diff --git a/libc/src/__support/CMakeLists.txt b/libc/src/__support/CMakeLists.txt
--- a/libc/src/__support/CMakeLists.txt
+++ b/libc/src/__support/CMakeLists.txt
@@ -30,6 +30,14 @@
     sanitizer.h
 )
 
+add_header_library(
+  error_or
+  HDRS
+    error_or.h
+  DEPENDS
+    libc.src.__support.CPP.expected
+)
+
 add_header_library(
   ctype_utils
   HDRS
diff --git a/libc/src/__support/CPP/CMakeLists.txt b/libc/src/__support/CPP/CMakeLists.txt
--- a/libc/src/__support/CPP/CMakeLists.txt
+++ b/libc/src/__support/CPP/CMakeLists.txt
@@ -94,7 +94,7 @@
 )
 
 add_header_library(
-  error
+  expected
   HDRS
-    error.h
+    expected.h
 )
diff --git a/libc/src/__support/CPP/error.h b/libc/src/__support/CPP/error.h
deleted file mode 100644
--- a/libc/src/__support/CPP/error.h
+++ /dev/null
@@ -1,51 +0,0 @@
-//===-- A simple classes to manage error return vals ----------*- 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_CPP_ERROR_H
-#define LLVM_LIBC_SRC_SUPPORT_CPP_ERROR_H
-
-namespace __llvm_libc {
-namespace cpp {
-// Many C functions return an error val and/or the actual result of the
-// evaluation/operation performed by the function. This file defines a simple
-// convenience data structure to encapsulate the error and the actual val in
-// a single place.
-
-struct Error {
-  int error_code;
-};
-
-// This class is implemented in a simple fashion as the intention is it add
-// more generality as required. Currently, it only supports simple copyable
-// types for T.
-template <typename T> class ErrorOr {
-  bool is_error;
-
-  union {
-    T val;
-    Error error;
-  };
-
-public:
-  ErrorOr(const T &value) : is_error(false), val(value) {}
-
-  ErrorOr(const Error &error) : is_error(true), error(error) {}
-
-  operator bool() { return !is_error; }
-
-  operator T &() { return val; }
-
-  T &value() { return val; }
-
-  int error_code() { return is_error ? error.error_code : 0; }
-};
-
-} // namespace cpp
-} // namespace __llvm_libc
-
-#endif // LLVM_LIBC_SRC_SUPPORT_CPP_ERROR_H
diff --git a/libc/src/__support/CPP/expected.h b/libc/src/__support/CPP/expected.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/CPP/expected.h
@@ -0,0 +1,52 @@
+//===-- Holds an expected or unexpected value -------------------*- 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_SUPPORT_CPP_EXPECTED_H
+#define LLVM_LIBC_SUPPORT_CPP_EXPECTED_H
+
+namespace __llvm_libc::cpp {
+
+// This is used to hold an unexpected value so that a different constructor is
+// selected.
+template <class T> class unexpected {
+  T value;
+
+public:
+  constexpr explicit unexpected(T value) : value(value) {}
+  constexpr T error() { return value; }
+};
+
+template <class T, class E> class expected {
+  union {
+    T exp;
+    E unexp;
+  };
+  bool is_expected;
+
+public:
+  constexpr expected(T exp) : exp(exp), is_expected(true) {}
+  constexpr expected(unexpected<E> unexp)
+      : unexp(unexp.error()), is_expected(false) {}
+
+  constexpr bool has_value() { return is_expected; }
+
+  constexpr T value() { return exp; }
+  constexpr E error() { return unexp; }
+
+  constexpr operator T() { return exp; }
+  constexpr operator bool() { return is_expected; }
+
+  constexpr T &operator*() { return exp; }
+  constexpr const T &operator*() const { return exp; }
+  constexpr T *operator->() { return &exp; }
+  constexpr const T *operator->() const { return &exp; }
+};
+
+} // namespace __llvm_libc::cpp
+
+#endif // LLVM_LIBC_SUPPORT_CPP_EXPECTED_H
diff --git a/libc/src/__support/File/CMakeLists.txt b/libc/src/__support/File/CMakeLists.txt
--- a/libc/src/__support/File/CMakeLists.txt
+++ b/libc/src/__support/File/CMakeLists.txt
@@ -14,7 +14,7 @@
     libc.include.errno
     libc.src.__support.CPP.span
     libc.src.__support.threads.mutex
-    libc.src.errno.errno
+    libc.src.__support.error_or
 )
 
