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errno and libc_errno in LLVM-libc

Many C and POSIX functions report errors by setting errno. Because LLVM-libc supports multiple build and deployment modes ({doc}../full_host_build, {doc}../full_cross_build, {doc}../overlay_mode, baremetal, GPU, and header-only libc/shared/), internal code and unit tests access errno through libc_errno.

Targets Overview

errno support is split across three targets:

CMake Target	Bazel Target	Header / Source	Purpose
libc.hdr.errno_macros	:hdr_errno_macros	libc/hdr/errno_macros.h	Error constants (EINVAL, ERANGE, EDOM, ENOMEM, etc.) without libc_errno.
libc.src.__support.libc_errno	:__support_libc_errno	libc/src/__support/libc_errno.h	Declares libc_errno (either as #define libc_errno errno or as LIBC_INLINE_VAR constexpr Errno libc_errno{}).
libc.src.errno.errno	:errno	libc/src/errno/libc_errno.cpp	Defines the backing storage (thread_errno or shared_errno), extern "C" int *__llvm_libc_errno() noexcept, and (when LIBC_COPT_PROVIDE_GNU_ALIASES is enabled) the __errno_location alias.

Usage Guidelines

1. Header Inclusion

• Never #include <errno.h> in libc/src/** or unit tests (libc/test/src/**).

• Only need error constants (EINVAL, ERANGE, EDOM, etc.)? Include "hdr/errno_macros.h":

  #include "hdr/errno_macros.h"
  

• Read or write errno? Include "src/__support/libc_errno.h" (which also includes "hdr/errno_macros.h") and use libc_errno:

  #include "src/__support/libc_errno.h"
  
  libc_errno = EINVAL;
  

2. Where to Set libc_errno

• Set libc_errno only at the public entrypoint boundary (LLVM_LIBC_FUNCTION) just before returning an error, or via fputil::set_errno_if_required in math implementations.

• Internal helpers in libc/src/__support/ should not set libc_errno directly; return ErrorOr<T> or cpp::optional<T> instead and set libc_errno in the entrypoint:

  LLVM_LIBC_FUNCTION(int, close, (int fd)) {
    auto res = linux_syscalls::close(fd);
    if (!res) {
      libc_errno = res.error();
      return -1;
    }
    return 0;
  }
  

3. Build Dependencies

Target Type	Only Needs Error Constants (EINVAL, etc.)	Reads or Writes libc_errno
libc/src/__support/**	libc.hdr.errno_macros	libc.src.__support.libc_errno (never libc.src.errno.errno)
Public Entrypoints (libc/src/**)	libc.hdr.errno_macros	libc.src.__support.libc_errno (and libc.src.errno.errno for non-math libc.a entrypoints)
Tests / Fuzzers / Benchmarks	libc.hdr.errno_macros	Linked automatically (libc.src.__support.libc_errno and libc.src.errno.errno)

• While public entrypoints normally cannot depend on other entrypoints, libc.src.errno.errno is explicitly allowed in ALLOWED_DEPS in libc/cmake/modules/LLVMLibCObjectRules.cmake.

4. Testing errno

• Test rules (add_libc_test, add_fp_unittest, add_integration_test, add_libc_fuzzer, and perf_test) automatically link libc.src.errno.errno.
• Unit tests that check errno should use ErrnoCheckingTest (test/UnitTest/ErrnoCheckingTest.h) or ErrnoSetterMatcher (test/UnitTest/ErrnoSetterMatcher.h):
  • ErrnoCheckingTest resets libc_errno = 0 before each test and verifies libc_errno == 0 at the end.
  • ErrnoSetterMatcher (ASSERT_THAT(..., Fails(EINVAL)) / Succeeds()) checks and resets libc_errno automatically.
• Header-only libc/shared/* tests (libc/test/shared/*): Compiled with LIBC_ERRNO_MODE_SYSTEM_INLINE in overlay mode; check errno directly when testing error conditions.

