[libc] Add POSIX functions pread and pwrite.

[michaelrj]

LGTM

libc/config/linux/aarch64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.dup2
     libc.src.unistd.dup3
     libc.src.unistd.fchdir
     libc.src.unistd.fsync
     libc.src.unistd.ftruncate
     libc.src.unistd.link
     libc.src.unistd.linkat
     libc.src.unistd.lseek
+    libc.src.unistd.pread
+    libc.src.unistd.pwrite
     libc.src.unistd.read
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.truncate
     libc.src.unistd.unlink

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.dup2
     libc.src.unistd.dup3
     libc.src.unistd.fchdir
     libc.src.unistd.fsync
     libc.src.unistd.ftruncate
     libc.src.unistd.link
     libc.src.unistd.linkat
     libc.src.unistd.lseek
+    libc.src.unistd.pread
+    libc.src.unistd.pwrite
     libc.src.unistd.read
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.truncate
     libc.src.unistd.unlink

libc/spec/posix.td

[
           [ArgSpec<IntType>, ArgSpec<ConstCharPtr>, ArgSpec<IntType>, ArgSpec<ConstCharPtr>, ArgSpec<IntType>]
         >,
         FunctionSpec<
           "lseek",
           RetValSpec<OffTType>,
           [ArgSpec<IntType>, ArgSpec<OffTType>, ArgSpec<IntType>]
         >,
         FunctionSpec<
+          "pread",
+          RetValSpec<SSizeTType>,
+          [ArgSpec<IntType>, ArgSpec<VoidPtr>, ArgSpec<SizeTType>, ArgSpec<OffTType>]
+        >,
+        FunctionSpec<
+          "pwrite",
+          RetValSpec<SSizeTType>,
+          [ArgSpec<IntType>, ArgSpec<ConstVoidPtr>, ArgSpec<SizeTType>, ArgSpec<OffTType>]
+        >,
+        FunctionSpec<
           "read",
           RetValSpec<SSizeTType>,
           [ArgSpec<IntType>, ArgSpec<VoidPtr>, ArgSpec<SizeTType>]
         >,
         FunctionSpec<
           "readlink",
           RetValSpec<SSizeTType>,
           [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtr>, ArgSpec<SizeTType>]

libc/src/unistd/CMakeLists.txt

 add_entrypoint_object(
   lseek
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}.lseek
 )
 
 add_entrypoint_object(
+  pread
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.pread
+)
+
+add_entrypoint_object(
+  pwrite
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.pwrite
+)
+
+add_entrypoint_object(
   read
   ALIAS
   DEPENDS
     .${LIBC_TARGET_OS}.read
 )
 
 add_entrypoint_object(
   readlink

libc/src/unistd/linux/CMakeLists.txt

add_entrypoint_object(
   DEPENDS
     libc.include.unistd
     libc.include.sys_syscall
     libc.src.__support.OSUtil.osutil
     libc.src.errno.errno
 )
 
 add_entrypoint_object(
+  pread
+  SRCS
+    pread.cpp
+  HDRS
+    ../pread.h
+  DEPENDS
+    libc.include.unistd
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
+  pwrite
+  SRCS
+    pwrite.cpp
+  HDRS
+    ../pwrite.h
+  DEPENDS
+    libc.include.unistd
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
   read
   SRCS
     read.cpp
   HDRS
     ../read.h
   DEPENDS
     libc.include.unistd
     libc.include.sys_syscall

libc/src/unistd/linux/pread.cpp

+//===-- Linux implementation of pread -------------------------------------===//
+//
+// 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/unistd/pread.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+
+#include <errno.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(ssize_t, pread,
+                   (int fd, void *buf, size_t count, off_t offset)) {
+  long ret = __llvm_libc::syscall(SYS_pread64, fd, buf, count, offset);
+  if (ret < 0) {
+    errno = -ret;
+    return -1;
+  }
+  return ret;
+}
+
+} // namespace __llvm_libc

libc/src/unistd/linux/pwrite.cpp

+//===-- Linux implementation of pwrite ------------------------------------===//
+//
+// 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/unistd/pwrite.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+
+#include <errno.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(ssize_t, pwrite,
+                   (int fd, const void *buf, size_t count, off_t offset)) {
+  long ret = __llvm_libc::syscall(SYS_pwrite64, fd, buf, count, offset);
+  if (ret < 0) {
+    errno = -ret;
+    return -1;
+  }
+  return ret;
+}
+
+} // namespace __llvm_libc

libc/src/unistd/pread.h

+//===-- Implementation header for pread -------------------------*- 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_UNISTD_PREAD_H
+#define LLVM_LIBC_SRC_UNISTD_PREAD_H
+
+#include <unistd.h>
+
+namespace __llvm_libc {
+
+ssize_t pread(int fd, void *buf, size_t count, off_t offset);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UNISTD_PREAD_H