 add_object_library(
@@ -41,6 +41,7 @@
       libc.include.sys_syscall
       libc.src.__support.OSUtil.osutil
       libc.src.errno.errno
+      libc.src.__support.error_or
   )
 endif()
 
diff --git a/libc/src/__support/File/file.h b/libc/src/__support/File/file.h
--- a/libc/src/__support/File/file.h
+++ b/libc/src/__support/File/file.h
@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SRC_SUPPORT_OSUTIL_FILE_H
 #define LLVM_LIBC_SRC_SUPPORT_OSUTIL_FILE_H
 
+#include "src/__support/error_or.h"
 #include "src/__support/threads/mutex.h"
 
 #include <stddef.h>
@@ -16,6 +17,18 @@
 
 namespace __llvm_libc {
 
+struct FileIOResult {
+  size_t value;
+  int error;
+
+  constexpr FileIOResult(size_t val) : value(val), error(0) {}
+  constexpr FileIOResult(size_t val, int error) : value(val), error(error) {}
+
+  constexpr bool has_error() { return error != 0; }
+
+  constexpr operator size_t() { return value; }
+};
+
 // This a generic base class to encapsulate a platform independent file data
 // structure. Platform specific specializations should create a subclass as
 // suitable for their platform.
@@ -26,11 +39,11 @@
   using LockFunc = void(File *);
   using UnlockFunc = void(File *);
 
-  using WriteFunc = size_t(File *, const void *, size_t);
-  using ReadFunc = size_t(File *, void *, size_t);
+  using WriteFunc = FileIOResult(File *, const void *, size_t);
+  using ReadFunc = FileIOResult(File *, void *, size_t);
   // The SeekFunc is expected to return the current offset of the external
   // file position indicator.
-  using SeekFunc = long(File *, long, int);
+  using SeekFunc = ErrorOr<long>(File *, long, int);
   using CloseFunc = int(File *);
   using FlushFunc = int(File *);
 
@@ -174,26 +187,26 @@
   }
 
   // Buffered write of |len| bytes from |data| without the file lock.
-  size_t write_unlocked(const void *data, size_t len);
+  FileIOResult write_unlocked(const void *data, size_t len);
 
   // Buffered write of |len| bytes from |data| under the file lock.
-  size_t write(const void *data, size_t len) {
+  FileIOResult write(const void *data, size_t len) {
     FileLock l(this);
     return write_unlocked(data, len);
   }
 
   // Buffered read of |len| bytes into |data| without the file lock.
-  size_t read_unlocked(void *data, size_t len);
+  FileIOResult read_unlocked(void *data, size_t len);
 
   // Buffered read of |len| bytes into |data| under the file lock.
-  size_t read(void *data, size_t len) {
+  FileIOResult read(void *data, size_t len) {
     FileLock l(this);
     return read_unlocked(data, len);
   }
 
-  int seek(long offset, int whence);
+  ErrorOr<int> seek(long offset, int whence);
 
-  long tell();
+  ErrorOr<long> tell();
 
   // If buffer has data written to it, flush it out. Does nothing if the
   // buffer is currently being used as a read buffer.
@@ -256,9 +269,9 @@
   static ModeFlags mode_flags(const char *mode);
 
 private:
-  size_t write_unlocked_lbf(const uint8_t *data, size_t len);
-  size_t write_unlocked_fbf(const uint8_t *data, size_t len);
-  size_t write_unlocked_nbf(const uint8_t *data, size_t len);
+  FileIOResult write_unlocked_lbf(const uint8_t *data, size_t len);
+  FileIOResult write_unlocked_fbf(const uint8_t *data, size_t len);
+  FileIOResult write_unlocked_nbf(const uint8_t *data, size_t len);
 
   constexpr void adjust_buf() {
     if (read_allowed() && (buf == nullptr || bufsize == 0)) {
@@ -285,7 +298,7 @@
 
