[libc][POSIX] Add clock_settime() function for Linux #161729

amemov · 2025-10-02T20:24:51Z

Closes #161461

This is my first time contributing to libc's POSIX, so for reference I used clock_gettime implementation for Linux. For convenience, here is the description of clock_settime function behavior

- Implemented unit-tests for the feature

amemov · 2025-10-02T20:26:03Z

Here are some questions that popped up when I started working on my PR:

When I was looking at the clock_gettime implementation, I saw that it is implemented for GPUs, but in time.rst file it is not marked as check. Is it correct or I should change that in my PR?
About testing: from my understanding to call clock_settime() and actually change the time succesfully, a caller needs to have appropriate permission. How to emulate this in tests or it can be ignored?

llvmbot · 2025-10-02T21:28:47Z

@llvm/pr-subscribers-libc

@llvm/pr-subscribers-backend-risc-v

Author: Anton Shepelev (amemov)

Changes

Closes #161461

This is my first time contributing to libc's POSIX, so for reference I used clock_gettime implementation for Linux. For convenience, here is the description of clock_settime function behavior

Full diff: https://github.com/llvm/llvm-project/pull/161729.diff

15 Files Affected:

(modified) libc/config/linux/aarch64/entrypoints.txt (+1)
(modified) libc/config/linux/riscv/entrypoints.txt (+1)
(modified) libc/config/linux/x86_64/entrypoints.txt (+1)
(modified) libc/docs/headers/time.rst (+1-1)
(modified) libc/include/time.yaml (+7)
(modified) libc/src/__support/time/CMakeLists.txt (+9)
(added) libc/src/__support/time/clock_settime.h (+22)
(modified) libc/src/__support/time/linux/CMakeLists.txt (+15)
(added) libc/src/__support/time/linux/clock_settime.cpp (+53)
(modified) libc/src/time/CMakeLists.txt (+8)
(added) libc/src/time/clock_settime.h (+22)
(modified) libc/src/time/linux/CMakeLists.txt (+13)
(added) libc/src/time/linux/clock_settime.cpp (+30)
(modified) libc/test/src/time/CMakeLists.txt (+15)
(added) libc/test/src/time/clock_settime_test.cpp (+31)

diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
index 00c4b2ec0f828..605b76718fd64 100644
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -1142,6 +1142,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
+    libc.src.time.clock_settime
     libc.src.time.difftime
     libc.src.time.gettimeofday
     libc.src.time.gmtime
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index 89e3653186d13..97cd7fdef7a07 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -1267,6 +1267,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
+    libc.src.time.clock_settime
     libc.src.time.difftime
     libc.src.time.gettimeofday
     libc.src.time.gmtime
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 0bb8a683c5b01..59357822ec537 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1305,6 +1305,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.time.ctime_r
     libc.src.time.clock
     libc.src.time.clock_gettime
+    libc.src.time.clock_settime
     libc.src.time.difftime
     libc.src.time.gettimeofday
     libc.src.time.gmtime
diff --git a/libc/docs/headers/time.rst b/libc/docs/headers/time.rst
index 55bc1a17ee285..00b0dd472de5c 100644
--- a/libc/docs/headers/time.rst
+++ b/libc/docs/headers/time.rst
@@ -71,7 +71,7 @@ Implementation Status
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+
 | clock_nanosleep     |         |         |         |                 |         |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+
-| clock_settime       |         |         |         |                 |         |         |         |         |         |         |         |         |         |
+| clock_settime       | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+
 | ctime               | |check| | |check| |         |     |check|     |         |         |         |         |         |         |         |         |         |
 +---------------------+---------+---------+---------+-----------------+---------+---------+---------+---------+---------+---------+---------+---------+---------+
diff --git a/libc/include/time.yaml b/libc/include/time.yaml
index 2f8024298fad1..7b86838a7dca2 100644
--- a/libc/include/time.yaml
+++ b/libc/include/time.yaml
@@ -67,6 +67,13 @@ functions:
     arguments:
       - type: clockid_t
       - type: struct timespec *
+  - name: clock_settime
+    standard:
+      - POSIX
+    return_type: int
+    arguments:
+      - type: clockid_t
+      - type: const struct timespec *
   - name: difftime
     standard:
       - stdc
diff --git a/libc/src/__support/time/CMakeLists.txt b/libc/src/__support/time/CMakeLists.txt
index 8247e792e8410..04744da3fc981 100644
--- a/libc/src/__support/time/CMakeLists.txt
+++ b/libc/src/__support/time/CMakeLists.txt
@@ -19,3 +19,12 @@ add_object_library(
   DEPENDS
     libc.src.__support.time.${LIBC_TARGET_OS}.clock_gettime
 )
+
+if(LIBC_TARGET_OS_IS_LINUX)
+  add_object_library(
+    clock_settime
+    ALIAS
+    DEPENDS
+      libc.src.__support.time.${LIBC_TARGET_OS}.clock_settime
+  )
+endif()
diff --git a/libc/src/__support/time/clock_settime.h b/libc/src/__support/time/clock_settime.h
new file mode 100644
index 0000000000000..d8d305cadf4b9
--- /dev/null
+++ b/libc/src/__support/time/clock_settime.h
@@ -0,0 +1,22 @@
+//===--- clock_settime linux implementation ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_TIME_CLOCK_SETTIME_H
+#define LLVM_LIBC_SRC___SUPPORT_TIME_CLOCK_SETTIME_H
+
+#include "hdr/types/clockid_t.h"
+#include "hdr/types/struct_timespec.h"
+#include "src/__support/error_or.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+ErrorOr<int> clock_settime(clockid_t clockid, const timespec *ts);
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_TIME_CLOCK_SETTIME_H
diff --git a/libc/src/__support/time/linux/CMakeLists.txt b/libc/src/__support/time/linux/CMakeLists.txt
index 6fec7eeba99ad..478529502b403 100644
--- a/libc/src/__support/time/linux/CMakeLists.txt
+++ b/libc/src/__support/time/linux/CMakeLists.txt
@@ -14,6 +14,21 @@ add_object_library(
     libc.src.__support.OSUtil.linux.vdso
 )
 
