[libc] Implement fcntl() function (#89507)

Fixes #84968. 

Implements the `fcntl()` function defined in the `fcntl.h` header.

22 files changed, 545 insertions, 0 deletions

diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
index eedd9342a09f..ad50b6f59cdc 100644
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -22,6 +22,7 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # fcntl.h entrypoints
     libc.src.fcntl.creat
+    libc.src.fcntl.fcntl
     libc.src.fcntl.open
     libc.src.fcntl.openat
 
diff --git a/libc/config/linux/riscv/entrypoints.txt b/libc/config/linux/riscv/entrypoints.txt
index 4ddc1fb365e1..479af40b5b26 100644
--- a/libc/config/linux/riscv/entrypoints.txt
+++ b/libc/config/linux/riscv/entrypoints.txt
@@ -22,6 +22,7 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # fcntl.h entrypoints
     libc.src.fcntl.creat
+    libc.src.fcntl.fcntl
     libc.src.fcntl.open
     libc.src.fcntl.openat
 
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index 2576e4a92e85..5e3ddd34fb4d 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -22,6 +22,7 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # fcntl.h entrypoints
     libc.src.fcntl.creat
+    libc.src.fcntl.fcntl
     libc.src.fcntl.open
     libc.src.fcntl.openat
 
diff --git a/libc/hdr/CMakeLists.txt b/libc/hdr/CMakeLists.txt
index fb7c342f92b7..179b05e6ee96 100644
--- a/libc/hdr/CMakeLists.txt
+++ b/libc/hdr/CMakeLists.txt
@@ -33,6 +33,15 @@ add_proxy_header_library(
 )
 
 add_proxy_header_library(
+  fcntl_macros
+  HDRS
+    fcntl_macros.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-macros.fcntl_macros
+    libc.include.fcntl
+)
+
+add_proxy_header_library(
   fenv_macros
   HDRS
     fenv_macros.h
diff --git a/libc/hdr/fcntl_macros.h b/libc/hdr/fcntl_macros.h
new file mode 100644
index 000000000000..828cb984c0cb
--- /dev/null
+++ b/libc/hdr/fcntl_macros.h
@@ -0,0 +1,22 @@
+//===-- Definition of macros from fcntl/fcntl.h ---------------------------===//
+//
+// 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_HDR_FCNTL_MACROS_H
+#define LLVM_LIBC_HDR_FCNTL_MACROS_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-macros/fcntl-macros.h"
+
+#else // Overlay mode
+
+#include <fcntl.h>
+
+#endif // LLVM_LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_FCNTL_MACROS_H
diff --git a/libc/hdr/types/CMakeLists.txt b/libc/hdr/types/CMakeLists.txt
index f53766777e75..46a66ec59020 100644
--- a/libc/hdr/types/CMakeLists.txt
+++ b/libc/hdr/types/CMakeLists.txt
@@ -15,6 +15,30 @@ add_proxy_header_library(
 )
 