 // The implementaiton of this function is provided by the platfrom_file
 // library.
-File *openfile(const char *path, const char *mode);
+ErrorOr<File *> openfile(const char *path, const char *mode);
 
 // The platform_file library should implement it if it relevant for that
 // platform.
diff --git a/libc/src/__support/File/file.cpp b/libc/src/__support/File/file.cpp
--- a/libc/src/__support/File/file.cpp
+++ b/libc/src/__support/File/file.cpp
@@ -10,17 +10,16 @@
 
 #include "src/__support/CPP/span.h"
 
-#include <errno.h>
+#include <errno.h> // For error macros
 #include <stdio.h>
 #include <stdlib.h>
 
 namespace __llvm_libc {
 
-size_t File::write_unlocked(const void *data, size_t len) {
+FileIOResult File::write_unlocked(const void *data, size_t len) {
   if (!write_allowed()) {
-    errno = EBADF;
     err = true;
-    return 0;
+    return {0, EBADF};
   }
 
   prev_op = FileOp::WRITE;
@@ -37,26 +36,27 @@
   }
 }
 
-size_t File::write_unlocked_nbf(const uint8_t *data, size_t len) {
+FileIOResult File::write_unlocked_nbf(const uint8_t *data, size_t len) {
   if (pos > 0) { // If the buffer is not empty
     // Flush the buffer
     const size_t write_size = pos;
-    size_t bytes_written = platform_write(this, buf, write_size);
+    auto write_result = platform_write(this, buf, write_size);
     pos = 0; // Buffer is now empty so reset pos to the beginning.
     // If less bytes were written than expected, then an error occurred.
-    if (bytes_written < write_size) {
+    if (write_result < write_size) {
       err = true;
-      return 0; // No bytes from data were written, so return 0.
+      // No bytes from data were written, so return 0.
+      return {0, write_result.error};
     }
   }
 
-  size_t written = platform_write(this, data, len);
-  if (written < len)
+  auto write_result = platform_write(this, data, len);
+  if (write_result < len)
     err = true;
-  return written;
+  return write_result;
 }
 
-size_t File::write_unlocked_fbf(const uint8_t *data, size_t len) {
+FileIOResult File::write_unlocked_fbf(const uint8_t *data, size_t len) {
   const size_t init_pos = pos;
   const size_t bufspace = bufsize - pos;
 
@@ -96,13 +96,17 @@
   // We need to flush the buffer now, since there is still data and the buffer
   // is full.
   const size_t write_size = pos;
-  size_t bytes_written = platform_write(this, buf, write_size);
+
+  auto buf_result = platform_write(this, buf, write_size);
+  size_t bytes_written = buf_result.value;
+
   pos = 0; // Buffer is now empty so reset pos to the beginning.
   // If less bytes were written than expected, then an error occurred. Return
   // the number of bytes that have been written from |data|.
-  if (bytes_written < write_size) {
+  if (buf_result.has_error() || bytes_written < write_size) {
     err = true;
-    return bytes_written <= init_pos ? 0 : bytes_written - init_pos;
+    return {bytes_written <= init_pos ? 0 : bytes_written - init_pos,
+            buf_result.error};
   }
 
   // The second piece is handled basically the same as the first, although we
@@ -114,21 +118,22 @@
       bufref[i] = remainder[i];
     pos = remainder.size();
   } else {
-    size_t bytes_written =
-        platform_write(this, remainder.data(), remainder.size());
+
+    auto result = platform_write(this, remainder.data(), remainder.size());
+    size_t bytes_written = buf_result.value;
 