libc/src/unistd/pwrite.h

+//===-- Implementation header for pwrite ------------------------*- 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_UNISTD_PWRITE_H
+#define LLVM_LIBC_SRC_UNISTD_PWRITE_H
+
+#include <unistd.h>
+
+namespace __llvm_libc {
+
+ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UNISTD_PWRITE_H

libc/test/src/unistd/CMakeLists.txt

DEPENDS
     libc.src.unistd.read
     libc.src.unistd.ftruncate
     libc.src.unistd.unlink
     libc.src.unistd.write
     libc.src.__support.CPP.string_view
 )
 
 add_libc_unittest(
+  pread_pwrite_test
+  SUITE
+    libc_unistd_unittests
+  SRCS
+    pread_pwrite_test.cpp
+  DEPENDS
+    libc.include.errno
+    libc.include.unistd
+    libc.src.fcntl.open
+    libc.src.unistd.close
+    libc.src.unistd.fsync
+    libc.src.unistd.pread
+    libc.src.unistd.pwrite
+    libc.src.unistd.unlink
+    libc.src.unistd.write
+    libc.test.errno_setter_matcher
+)
+
+add_libc_unittest(
   read_write_test
   SUITE
     libc_unistd_unittests
   SRCS
     read_write_test.cpp
   DEPENDS
     libc.include.errno
     libc.include.unistd

libc/test/src/unistd/pread_pwrite_test.cpp

+//===-- Unittests for pread and pwrite ------------------------------------===//
+//
+// 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/open.h"
+#include "src/unistd/close.h"
+#include "src/unistd/fsync.h"
+#include "src/unistd/pread.h"
+#include "src/unistd/pwrite.h"
+#include "src/unistd/unlink.h"
+#include "src/unistd/write.h"
+#include "test/ErrnoSetterMatcher.h"
+#include "utils/UnitTest/Test.h"
+#include "utils/testutils/FDReader.h"
+
+#include <errno.h>
+
+TEST(LlvmLibcUniStd, PWriteAndPReadBackTest) {
+  // The strategy here is that we first create a file and write to it. Next,
+  // we open that file again and write at an offset. Finally, we open the
+  // file again and pread at an offset and make sure that only expected data
+  // is being read back. This also confirms that pwrite happened successfully.
+  constexpr const char HELLO[] = "hello";
+  constexpr int HELLO_SIZE = sizeof(HELLO);
+  constexpr off_t OFFSET = 3;
+  constexpr const char OFFSET_TEXT[] = "helhello";
+  constexpr int OFFSET_TEXT_SIZE = sizeof(OFFSET_TEXT);
+
+  using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
+
+  constexpr const char *TEST_FILE = "testdata/pread_pwrite.test";
+  int fd = __llvm_libc::open(TEST_FILE, O_WRONLY | O_CREAT, S_IRWXU);
+  ASSERT_EQ(errno, 0);
+  ASSERT_GT(fd, 0);
+  ASSERT_THAT(__llvm_libc::write(fd, HELLO, HELLO_SIZE), Succeeds(HELLO_SIZE));
+  ASSERT_THAT(__llvm_libc::fsync(fd), Succeeds(0));
+  ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
+
+  fd = __llvm_libc::open(TEST_FILE, O_WRONLY);
+  ASSERT_EQ(errno, 0);
+  ASSERT_GT(fd, 0);
+  ASSERT_THAT(__llvm_libc::pwrite(fd, HELLO, HELLO_SIZE, OFFSET),
+              Succeeds(HELLO_SIZE));
+  ASSERT_THAT(__llvm_libc::fsync(fd), Succeeds(0));
+  ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
+
+  fd = __llvm_libc::open(TEST_FILE, O_RDONLY);
+  ASSERT_EQ(errno, 0);
+  ASSERT_GT(fd, 0);
+  char read_buf[OFFSET_TEXT_SIZE];
+  ASSERT_THAT(__llvm_libc::pread(fd, read_buf, HELLO_SIZE, OFFSET),
+              Succeeds(HELLO_SIZE));
+  EXPECT_STREQ(read_buf, HELLO);
+  ASSERT_THAT(__llvm_libc::pread(fd, read_buf, OFFSET_TEXT_SIZE, 0),
+              Succeeds(OFFSET_TEXT_SIZE));
+  EXPECT_STREQ(read_buf, OFFSET_TEXT);
+  ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
+
+  ASSERT_THAT(__llvm_libc::unlink(TEST_FILE), Succeeds(0));
+}
+
+TEST(LlvmLibcUniStd, PWriteFails) {
+  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
+  EXPECT_THAT(__llvm_libc::pwrite(-1, "", 1, 0), Fails(EBADF));
+}
+
+TEST(LlvmLibcUniStd, PReadFails) {
+  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
+  EXPECT_THAT(__llvm_libc::pread(-1, nullptr, 1, 0), Fails(EBADF));
+}