[libc][linux] provide `sys/` headers

[RossComputerGuy]

The sys/ headers are required for LLVM libc but in a full build environment with no existing libc, the build fails due to the sys/ headers not being present. These are not provided in the Linux headers package.

Headers:

• sys/auxv.h
• sys/ioctl.h
• sys/mman.h
• sys/prctl.h
• sys/random.h
• sys/resource.h
• sys/select.h
• sys/sendfile.h
• sys/socket.h
• sys/stat.h
• sys/syscall.h
• sys/types.h
• sys/utsname.h
• sys/wait.h

[llvmbot]

@llvm/issue-subscribers-libc

Author: Tristan Ross (RossComputerGuy)

The `sys/` headers are required for LLVM libc but in a full build environment with no existing libc, the build fails due to the `sys/` headers not being present. These are not provided in the Linux headers package.

Headers:

• sys/auxv.h
• sys/ioctl.h
• sys/mman.h
• sys/prctl.h
• sys/random.h
• sys/resource.h
• sys/select.h
• sys/sendfile.h
• sys/socket.h
• sys/stat.h
• sys/syscall.h
• sys/types.h
• sys/utsname.h
• sys/wait.h

[michaelrj-google]

Most of these should already be available, for example: https://github.com/llvm/llvm-project/blob/51bceb46f8eeb7c3d060387be315ca41855933c2/libc/include/sys/auxv.h.def

When you do ninja install-libc does it generate these headers?

[RossComputerGuy]

I'm not sure since I can't build without the sys/syscall.h header at least.

[michaelrj-google]

That header is defined here: https://github.com/llvm/llvm-project/blob/51bceb46f8eeb7c3d060387be315ca41855933c2/libc/include/sys/syscall.h.def

[RossComputerGuy]

FAILED: libc/src/math/generic/CMakeFiles/libc.src.math.generic.issignalingl.dir/issignalingl.cpp.o
/nix/store/rsdq8qwhg80fnmkv1frwdplzqjbrlx68-aarch64-unknown-linux-llvm-clang-wrapper-21.0.0-unstable-2025-03-16/bin/aarch64-unknown-linux-llvm-clang++ -DLIBC_NAMESPACE=__llvm_
libc_21_0_0_git -I/build/libc-src-21.0.0-unstable-2025-03-16/libc -isystem /build/libc-src-21.0.0-unstable-2025-03-16/runtimes/build/libc/include -Werror=date-time -Werror=ung
uarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexc
ept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wno-comment -Wstring-conversion -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 -DLIBC
_QSORT_IMPL=LIBC_QSORT_QUICK_SORT -DLIBC_ADD_NULL_CHECKS -fpie -ffreestanding -DLIBC_FULL_BUILD -nostdlibinc -idirafter=/usr/include -ffixed-point -fno-builtin -fno-exceptions
 -fno-lax-vector-conversions -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-rtti -ftrivial-auto-var-init=pattern -fno-omit-frame-pointer -Wall -Wextra -Werror -Wconve
rsion -Wno-sign-conversion -Wdeprecated -Wno-c99-extensions -Wno-gnu-imaginary-constant -Wno-pedantic -Wimplicit-fallthrough -Wwrite-strings -Wextra-semi -Wnewline-eof -Wnonpo
rtable-system-include-path -Wstrict-prototypes -Wthread-safety -Wglobal-constructors -DLIBC_COPT_PUBLIC_PACKAGING -MD -MT libc/src/math/generic/CMakeFiles/libc.src.math.generi
c.issignalingl.dir/issignalingl.cpp.o -MF libc/src/math/generic/CMakeFiles/libc.src.math.generic.issignalingl.dir/issignalingl.cpp.o.d -o libc/src/math/generic/CMakeFiles/libc
.src.math.generic.issignalingl.dir/issignalingl.cpp.o -c /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/math/generic/issignalingl.cpp
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/math/generic/issignalingl.cpp:10:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/BasicOperations.h:12:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/FEnvImpl.h:24:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/aarch64/FEnvImpl.h:25:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/FPUtil/FPBits.h:21:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/libc_assert.h:25:
In file included from /build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/OSUtil/io.h:19:
/build/libc-src-21.0.0-unstable-2025-03-16/libc/src/__support/OSUtil/linux/io.h:16:10: fatal error: 'sys/syscall.h' file not found
   16 | #include <sys/syscall.h> // For syscall numbers.
      |          ^~~~~~~~~~~~~~~

[michaelrj-google]

could you share the cmake command? That build error isn't super helpful.

[RossComputerGuy]

I don't have the full CMake command unless I want to wait like a couple hours but I have the flags:

-DLLVM_LIBC_FULL_BUILD:BOOL=TRUE -DLLVM_ENABLE_RUNTIMES:STRING=libc;compiler-rt -DLLVM_INCLUDE_TESTS:BOOL=FALSE -DLLVM_LIBC_INCLUDE_SCUDO:BOOL=TRUE -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC:BOOL=TRUE -DCOMPILER_RT_BUILD_GWP_ASAN:BOOL=FALSE -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED:BOOL=FALSE -DCMAKE_C_COMPILER_WORKS:BOOL=TRUE -DCMAKE_CXX_COMPILER_WORKS:BOOL=TRUE -DCMAKE_SYSTEM_NAME=Linux -DCMAKE_SYSTEM_PROCESSOR=aarch64 -DCMAKE_HOST_SYSTEM_NAME=Linux -DCMAKE_HOST_SYSTEM_PROCESSOR=aarch64 -DCMAKE_CROSSCOMPILING_EMULATOR=env

Nothing special aside from using Ninja and building in a build directory.

[michaelrj-google]

Alright, that seems fine. Can you try applying this diff:

diff --git a/libc/src/__support/OSUtil/linux/CMakeLists.txt b/libc/src/__support/OSUtil/linux/CMakeLists.txt
index 6c7014940407..b9704d42cd33 100644
--- a/libc/src/__support/OSUtil/linux/CMakeLists.txt
+++ b/libc/src/__support/OSUtil/linux/CMakeLists.txt
@@ -22,6 +22,7 @@ add_object_library(
     libc.hdr.types.struct_flock64
     libc.hdr.types.struct_f_owner_ex
     libc.hdr.types.off_t
+    libc.include.sys_syscall
 )

Related: A bunch of the headers are implemented, just not enabled on aarch64 (see https://github.com/llvm/llvm-project/blob/51bceb46f8eeb7c3d060387be315ca41855933c2/libc/config/linux/aarch64/headers.txt)
That definitely is an issue we should fix.

[RossComputerGuy]

I can't copy that without it getting messed up through my SSH session, think you could provide it as something I can just curl?

[michaelrj-google]

if all you have is ssh it might be easier to just apply manually: Add libc.include.sys_syscall to line 24 of libc/src/__support/OSUtil/linux/CMakeLists.txt

[RossComputerGuy]

I managed to hopefully recreate the patch by hand. I need patches to change the source from within the nix sandbox.

[michaelrj-google]

(a brief explanation of why this might fix the issue: ninja/cmake do lazy evaluation of dependencies, and also may reorder build steps, but don't strictly enforce that every dependency is explicit. This means that the sys/syscall.h header generation step may have been put after building the OSUtils, causing it to fail inconsistently and only on clean builds. The cmake build is full of this sort of thing because it's easy to miss. This is also why I think Bazel is a better build system overall because it's much more aggressive about forcing explicit dependencies.)

[RossComputerGuy]

Ok, the patch works

[michaelrj-google]

excellent, I'll put up a PR fixing that issue and also adding some more headers to aarch64