     // If less bytes were written than expected, then an error occurred. Return
     // the number of bytes that have been written from |data|.
-    if (bytes_written < remainder.size()) {
+    if (result.has_error() || bytes_written < remainder.size()) {
       err = true;
-      return primary.size() + bytes_written;
+      return {primary.size() + bytes_written, result.error};
     }
   }
 
   return len;
 }
 
-size_t File::write_unlocked_lbf(const uint8_t *data, size_t len) {
+FileIOResult File::write_unlocked_lbf(const uint8_t *data, size_t len) {
   constexpr uint8_t NEWLINE_CHAR = '\n';
   size_t last_newline = len;
   for (size_t i = len; i >= 1; --i) {
@@ -175,11 +180,10 @@
   return len;
 }
 
-size_t File::read_unlocked(void *data, size_t len) {
+FileIOResult File::read_unlocked(void *data, size_t len) {
   if (!read_allowed()) {
-    errno = EBADF;
     err = true;
-    return 0;
+    return {0, EBADF};
   }
 
   prev_op = FileOp::READ;
@@ -210,31 +214,33 @@
 
   size_t to_fetch = len - available_data;
   if (to_fetch > bufsize) {
-    size_t fetched_size = platform_read(this, dataref.data(), to_fetch);
-    if (fetched_size < to_fetch) {
-      if (errno == 0)
+    auto result = platform_read(this, dataref.data(), to_fetch);
+    size_t fetched_size = result.value;
+    if (result.has_error() || fetched_size < to_fetch) {
+      if (!result.has_error())
         eof = true;
       else
         err = true;
-      return available_data + fetched_size;
+      return {available_data + fetched_size, result.has_error()};
     }
     return len;
   }
 
   // Fetch and buffer another buffer worth of data.
-  size_t fetched_size = platform_read(this, buf, bufsize);
+  auto result = platform_read(this, buf, bufsize);
+  size_t fetched_size = result.value;
   read_limit += fetched_size;
   size_t transfer_size = fetched_size >= to_fetch ? to_fetch : fetched_size;
   for (size_t i = 0; i < transfer_size; ++i)
     dataref[i] = bufref[i];
   pos += transfer_size;
-  if (fetched_size < to_fetch) {
-    if (errno == 0)
+  if (result.has_error() || fetched_size < to_fetch) {
+    if (!result.has_error())
       eof = true;
     else
       err = true;
   }
-  return transfer_size + available_data;
+  return {transfer_size + available_data, result.error};
 }
 
 int File::ungetc_unlocked(int c) {
@@ -275,13 +281,14 @@
   return c;
 }
 
-int File::seek(long offset, int whence) {
+ErrorOr<int> File::seek(long offset, int whence) {
   FileLock lock(this);
   if (prev_op == FileOp::WRITE && pos > 0) {
-    size_t transferred_size = platform_write(this, buf, pos);
-    if (transferred_size < pos) {
+
+    auto buf_result = platform_write(this, buf, pos);
+    if (buf_result.has_error() || buf_result.value < pos) {
       err = true;
-      return -1;
+      return Error(buf_result.error);
     }
   } else if (prev_op == FileOp::READ && whence == SEEK_CUR) {
     // More data could have been read out from the platform file than was
@@ -294,22 +301,20 @@
   // Reset the eof flag as a seek might move the file positon to some place
   // readable.
   eof = false;
-  long platform_pos = platform_seek(this, offset, whence);
-  if (platform_pos >= 0)
-    return 0;
+  auto result = platform_seek(this, offset, whence);
+  if (!result.has_value())
+    return Error(result.error());
   else
-    return -1;
+    return 0;
 }
 