 add_proxy_header_library(
+  struct_flock
+  HDRS
+    struct_flock.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.struct_flock
+)
+
+add_proxy_header_library(
+  struct_flock64
+  HDRS
+    struct_flock64.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.struct_flock64
+)
+
+add_proxy_header_library(
+  struct_f_owner_ex
+  HDRS
+    struct_f_owner_ex.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.struct_f_owner_ex
+)
+
+add_proxy_header_library(
   struct_timespec
   HDRS
     struct_timespec.h
diff --git a/libc/hdr/types/struct_f_owner_ex.h b/libc/hdr/types/struct_f_owner_ex.h
new file mode 100644
index 000000000000..49985115ae4b
--- /dev/null
+++ b/libc/hdr/types/struct_f_owner_ex.h
@@ -0,0 +1,21 @@
+//===-- Proxy for struct f_owner_ex  --------------------------------------===//
+//
+// 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_HDR_TYPES_STRUCT_F_OWNER_EX_H
+#define LLVM_LIBC_HDR_TYPES_STRUCT_F_OWNER_EX_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/struct_f_owner_ex.h"
+
+#else
+
+#include <fcntl.h>
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_STRUCT_F_OWNER_EX_H
diff --git a/libc/hdr/types/struct_flock.h b/libc/hdr/types/struct_flock.h
new file mode 100644
index 000000000000..a552b91c432b
--- /dev/null
+++ b/libc/hdr/types/struct_flock.h
@@ -0,0 +1,21 @@
+//===-- Proxy for struct flock  -------------------------------------------===//
+//
+// 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_HDR_TYPES_STRUCT_FLOCK_H
+#define LLVM_LIBC_HDR_TYPES_STRUCT_FLOCK_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/struct_flock.h"
+
+#else
+
+#include <fcntl.h>
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_STRUCT_FLOCK_H
diff --git a/libc/hdr/types/struct_flock64.h b/libc/hdr/types/struct_flock64.h
new file mode 100644
index 000000000000..84fe67816c33
--- /dev/null
+++ b/libc/hdr/types/struct_flock64.h
@@ -0,0 +1,21 @@
+//===-- Proxy for struct flock64  -----------------------------------------===//
+//
+// 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_HDR_TYPES_STRUCT_FLOCK64_H
+#define LLVM_LIBC_HDR_TYPES_STRUCT_FLOCK64_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/struct_flock64.h"
+
+#else
+
+#include <fcntl.h>
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_STRUCT_FLOCK64_H
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
index 6dea8e539969..6101ec136b26 100644
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -43,6 +43,10 @@ add_gen_header(
   DEPENDS
     .llvm-libc-macros.fcntl_macros
     .llvm-libc-types.mode_t
+    .llvm-libc-types.struct_flock
+    .llvm-libc-types.struct_flock64
+    .llvm-libc-types.off64_t
+    .llvm-libc-types.pid_t
     .llvm-libc-types.off_t
     .llvm_libc_common_h
 )
diff --git a/libc/include/llvm-libc-macros/linux/fcntl-macros.h b/libc/include/llvm-libc-macros/linux/fcntl-macros.h
index 1d4e5bbbdc77..8ee95863728e 100644
--- a/libc/include/llvm-libc-macros/linux/fcntl-macros.h
+++ b/libc/include/llvm-libc-macros/linux/fcntl-macros.h
@@ -67,5 +67,36 @@
 #define F_SETFD 2
 #define F_GETFL 3
 #define F_SETFL 4
+#define F_GETLK 5
+#define F_SETLK 6
+#define F_SETLKW 7
+#define F_SETOWN 8
+#define F_GETOWN 9
+#define F_SETSIG 10
+#define F_GETSIG 11
+#define F_GETLK64 12
+#define F_SETLK64 13
+#define F_SETLKW64 14
+#define F_SETOWN_EX 15
+#define F_GETOWN_EX 16
+
+// Open File Description Locks.
+#define F_OFD_GETLK 36
+#define F_OFD_SETLK 37
+#define F_OFD_SETLKW 38
+
+// Close on succesful
+#define F_CLOEXEC 1
+
+#define F_RDLCK 0
+#define F_WRLCK 1
+#define F_UNLCK 2
+
+// For Large File Support
+#if defined(_LARGEFILE64_SOURCE)
+#define F_GETLK F_GETLK64
+#define F_SETLK F_SETLK64
+#define F_SETLKW F_SETLKW64
+#endif
 