+add_object_library(
+  clock_settime
+  HDRS
+    ../clock_settime.h
+  SRCS
+    clock_settime.cpp
+  DEPENDS
+    libc.include.sys_syscall
+    libc.hdr.types.struct_timespec
+    libc.hdr.types.clockid_t
+    libc.src.__support.common
+    libc.src.__support.error_or
+    libc.src.__support.OSUtil.osutil
+)
+
 add_header_library(
   clock_conversion
   HDRS
diff --git a/libc/src/__support/time/linux/clock_settime.cpp b/libc/src/__support/time/linux/clock_settime.cpp
new file mode 100644
index 0000000000000..d81889ebc469d
--- /dev/null
+++ b/libc/src/__support/time/linux/clock_settime.cpp
@@ -0,0 +1,53 @@
+//===--- clock_settime linux implementation ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/time/clock_settime.h"
+#include "hdr/types/clockid_t.h"
+#include "hdr/types/struct_timespec.h"
+#include "src/__support/OSUtil/syscall.h"
+#include "src/__support/common.h"
+#include "src/__support/error_or.h"
+#include "src/__support/macros/config.h"
+#include <sys/syscall.h>
+
+#if defined(SYS_clock_settime64)
+#include <linux/time_types.h>
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+ErrorOr<int> clock_settime(clockid_t clockid, const timespec *ts) {
+  int ret;
+#if defined(SYS_clock_settime)
+  ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_clock_settime,
+                                          static_cast<long>(clockid),
+                                          reinterpret_cast<long>(ts));
+#elif defined(SYS_clock_settime64)
+  static_assert(
+      sizeof(time_t) == sizeof(int64_t),
+      "SYS_clock_settime64 requires struct timespec with 64-bit members.");
+
+  __kernel_timespec ts64{};
+
+  ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_clock_settime64,
+                                          static_cast<long>(clockid),
+                                          reinterpret_cast<long>(&ts64));
+  if (ret == 0) {
+    ts->tv_sec = static_cast<decltype(ts->tv_sec)>(ts64.tv_sec);
+    ts->tv_nsec = static_cast<decltype(ts->tv_nsec)>(ts64.tv_nsec);
+  }
+#else
+#error "SYS_clock_settime and SYS_clock_settime64 syscalls not available."
+#endif
+  if (ret < 0)
+    return Error(-ret);
+  return ret;
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/time/CMakeLists.txt b/libc/src/time/CMakeLists.txt
index ec942e38d1af5..4d647c22c3239 100644
--- a/libc/src/time/CMakeLists.txt
+++ b/libc/src/time/CMakeLists.txt
@@ -245,3 +245,11 @@ add_entrypoint_object(
   DEPENDS
     .${LIBC_TARGET_OS}.clock_getres
 )
+
+add_entrypoint_object(
+  clock_settime
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.clock_settime
+)
+
diff --git a/libc/src/time/clock_settime.h b/libc/src/time/clock_settime.h
new file mode 100644
index 0000000000000..9321dd1074101
--- /dev/null
+++ b/libc/src/time/clock_settime.h
@@ -0,0 +1,22 @@
+//===-- Implementation header for clock_settime function --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_TIME_CLOCK_SETTIME_H
+#define LLVM_LIBC_SRC_TIME_CLOCK_SETTIME_H
+
+#include "hdr/types/clockid_t.h"
+#include "hdr/types/struct_timespec.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+int clock_settime(clockid_t clockid, const timespec *tp);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_TIME_CLOCK_SETTIME_H
diff --git a/libc/src/time/linux/CMakeLists.txt b/libc/src/time/linux/CMakeLists.txt
index a6ec7c7c06963..6ea04597063cb 100644
--- a/libc/src/time/linux/CMakeLists.txt
+++ b/libc/src/time/linux/CMakeLists.txt
@@ -54,6 +54,19 @@ add_entrypoint_object(
     libc.src.errno.errno
 )
 
