libc`, `gcc-avr`, `clang-19`). `clang` fails to find avr-libc when linking:

clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include   -c -o main.o main.c
main.c:18:1: warning: unused function 'wdr' [-Wunused-function]
   18 | wdr(void)
      | ^~~
1 warning generated.
clang -I. -g -Os -Wall -mmcu=attiny85 -DF_CPU=16500000 -DBOOTLOADER_ADDRESS=0x1980 --target=avr --sysroot=/usr/lib/avr -I/usr/lib/avr/include -Wl,-Map=main.map -v -o main.elf main.o
Debian clang version 19.1.7 (10.1)
Target: avr
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: warning: no avr-libc installation can be found on the system, cannot link standard libraries [-Wavr-rtlib-linking-quirks]
clang: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked [-Wavr-rtlib-linking-quirks]
 "/usr/bin/avr-ld" -o main.elf --gc-sections --defsym=__DATA_REGION_ORIGIN__=0x800060 -Map=main.map main.o -mavr25

Looking at

llvm-project/clang/lib/Driver/ToolChains/AVR.cpp

Lines 455 to 474 in b31199b

	llvm::Optional<std::string> AVRToolChain::findAVRLibcInstallation() const {
	// Search avr-libc installation according to avr-gcc installation.
	std::string GCCParent(GCCInstallation.getParentLibPath());
	std::string Path(GCCParent + "/avr");
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	Path = GCCParent + "/../avr";
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	// Search avr-libc installation from possible locations, and return the first
	// one that exists, if there is no avr-gcc installed.
	for (StringRef PossiblePath : PossibleAVRLibcLocations) {
	std::string Path = getDriver().SysRoot + PossiblePath.str();
	if (llvm::sys::fs::is_directory(Path))
	return Path;
	}
	return llvm::None;
	}

I suspect part of the issue may be that the Debian package install avr-libc in /usr/lib/avr/, not within /usr/lib/gcc/avr/14.2.0/, but I don't understand the code well enough to be certain yet.

The comment #50567 (comment) indicates I'm not the only one hitting this problem.