-long File::tell() {
+ErrorOr<long> File::tell() {
   FileLock lock(this);
-  long platform_offset;
-  if (eof)
-    platform_offset = platform_seek(this, 0, SEEK_END);
-  else
-    platform_offset = platform_seek(this, 0, SEEK_CUR);
-  if (platform_offset < 0)
-    return -1;
+  auto seek_target = eof ? SEEK_END : SEEK_CUR;
+  auto result = platform_seek(this, 0, seek_target);
+  if (!result.has_value() || result.value() < 0)
+    return Error(result.error());
+  long platform_offset = result.value();
   if (prev_op == FileOp::READ)
     return platform_offset - (read_limit - pos);
   else if (prev_op == FileOp::WRITE)
@@ -320,10 +325,10 @@
 
 int File::flush_unlocked() {
   if (prev_op == FileOp::WRITE && pos > 0) {
-    size_t transferred_size = platform_write(this, buf, pos);
-    if (transferred_size < pos) {
+    auto buf_result = platform_write(this, buf, pos);
+    if (buf_result.has_error() || buf_result.value < pos) {
       err = true;
-      return -1;
+      return buf_result.error;
     }
     pos = 0;
     return platform_flush(this);
@@ -336,14 +341,15 @@
   {
     FileLock lock(this);
     if (prev_op == FileOp::WRITE && pos > 0) {
-      size_t transferred_size = platform_write(this, buf, pos);
-      if (transferred_size < pos) {
+      auto buf_result = platform_write(this, buf, pos);
+      if (buf_result.has_error() || buf_result.value < pos) {
         err = true;
-        return -1;
+        return buf_result.error;
       }
     }
-    if (platform_close(this) != 0)
-      return -1;
+    int result = platform_close(this);
+    if (result != 0)
+      return result;
     if (own_buf)
       free(buf);
   }
diff --git a/libc/src/__support/File/linux_file.cpp b/libc/src/__support/File/linux_file.cpp
--- a/libc/src/__support/File/linux_file.cpp
+++ b/libc/src/__support/File/linux_file.cpp
@@ -10,7 +10,7 @@
 
 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
 
-#include <errno.h>
+#include <errno.h> // For error macros
 #include <fcntl.h> // For mode_t and other flags to the open syscall
 #include <stdio.h>
 #include <stdlib.h>      // For malloc
@@ -20,9 +20,9 @@
 
 namespace {
 
-size_t write_func(File *, const void *, size_t);
-size_t read_func(File *, void *, size_t);
-long seek_func(File *, long, int);
+FileIOResult write_func(File *, const void *, size_t);
+FileIOResult read_func(File *, void *, size_t);
+ErrorOr<long> seek_func(File *, long, int);
 int close_func(File *);
 int flush_func(File *);
 
@@ -51,75 +51,70 @@
 
 namespace {
 
-size_t write_func(File *f, const void *data, size_t size) {
+FileIOResult write_func(File *f, const void *data, size_t size) {
   auto *lf = reinterpret_cast<LinuxFile *>(f);
-  long ret = __llvm_libc::syscall_impl(SYS_write, lf->get_fd(), data, size);
+  int ret = __llvm_libc::syscall_impl(SYS_write, lf->get_fd(), data, size);
   if (ret < 0) {
-    errno = -ret;
-    return 0;
+    return {0, -ret};
   }
   return ret;
 }
 
-size_t read_func(File *f, void *buf, size_t size) {
+FileIOResult read_func(File *f, void *buf, size_t size) {
   auto *lf = reinterpret_cast<LinuxFile *>(f);
-  long ret = __llvm_libc::syscall_impl(SYS_read, lf->get_fd(), buf, size);
+  int ret = __llvm_libc::syscall_impl(SYS_read, lf->get_fd(), buf, size);
   if (ret < 0) {
-    errno = -ret;
-    return 0;
+    return {0, -ret};
   }
   return ret;
 }
 