 #endif // LLVM_LIBC_MACROS_LINUX_FCNTL_MACROS_H
diff --git a/libc/include/llvm-libc-types/CMakeLists.txt b/libc/include/llvm-libc-types/CMakeLists.txt
index 16e343d6f344..018b6c58316c 100644
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@@ -60,6 +60,9 @@ add_header(rlim_t HDR rlim_t.h)
 add_header(time_t HDR time_t.h)
 add_header(stack_t HDR stack_t.h)
 add_header(suseconds_t HDR suseconds_t.h)
+add_header(struct_flock HDR struct_flock.h DEPENDS .off_t .pid_t)
+add_header(struct_flock64 HDR struct_flock64.h DEPENDS .off64_t .pid_t)
+add_header(struct_f_owner_ex HDR struct_f_owner_ex.h DEPENDS .pid_t)
 add_header(struct_timeval HDR struct_timeval.h DEPENDS .suseconds_t .time_t)
 add_header(struct_rlimit HDR struct_rlimit.h DEPENDS .rlim_t)
 add_header(struct_rusage HDR struct_rusage.h DEPENDS .struct_timeval)
diff --git a/libc/include/llvm-libc-types/struct_f_owner_ex.h b/libc/include/llvm-libc-types/struct_f_owner_ex.h
new file mode 100644
index 000000000000..c9cc85f69d2b
--- /dev/null
+++ b/libc/include/llvm-libc-types/struct_f_owner_ex.h
@@ -0,0 +1,25 @@
+//===-- Definition of type struct f_owner_ex ------------------------------===//
+//
+// 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_TYPES_STRUCT_F_OWNER_EX_H
+#define LLVM_LIBC_TYPES_STRUCT_F_OWNER_EX_H
+
+#include "llvm-libc-types/pid_t.h"
+
+enum pid_type {
+  F_OWNER_TID = 0,
+  F_OWNER_PID,
+  F_OWNER_PGRP,
+};
+
+struct f_owner_ex {
+  enum pid_type type;
+  pid_t pid;
+};
+
+#endif // LLVM_LIBC_TYPES_STRUCT_F_OWNER_EX_H
diff --git a/libc/include/llvm-libc-types/struct_flock.h b/libc/include/llvm-libc-types/struct_flock.h
new file mode 100644
index 000000000000..51c9d27640ea
--- /dev/null
+++ b/libc/include/llvm-libc-types/struct_flock.h
@@ -0,0 +1,25 @@
+//===-- Definition of type struct flock64 ---------------------------------===//
+//
+// 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_TYPES_STRUCT_FLOCK_H
+#define LLVM_LIBC_TYPES_STRUCT_FLOCK_H
+
+#include "llvm-libc-types/off_t.h"
+#include "llvm-libc-types/pid_t.h"
+
+#include <stdint.h>
+
+struct flock {
+  int16_t l_type;
+  int16_t l_whence;
+  off_t l_start;
+  off_t l_len;
+  pid_t l_pid;
+};
+
+#endif // LLVM_LIBC_TYPES_STRUCT_FLOCK_H
diff --git a/libc/include/llvm-libc-types/struct_flock64.h b/libc/include/llvm-libc-types/struct_flock64.h
new file mode 100644
index 000000000000..ac50003ca62f
--- /dev/null
+++ b/libc/include/llvm-libc-types/struct_flock64.h
@@ -0,0 +1,25 @@
+//===-- Definition of type struct flock64 ---------------------------------===//
+//
+// 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_TYPES_STRUCT_FLOCK64_H
+#define LLVM_LIBC_TYPES_STRUCT_FLOCK64_H
+
+#include "llvm-libc-types/off64_t.h"
+#include "llvm-libc-types/pid_t.h"
+
+#include <stdint.h>
+
+struct flock64 {
+  int16_t l_type;
+  int16_t l_whence;
+  off64_t l_start;
+  off64_t l_len;
+  pid_t l_pid;
+};
+
+#endif // LLVM_LIBC_TYPES_STRUCT_FLOCK64_H
diff --git a/libc/spec/posix.td b/libc/spec/posix.td
index d428d54e32a3..e7a0cf883c60 100644
--- a/libc/spec/posix.td
+++ b/libc/spec/posix.td
@@ -231,6 +231,11 @@ def POSIX : StandardSpec<"POSIX"> {
           [ArgSpec<ConstCharPtr>, ArgSpec<ModeTType>]
       >,
       FunctionSpec<
+          "fcntl",
+          RetValSpec<IntType>,
+          [ArgSpec<IntType>, ArgSpec<IntType>, ArgSpec<VarArgType>]
+      >,
+      FunctionSpec<
           "open",
           RetValSpec<IntType>,
           [ArgSpec<ConstCharPtr>, ArgSpec<IntType>, ArgSpec<VarArgType>]
diff --git a/libc/src/fcntl/CMakeLists.txt b/libc/src/fcntl/CMakeLists.txt
index 0b9ee47c4f7c..77400e9050d0 100644
--- a/libc/src/fcntl/CMakeLists.txt
+++ b/libc/src/fcntl/CMakeLists.txt
@@ -10,6 +10,13 @@ add_entrypoint_object(
 )
 
