Link error when using `getrandom`

[stevenwdv]

When trying to use getrandom from MUSL's <sys/random.h>, it fails to link with "undefined symbol: getrandom".

Note that getentropy works fine, as it seems to be defined in terms of __wasi_random_get for __EMSCRIPTEN__.

I'm not sure if getrandom is actually implemented? It just implemented as syscall_cp(SYS_getrandom, ...) at the moment. But then why is the header present?

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 6.0.2 (7a2d97d)

Failing command line in full:

getrandom.cpp

#include <array>
#include <iostream>
#include <cstddef>
#include <cstdio>
#include <sys/random.h>

int main() {
	std::array<unsigned char, 42> buf;
	if (::getrandom(buf.data(), buf.size(), 0) < 0) {
		::perror("random failed");
	}
	for (unsigned char c : buf) {
		std::cout << unsigned{c} << ' ';
	}
	std::cout << std::endl;
}

$ em++ getrandom.cpp -o getrandom -v
 "/home/swdv/emsdk/upstream/bin/clang++" -target wasm32-unknown-emscripten -fignore-exceptions -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=/home/swdv/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -v -c getrandom.cpp -o /tmp/emscripten_temp_jack687l/getrandom.o
clang version 23.0.0git (https:/github.com/llvm/llvm-project 787619a4072e0eb7887357d5d284e86c17548aed)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/swdv/emsdk/upstream/bin
 (in-process)
 "/home/swdv/emsdk/upstream/bin/clang-23" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name getrandom.cpp -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -fvisibility=hidden -debugger-tuning=gdb -fdebug-compilation-dir=/home/swdv -v -fcoverage-compilation-dir=/home/swdv -resource-dir /home/swdv/emsdk/upstream/lib/clang/23 -isysroot /home/swdv/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/noeh/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1 -internal-isystem /home/swdv/emsdk/upstream/lib/clang/23/include -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include -fdeprecated-macro -ferror-limit 19 -fmessage-length=214 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fignore-exceptions -fexceptions -fcolor-diagnostics -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_jack687l/getrandom.o -x c++ getrandom.cpp
clang -cc1 version 23.0.0git based upon LLVM 23.0.0git default target x86_64-unknown-linux-gnu
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/noeh/c++/v1"
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten/c++/v1"
ignoring nonexistent directory "/home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/compat
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include/c++/v1
 /home/swdv/emsdk/upstream/lib/clang/23/include
 /home/swdv/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
 /home/swdv/emsdk/upstream/bin/clang --version
 /home/swdv/emsdk/upstream/bin/wasm-ld -o getrandom.wasm /tmp/tmprijrpay7libemscripten_js_symbols.so -Bstatic --strip-debug --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__wasm_call_ctors --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --stack-first --table-base=1 /tmp/emscripten_temp_jack687l/getrandom.o -L/home/swdv/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -L/home/swdv/emsdk/upstream/emscripten/src/lib -lGL-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc-debug -lclang_rt.builtins -lc++-debug-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
wasm-ld: error: /tmp/emscripten_temp_jack687l/getrandom.o: undefined symbol: getrandom
em++: error: '/home/swdv/emsdk/upstream/bin/wasm-ld -o getrandom.wasm /tmp/tmprijrpay7libemscripten_js_symbols.so -Bstatic --strip-debug --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=__wasm_call_ctors --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --stack-first --table-base=1 /tmp/emscripten_temp_jack687l/getrandom.o -L/home/swdv/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -L/home/swdv/emsdk/upstream/emscripten/src/lib -lGL-getprocaddr -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc-debug -lclang_rt.builtins -lc++-debug-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr' failed (returned 1)

[kleisauke]

This file:
https://github.com/emscripten-core/emscripten/blob/6.0.2/system/lib/libc/musl/arch/emscripten/bits/syscall.h

tracks the syscalls that are implemented. SYS_getrandom is not listed there and getrandom.c is also not compiled here:

emscripten/tools/system_libs.py

Lines 1339 to 1341 in 3775844

	libc_files += files_in_path(
	path='system/lib/libc/musl/src/linux',
	filenames=['getdents.c', 'gettid.c', 'utimes.c', 'statx.c', 'wait4.c', 'wait3.c', 'epoll.c'])

I think this means we can remove the Linux-specific <sys/random.h> header, similar to what was done in commit 655ad88. We should probably also remove this line:

emscripten/system/include/compat/sys/random.h

Line 12 in 3775844

#include_next <sys/random.h>

[sbc100]

I guess that question is: Is it worth implementing the linux-specific getrandom when we already have getentropy?

I'm guessing that answer is probably, no, given that we have not had a lot of requests for in the last 15 years.

So maybe we do with @kleisauke suggestion, unless @stevenwdv you have a good reason to implement this for compat reasons?

[stevenwdv]

Not necessarily, just thought it was kind-of weird that getentropy works but getrandom doesn't, while they seem very similar. I don't really know how both came to exist, I guess one was FreeBSD among others and the other was Linux-specific, as @sbc100 mentions? I also don't really know which is most commonly used, but it does sound like getentropy is more general.