-long seek_func(File *f, long offset, int whence) {
+ErrorOr<long> seek_func(File *f, long offset, int whence) {
   auto *lf = reinterpret_cast<LinuxFile *>(f);
   long result;
 #ifdef SYS_lseek
-  long ret = __llvm_libc::syscall_impl(SYS_lseek, lf->get_fd(), offset, whence);
+  int ret = __llvm_libc::syscall_impl(SYS_lseek, lf->get_fd(), offset, whence);
   result = ret;
 #elif defined(SYS__llseek)
   long result;
-  long ret = __llvm_libc::syscall_impl(SYS__llseek, lf->get_fd(), offset >> 32,
-                                       offset, &result, whence);
+  int ret = __llvm_libc::syscall_impl(SYS__llseek, lf->get_fd(), offset >> 32,
+                                      offset, &result, whence);
 #else
 #error "lseek and _llseek syscalls not available to perform a seek operation."
 #endif
 
-  if (ret < 0) {
-    errno = -ret;
-    return -1;
-  }
+  if (ret < 0)
+    return Error(-ret);
+
   return result;
 }
 
 int close_func(File *f) {
   auto *lf = reinterpret_cast<LinuxFile *>(f);
-  long ret = __llvm_libc::syscall_impl(SYS_close, lf->get_fd());
+  int ret = __llvm_libc::syscall_impl(SYS_close, lf->get_fd());
   if (ret < 0) {
-    errno = -ret;
-    return -1;
+    return -ret;
   }
   return 0;
 }
 
 int flush_func(File *f) {
   auto *lf = reinterpret_cast<LinuxFile *>(f);
-  long ret = __llvm_libc::syscall_impl(SYS_fsync, lf->get_fd());
+  int ret = __llvm_libc::syscall_impl(SYS_fsync, lf->get_fd());
   if (ret < 0) {
-    errno = -ret;
-    return -1;
+    return -ret;
   }
   return 0;
 }
 
 } // anonymous namespace
 
-File *openfile(const char *path, const char *mode) {
+ErrorOr<File *> openfile(const char *path, const char *mode) {
   using ModeFlags = File::ModeFlags;
   auto modeflags = File::mode_flags(mode);
   if (modeflags == 0) {
-    errno = EINVAL;
-    return nullptr;
+    // return {nullptr, EINVAL};
+    return Error(EINVAL);
   }
   long open_flags = 0;
   if (modeflags & ModeFlags(File::OpenMode::APPEND)) {
@@ -155,8 +150,8 @@
 #endif
 
   if (fd < 0) {
-    errno = -fd;
-    return nullptr;
+    return Error(-fd);
+    // return {nullptr, -fd};
   }
 
   void *buffer = malloc(File::DEFAULT_BUFFER_SIZE);
diff --git a/libc/src/__support/error_or.h b/libc/src/__support/error_or.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/error_or.h
@@ -0,0 +1,39 @@
+//===-- A data structure for returning an error or a value ------*- 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_SUPPORT_ERROR_OR_RESULT_H
+#define LLVM_LIBC_SUPPORT_ERROR_OR_RESULT_H
+
+#include "src/__support/CPP/expected.h"
+
+namespace __llvm_libc {
+
+template <class T> using ErrorOr = cpp::expected<T, int>;
+
+using Error = cpp::unexpected<int>;
+
+// template <typename T> struct ErrorOr {
+//   union {
+//     T value;
+//     int error;
+//   };
+//   bool is_error;
+
+//   constexpr ErrorOr(T value) : value(value), is_error(false) {}
+//   constexpr ErrorOr(int error, bool is_error)
+//       : error(error), is_error(is_error) {}
+
+//   constexpr bool has_error() { return is_error; }
+
+//   constexpr operator bool() { return is_error; }
+//   constexpr operator T() { return value; }
+// };
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SUPPORT_ERROR_OR_RESULT_H
diff --git a/libc/src/__support/threads/linux/CMakeLists.txt b/libc/src/__support/threads/linux/CMakeLists.txt
--- a/libc/src/__support/threads/linux/CMakeLists.txt
+++ b/libc/src/__support/threads/linux/CMakeLists.txt
@@ -29,7 +29,7 @@
     libc.config.linux.app_h
     libc.include.sys_syscall
     libc.src.__support.CPP.atomic
-    libc.src.__support.CPP.error
+    libc.src.__support.error_or
     libc.src.__support.CPP.stringstream
     libc.src.__support.CPP.string_view
     libc.src.__support.threads.thread_common
diff --git a/libc/src/__support/threads/linux/thread.cpp b/libc/src/__support/threads/linux/thread.cpp
--- a/libc/src/__support/threads/linux/thread.cpp
+++ b/libc/src/__support/threads/linux/thread.cpp
@@ -8,11 +8,11 @@
 