 add_entrypoint_object(
+  fcntl
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.fcntl
+)
+
+add_entrypoint_object(
   open
   ALIAS
   DEPENDS
diff --git a/libc/src/fcntl/fcntl.h b/libc/src/fcntl/fcntl.h
new file mode 100644
index 000000000000..8fe3fb3146b9
--- /dev/null
+++ b/libc/src/fcntl/fcntl.h
@@ -0,0 +1,18 @@
+//===-- Implementation header of fcntl --------------------------*- 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_FCNTL_FCNTL_H
+#define LLVM_LIBC_SRC_FCNTL_FCNTL_H
+
+namespace LIBC_NAMESPACE {
+
+int fcntl(int fd, int cmd, ...);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_FCNTL_FCNTL_H
diff --git a/libc/src/fcntl/linux/CMakeLists.txt b/libc/src/fcntl/linux/CMakeLists.txt
index 87b8d4695c4f..732b7beac41b 100644
--- a/libc/src/fcntl/linux/CMakeLists.txt
+++ b/libc/src/fcntl/linux/CMakeLists.txt
@@ -11,6 +11,22 @@ add_entrypoint_object(
 )
 
 add_entrypoint_object(
+  fcntl
+  SRCS
+    fcntl.cpp
+  HDRS
+    ../fcntl.h
+  DEPENDS
+    libc.include.fcntl
+    libc.hdr.types.struct_flock
+    libc.hdr.types.struct_flock64
+    libc.hdr.types.struct_f_owner_ex
+    libc.hdr.fcntl_macros
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
   open
   SRCS
     open.cpp
diff --git a/libc/src/fcntl/linux/fcntl.cpp b/libc/src/fcntl/linux/fcntl.cpp
new file mode 100644
index 000000000000..24a20fb36410
--- /dev/null
+++ b/libc/src/fcntl/linux/fcntl.cpp
@@ -0,0 +1,93 @@
+//===-- Implementation of fcntl -------------------------------------------===//
+//
+// 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/fcntl/fcntl.h"
+
+#include "hdr/fcntl_macros.h"
+#include "hdr/types/struct_f_owner_ex.h"
+#include "hdr/types/struct_flock.h"
+#include "hdr/types/struct_flock64.h"
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+#include "src/errno/libc_errno.h"
+
+#include <stdarg.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+// The OFD file locks require special handling for LARGEFILES
+namespace LIBC_NAMESPACE {
+LLVM_LIBC_FUNCTION(int, fcntl, (int fd, int cmd, ...)) {
+  void *arg;
+  va_list varargs;
+  va_start(varargs, cmd);
+  arg = va_arg(varargs, void *);
+  va_end(varargs);
+
+  switch (cmd) {
+  case F_SETLKW:
+    return syscall_impl<int>(SYS_fcntl, fd, cmd, arg);
+  case F_OFD_SETLKW: {
+    struct flock *flk = reinterpret_cast<struct flock *>(arg);
+    // convert the struct to a flock64
+    struct flock64 flk64;
+    flk64.l_type = flk->l_type;
+    flk64.l_whence = flk->l_whence;
+    flk64.l_start = flk->l_start;
+    flk64.l_len = flk->l_len;
+    flk64.l_pid = flk->l_pid;
+    // create a syscall
+    return syscall_impl<int>(SYS_fcntl, fd, cmd, &flk64);
+  }
+  case F_OFD_GETLK:
+  case F_OFD_SETLK: {
+    struct flock *flk = reinterpret_cast<struct flock *>(arg);
+    // convert the struct to a flock64
+    struct flock64 flk64;
+    flk64.l_type = flk->l_type;
+    flk64.l_whence = flk->l_whence;
+    flk64.l_start = flk->l_start;
+    flk64.l_len = flk->l_len;
+    flk64.l_pid = flk->l_pid;
+    // create a syscall
+    int retVal = syscall_impl<int>(SYS_fcntl, fd, cmd, &flk64);
+    // On failure, return
+    if (retVal == -1)
+      return -1;
+    // Check for overflow, i.e. the offsets are not the same when cast
+    // to off_t from off64_t.
+    if (static_cast<off_t>(flk64.l_len) != flk64.l_len ||
+        static_cast<off_t>(flk64.l_start) != flk64.l_start) {
+      libc_errno = EOVERFLOW;
+      return -1;
+    }
+    // Now copy back into flk, in case flk64 got modified
+    flk->l_type = flk64.l_type;
+    flk->l_whence = flk64.l_whence;
+    flk->l_start = flk64.l_start;
+    flk->l_len = flk64.l_len;
+    flk->l_pid = flk64.l_pid;
+    return retVal;
+  }
+  case F_GETOWN: {
+    struct f_owner_ex fex;
+    int retVal = syscall_impl<int>(SYS_fcntl, fd, F_GETOWN_EX, &fex);
+    if (retVal == -EINVAL)
+      return syscall_impl<int>(SYS_fcntl, fd, cmd,
+                               reinterpret_cast<void *>(arg));
+    if (static_cast<unsigned long>(retVal) <= -4096UL)
+      return fex.type == F_OWNER_PGRP ? -fex.pid : fex.pid;
+
+    libc_errno = -retVal;
+    return -1;
+  }
+  // The general case
+  default:
+    return syscall_impl<int>(SYS_fcntl, fd, cmd, reinterpret_cast<void *>(arg));
+  }
+}
+} // namespace LIBC_NAMESPACE
diff --git a/libc/test/src/fcntl/CMakeLists.txt b/libc/test/src/fcntl/CMakeLists.txt
index ae39d8d5f878..aae296f074be 100644
--- a/libc/test/src/fcntl/CMakeLists.txt
+++ b/libc/test/src/fcntl/CMakeLists.txt
@@ -18,6 +18,23 @@ add_libc_unittest(
 )
 