+add_entrypoint_object(
+  clock_settime
+  SRCS
+    clock_settime.cpp
+  HDRS
+    ../clock_settime.h
+  DEPENDS
+    libc.hdr.types.clockid_t
+    libc.hdr.types.struct_timespec
+    libc.src.__support.time.clock_settime
+    libc.src.errno.errno
+)
+
 add_entrypoint_object(
   gettimeofday
   SRCS
diff --git a/libc/src/time/linux/clock_settime.cpp b/libc/src/time/linux/clock_settime.cpp
new file mode 100644
index 0000000000000..1f39ca6efa02d
--- /dev/null
+++ b/libc/src/time/linux/clock_settime.cpp
@@ -0,0 +1,30 @@
+//===---------- Linux implementation of the POSIX clock_settime function --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/clock_settime.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/time/clock_settime.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, clock_settime,
+                   (clockid_t clockid, const struct timespec *ts)) {
+  auto result = internal::clock_settime(clockid, ts);
+
+  // A negative return value indicates an error with the magnitude of the
+  // value being the error code.
+  if (!result.has_value()) {
+    libc_errno = result.error();
+    return -1;
+  }
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/test/src/time/CMakeLists.txt b/libc/test/src/time/CMakeLists.txt
index 03e5428292418..c8e113f06d50b 100644
--- a/libc/test/src/time/CMakeLists.txt
+++ b/libc/test/src/time/CMakeLists.txt
@@ -124,6 +124,21 @@ add_libc_test(
     libc.src.time.clock_getres
 )
 
+add_libc_test(
+  clock_settime_test
+  SUITE
+    libc_time_unittests
+  SRCS
+    clock_settime_test.cpp
+  DEPENDS
+    libc.src.time.clock_settime
+    libc.hdr.types.time_t
+    libc.hdr.types.struct_timespec
+    libc.hdr.time_macros
+    libc.hdr.errno_macros
+    libc.test.UnitTest.ErrnoCheckingTest
+)
+
 add_libc_unittest(
   difftime_test
   SUITE
diff --git a/libc/test/src/time/clock_settime_test.cpp b/libc/test/src/time/clock_settime_test.cpp
new file mode 100644
index 0000000000000..881db1f75120d
--- /dev/null
+++ b/libc/test/src/time/clock_settime_test.cpp
@@ -0,0 +1,31 @@
+//===-- Unittests for clock_settime ---------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/time_macros.h"
+#include "hdr/types/struct_timespec.h"
+#include "src/time/clock_settime.h"
+#include "test/UnitTest/ErrnoCheckingTest.h"
+#include "test/UnitTest/Test.h"
+
+using LlvmLibcClockSetTime = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+
+#ifdef CLOCK_MONOTONIC
+TEST_F(LlvmLibcClockSetTime, MonotonicIsNotSettable) {
+  timespec ts = {0, 0};
+  int result = LIBC_NAMESPACE::clock_settime(CLOCK_MONOTONIC, &ts);
+  ASSERT_EQ(result, -1);
+  ASSERT_ERRNO_EQ(EINVAL);
+}
+#endif // CLOCK_MONOTONIC
+
+TEST_F(LlvmLibcClockSetTime, InvalidClockId) {
+  timespec ts = {0, 0};
+  int result = LIBC_NAMESPACE::clock_settime(static_cast<clockid_t>(-1), &ts);
+  ASSERT_EQ(result, -1);
+  ASSERT_ERRNO_EQ(EINVAL);
+}