 #include "src/__support/threads/thread.h"
 #include "config/linux/app.h"
-#include "src/__support/CPP/string_view.h"
 #include "src/__support/CPP/atomic.h"
-#include "src/__support/CPP/error.h"
+#include "src/__support/CPP/string_view.h"
 #include "src/__support/CPP/stringstream.h"
-#include "src/__support/OSUtil/syscall.h"           // For syscall functions.
+#include "src/__support/OSUtil/syscall.h" // For syscall functions.
+#include "src/__support/error_or.h"
 #include "src/__support/threads/linux/futex_word.h" // For FutexWordType
 
 #ifdef LLVM_LIBC_ARCH_AARCH64
@@ -54,7 +54,7 @@
                            // wake the joining thread.
     | CLONE_SETTLS;        // Setup the thread pointer of the new thread.
 
-static inline cpp::ErrorOr<void *> alloc_stack(size_t size) {
+static inline ErrorOr<void *> alloc_stack(size_t size) {
   long mmap_result =
       __llvm_libc::syscall_impl(MMAP_SYSCALL_NUMBER,
                                 0, // No special address
@@ -65,7 +65,7 @@
                                 0   // No offset
       );
   if (mmap_result < 0 && (uintptr_t(mmap_result) >= UINTPTR_MAX - size))
-    return cpp::Error{int(-mmap_result)};
+    return Error{int(-mmap_result)};
   return reinterpret_cast<void *>(mmap_result);
 }
 
@@ -113,8 +113,7 @@
 #endif
 }
 
-__attribute__((noinline))
-static void start_thread() {
+__attribute__((noinline)) static void start_thread() {
   auto *start_args = reinterpret_cast<StartArgs *>(get_start_args_addr());
   auto *attrib = start_args->thread_attrib;
   self.attrib = attrib;
@@ -141,7 +140,7 @@
       size = DEFAULT_STACK_SIZE;
     auto alloc = alloc_stack(size);
     if (!alloc)
-      return alloc.error_code();
+      return alloc.error();
     else
       stack = alloc.value();
     owned_stack = true;
@@ -374,7 +373,7 @@
   // These callbacks could be the ones registered by the language runtimes,
   // for example, the destructors of thread local objects. They can also
   // be destructors of the TSS objects set using API like pthread_setspecific.
-  // NOTE: We cannot call the atexit callbacks as part of the 
+  // NOTE: We cannot call the atexit callbacks as part of the
   // cleanup_thread_resources function as that function can be called from a
   // different thread. The destructors of thread local and TSS objects should
   // be called by the thread which owns them.
diff --git a/libc/src/pthread/pthread_setname_np.cpp b/libc/src/pthread/pthread_setname_np.cpp
--- a/libc/src/pthread/pthread_setname_np.cpp
+++ b/libc/src/pthread/pthread_setname_np.cpp
@@ -9,7 +9,6 @@
 #include "pthread_setname_np.h"
 
 #include "src/__support/CPP/string_view.h"
-#include "src/__support/CPP/error.h"
 #include "src/__support/common.h"
 #include "src/__support/threads/thread.h"
 