 add_libc_unittest(
+  fcntl_test
+  SUITE
+    libc_fcntl_unittests
+  SRCS
+    fcntl_test.cpp
+  DEPENDS
+    libc.include.fcntl
+    libc.src.errno.errno
+    libc.src.fcntl.fcntl
+    libc.src.fcntl.open
+    libc.src.unistd.close
+    libc.hdr.types.struct_flock
+    libc.hdr.fcntl_macros
+    libc.test.UnitTest.ErrnoSetterMatcher
+)
+
+add_libc_unittest(
   openat_test
   SUITE
     libc_fcntl_unittests
diff --git a/libc/test/src/fcntl/fcntl_test.cpp b/libc/test/src/fcntl/fcntl_test.cpp
new file mode 100644
index 000000000000..c5cbb61b4ed8
--- /dev/null
+++ b/libc/test/src/fcntl/fcntl_test.cpp
@@ -0,0 +1,155 @@
+//===-- Unittest for fcntl ------------------------------------------------===//
+//
+// 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/fcntl_macros.h"
+#include "hdr/types/struct_flock.h"
+#include "src/errno/libc_errno.h"
+#include "src/fcntl/fcntl.h"
+#include "src/fcntl/open.h"
+#include "src/unistd/close.h"
+#include "test/UnitTest/ErrnoSetterMatcher.h"
+#include "test/UnitTest/Test.h"
+
+#include <stdio.h>
+#include <sys/stat.h> // For S_IRWXU
+
+TEST(LlvmLibcFcntlTest, FcntlDupfd) {
+  using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
+  constexpr const char *TEST_FILE_NAME = "testdata/fcntl_dup.test";
+  auto TEST_FILE = libc_make_test_file_path(TEST_FILE_NAME);
+  int fd2, fd3;
+  int fd = LIBC_NAMESPACE::open(TEST_FILE, O_CREAT | O_TRUNC, S_IRWXU);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd, 0);
+
+  fd2 = LIBC_NAMESPACE::fcntl(fd, F_DUPFD, 0);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd2, 0);
+
+  fd3 = LIBC_NAMESPACE::fcntl(fd, F_DUPFD, 10);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd3, 0);
+
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd), Succeeds(0));
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd2), Succeeds(0));
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd3), Succeeds(0));
+}
+
+TEST(LlvmLibcFcntlTest, FcntlGetFl) {
+  using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
+  constexpr const char *TEST_FILE_NAME = "testdata/fcntl_getfl.test";
+  auto TEST_FILE = libc_make_test_file_path(TEST_FILE_NAME);
+  int retVal;
+  int fd = LIBC_NAMESPACE::open(TEST_FILE, O_CREAT | O_TRUNC, S_IRWXU);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd, 0);
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_GETFL);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd), Succeeds(0));
+}
+
+TEST(LlvmLibcFcntlTest, FcntlSetFl) {
+  using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
+  constexpr const char *TEST_FILE_NAME = "testdata/fcntl_setfl.test";
+  auto TEST_FILE = libc_make_test_file_path(TEST_FILE_NAME);