amemov · 2025-10-02T21:29:47Z

@lntue

libc/src/__support/time/CMakeLists.txt

libc/src/time/linux/clock_settime.cpp

…rgets

github-actions · 2025-10-03T16:33:08Z

✅ With the latest revision this PR passed the C/C++ code formatter.

michaelrj-google

Overall looks good

michaelrj-google · 2025-10-03T17:22:16Z

libc/test/src/time/clock_settime_test.cpp

+#include "test/UnitTest/Test.h"
+
+using LlvmLibcClockSetTime = LIBC_NAMESPACE::testing::ErrnoCheckingTest;
+


there should also be a test where the syscall succeeds, we want to check both sides of the branch.

Yes, but the problem with that is that it would require the user the user to run the test as privileged user (through sudo or as root).

That's the question that I mentioned above, and I'm not exactly sure how to ensure that when some unprivileged user runs it, they would be able to run the test successfully.

amemov · 2025-10-03T20:39:56Z

libc/src/__support/time/linux/clock_settime.cpp

+  __kernel_timespec ts64{};
+
+  ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_clock_settime64,
+                                          static_cast<long>(clockid),
+                                          reinterpret_cast<long>(&ts64));
+  if (ret == 0) {
+    ts->tv_sec = static_cast<decltype(ts->tv_sec)>(ts64.tv_sec);
+    ts->tv_nsec = static_cast<decltype(ts->tv_nsec)>(ts64.tv_nsec);
+  }


Question: shouldn't we copy values from ts into ts64 before calling doing syscall? E.g. :

#elif defined(SYS_clock_settime64) static_assert( sizeof(time_t) == sizeof(int64_t), "SYS_clock_settime64 requires struct timespec with 64-bit members."); __kernel_timespec ts64{}; // Populate the 64-bit kernel structure from the user-provided timespec. ts64.tv_sec = static_cast<decltype(ts64.tv_sec)>(ts->tv_sec); ts64.tv_nsec = static_cast<decltype(ts64.tv_nsec)>(ts->tv_nsec); ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_clock_settime64, static_cast<long>(clockid), reinterpret_cast<long>(&ts64)); #else ...

I think we should do the round trip?

@SchrodingerZhu , what do you mean?

- Implemented a clock_settime syscall wrapper for Linux

2c0ef10

- Implemented unit-tests for the feature

- Fixed CMake to include the implementation only on Linux for now

06ac55c

amemov changed the title ~~[libc][POSIX] Add clock_settime() function~~ [libc][POSIX] Add clock_settime() function for Linux Oct 2, 2025

amemov marked this pull request as ready for review October 2, 2025 21:28

llvmbot added backend:RISC-V libc labels Oct 2, 2025

lntue requested review from SchrodingerZhu, lntue and michaelrj-google October 3, 2025 14:38

lntue reviewed Oct 3, 2025

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lntue reviewed Oct 3, 2025

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amemov added 2 commits October 3, 2025 09:04

- Changed CMake for clock_settime() to be more flexible for future ta…

7105bef

…rgets

- Removed redundant 'struct' keyword

366ad9a

clang-format

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		#include "test/UnitTest/Test.h"

		using LlvmLibcClockSetTime = LIBC_NAMESPACE::testing::ErrnoCheckingTest;

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