LIBC_ERRNO_MODE Configurations

LIBC_CONF_ERRNO_MODE (passed to the compiler as -DLIBC_ERRNO_MODE=<mode>) controls how libc_errno is implemented:

Mode	libc_errno in libc_errno.h	Backing Storage in libc_errno.cpp	Typical Use Case
LIBC_ERRNO_MODE_DEFAULT	Resolves to THREAD_LOCAL or SYSTEM_INLINE (see below).	thread_errno	Default configuration.
LIBC_ERRNO_MODE_THREAD_LOCAL	Inline Errno wrapper calling *__llvm_libc_errno().	LIBC_THREAD_LOCAL int thread_errno;	Full builds; overlay unit tests.
LIBC_ERRNO_MODE_SHARED	Inline Errno wrapper calling *__llvm_libc_errno().	Global int shared_errno;	Single-threaded or thread-local-free targets (GPU, UEFI).
LIBC_ERRNO_MODE_EXTERNAL	Inline Errno wrapper calling *__llvm_libc_errno().	Provided by external embedder.	Baremetal (libc/config/baremetal/config.json).
LIBC_ERRNO_MODE_UNDEFINED	Inline no-op (operator= is a no-op, operator int() returns 0).	None (empty TU).	Minimal embedded environments without errno.
LIBC_ERRNO_MODE_SYSTEM_INLINE	#define libc_errno errno (from system <errno.h>).	None (empty TU).	Overlay public archive (libllvmlibc.a) and libc/shared/*.

How LIBC_ERRNO_MODE_DEFAULT Resolves

When LIBC_ERRNO_MODE is unset or set to LIBC_ERRNO_MODE_DEFAULT:

• Full build (LIBC_FULL_BUILD): resolves to LIBC_ERRNO_MODE_THREAD_LOCAL.
• Overlay public archive (LIBC_COPT_PUBLIC_PACKAGING): resolves to LIBC_ERRNO_MODE_SYSTEM_INLINE (libc/shared/libc_common.h and libc/test/shared/ also configure LIBC_ERRNO_MODE_SYSTEM_INLINE in non-full builds).
• Overlay tests, fuzzers, and benchmarks: resolves to LIBC_ERRNO_MODE_THREAD_LOCAL.

Why libc_errno Header and Runtime Storage Are Separated

Keeping libc.src.__support.libc_errno (header) separate from libc.src.errno.errno (runtime object) is necessary for several reasons:

1. Isolating unit tests from host errno in overlay mode: Public overlay archives (libllvmlibc.a) are built with -DLIBC_COPT_PUBLIC_PACKAGING and use the host‘s <errno.h> (LIBC_ERRNO_MODE_SYSTEM_INLINE). Unit tests are built without -DLIBC_COPT_PUBLIC_PACKAGING and use LLVM-libc’s internal thread_errno (LIBC_ERRNO_MODE_THREAD_LOCAL), preventing tested functions from clobbering the host errno used by the test harness. Conversely, if internal code or tests mistakenly #include <errno.h> and use errno directly, reads and writes will hit different thread-local variables in the same thread.
2. Header-only libc/shared/* consumption: External LLVM subprojects (such as libc++ and offload) include headers under libc/shared/, which set LIBC_ERRNO_MODE_SYSTEM_INLINE and map libc_errno to <errno.h> without compiling or linking libc_errno.cpp.
3. errno-free math and archive separation (libc.a vs. libm.a / libmvec.a): FEnvImpl.h includes src/__support/libc_errno.h to define fputil::set_errno_if_required, which is guarded by #ifndef LIBC_MATH_HAS_NO_ERRNO. Because Errno methods are inline, functions that never call set_errno_if_required (such as ceil, floor, copysign, vector math, or LIBC_MATH_NO_ERRNO builds) emit no reference to __llvm_libc_errno(), and libc.src.errno.errno stays in libc.a rather than being pulled into libm.a or libmvec.a.
4. CMake directory ordering: libc/src/CMakeLists.txt adds src/__support before src/errno because libc.src.errno.errno depends on libc.src.__support.*. Targets under libc/src/__support/ therefore cannot depend on libc.src.errno.errno.