+
+  int retVal;
+  int fd = LIBC_NAMESPACE::open(TEST_FILE, O_CREAT | O_TRUNC | O_RDWR, S_IRWXU);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd, 0);
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_GETFL);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  int oldFlags = LIBC_NAMESPACE::fcntl(fd, F_GETFL, 0);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(oldFlags, 0);
+
+  // Add the APPEND flag;
+  oldFlags |= O_APPEND;
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_SETFL, oldFlags);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  // Remove the APPEND flag;
+  oldFlags = -oldFlags & O_APPEND;
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_SETFL, oldFlags);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd), Succeeds(0));
+}
+
+TEST(LlvmLibcFcntlTest, FcntlGetLkRead) {
+  using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
+  constexpr const char *TEST_FILE_NAME = "testdata/fcntl_getlkread.test";
+  auto TEST_FILE = libc_make_test_file_path(TEST_FILE_NAME);
+
+  struct flock flk, svflk;
+  int retVal;
+  int fd =
+      LIBC_NAMESPACE::open(TEST_FILE, O_CREAT | O_TRUNC | O_RDONLY, S_IRWXU);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd, 0);
+
+  flk.l_type = F_RDLCK;
+  flk.l_start = 0;
+  flk.l_whence = SEEK_SET;
+  flk.l_len = 50;
+
+  // copy flk into svflk
+  svflk = flk;
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_GETLK, &svflk);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+  ASSERT_NE((int)flk.l_type, F_WRLCK); // File should not be write locked.
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_SETLK, &svflk);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd), Succeeds(0));
+}
+
+TEST(LlvmLibcFcntlTest, FcntlGetLkWrite) {
+  using LIBC_NAMESPACE::testing::ErrnoSetterMatcher::Succeeds;
+  constexpr const char *TEST_FILE_NAME = "testdata/fcntl_getlkwrite.test";
+  auto TEST_FILE = libc_make_test_file_path(TEST_FILE_NAME);
+
+  struct flock flk, svflk;
+  int retVal;
+  int fd = LIBC_NAMESPACE::open(TEST_FILE, O_CREAT | O_TRUNC | O_RDWR, S_IRWXU);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(fd, 0);
+
+  flk.l_type = F_WRLCK;
+  flk.l_start = 0;
+  flk.l_whence = SEEK_SET;
+  flk.l_len = 0;
+
+  // copy flk into svflk
+  svflk = flk;
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_GETLK, &svflk);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+  ASSERT_NE((int)flk.l_type, F_RDLCK); // File should not be read locked.
+
+  retVal = LIBC_NAMESPACE::fcntl(fd, F_SETLK, &svflk);
+  ASSERT_ERRNO_SUCCESS();
+  ASSERT_GT(retVal, -1);
+
+  ASSERT_THAT(LIBC_NAMESPACE::close(fd), Succeeds(0));
+}