diff --git a/libc/src/stdio/CMakeLists.txt b/libc/src/stdio/CMakeLists.txt
--- a/libc/src/stdio/CMakeLists.txt
+++ b/libc/src/stdio/CMakeLists.txt
@@ -25,6 +25,7 @@
     fclose.h
   DEPENDS
     libc.include.stdio
+    libc.include.errno
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
 )
@@ -108,6 +109,7 @@
   HDRS
     fgetc.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -120,6 +122,7 @@
   HDRS
     fgetc_unlocked.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -132,6 +135,7 @@
   HDRS
     getc.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -144,6 +148,7 @@
   HDRS
     getc_unlocked.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -156,6 +161,7 @@
   HDRS
     fgets.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -168,6 +174,7 @@
   HDRS
     fflush.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -204,6 +211,7 @@
   HDRS
     fread_unlocked.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -216,6 +224,7 @@
   HDRS
     fread.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -228,6 +237,7 @@
   HDRS
     fwrite_unlocked.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -240,6 +250,7 @@
   HDRS
     fwrite.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -252,6 +263,7 @@
   HDRS
     fputc.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -264,6 +276,7 @@
   HDRS
     putc.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -276,6 +289,7 @@
   HDRS
     putchar.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -288,6 +302,7 @@
   HDRS
     fputs.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -301,6 +316,7 @@
   HDRS
     puts.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -313,6 +329,7 @@
   HDRS
     fseek.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
@@ -494,6 +511,7 @@
   HDRS
     ftell.h
   DEPENDS
+    libc.include.errno
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
diff --git a/libc/src/stdio/fclose.cpp b/libc/src/stdio/fclose.cpp
--- a/libc/src/stdio/fclose.cpp
+++ b/libc/src/stdio/fclose.cpp
@@ -9,12 +9,18 @@
 #include "src/stdio/fclose.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fclose, (::FILE * stream)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->close();
+  int result = reinterpret_cast<__llvm_libc::File *>(stream)->close();
+  if (result != 0) {
+    errno = result;
+    return EOF;
+  }
+  return 0;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fflush.cpp b/libc/src/stdio/fflush.cpp
--- a/libc/src/stdio/fflush.cpp
+++ b/libc/src/stdio/fflush.cpp
@@ -9,12 +9,18 @@
 #include "src/stdio/fflush.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fflush, (::FILE * stream)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->flush();
+  int result = reinterpret_cast<__llvm_libc::File *>(stream)->flush();
+  if (result != 0) {
+    errno = result;
+    return EOF;
+  }
+  return 0;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fgetc.cpp b/libc/src/stdio/fgetc.cpp
--- a/libc/src/stdio/fgetc.cpp
+++ b/libc/src/stdio/fgetc.cpp
@@ -9,13 +9,18 @@
 #include "src/stdio/fgetc.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fgetc, (::FILE * stream)) {
   unsigned char c;
-  size_t r = reinterpret_cast<__llvm_libc::File *>(stream)->read(&c, 1);
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->read(&c, 1);
+  size_t r = result.value;
+  if (result.has_error())
+    errno = result.error;
+
   if (r != 1)
     return EOF;
   return c;
diff --git a/libc/src/stdio/fgetc_unlocked.cpp b/libc/src/stdio/fgetc_unlocked.cpp
--- a/libc/src/stdio/fgetc_unlocked.cpp
+++ b/libc/src/stdio/fgetc_unlocked.cpp
@@ -9,14 +9,18 @@
 #include "src/stdio/fgetc_unlocked.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fgetc_unlocked, (::FILE * stream)) {
   unsigned char c;
-  size_t r =
+  auto result =
       reinterpret_cast<__llvm_libc::File *>(stream)->read_unlocked(&c, 1);
+  size_t r = result.value;
+  if (result.has_error())
+    errno = result.error;
   if (r != 1)
     return EOF;
   return c;
diff --git a/libc/src/stdio/fgets.cpp b/libc/src/stdio/fgets.cpp
--- a/libc/src/stdio/fgets.cpp
+++ b/libc/src/stdio/fgets.cpp
@@ -9,6 +9,7 @@
 #include "src/stdio/fgets.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stddef.h>
 #include <stdio.h>
 
@@ -28,7 +29,11 @@
   int i = 0;
 
   for (; i < (count - 1) && c != '\n'; ++i) {
-    size_t r = stream->read_unlocked(&c, 1);
+    auto result = stream->read_unlocked(&c, 1);
+    size_t r = result.value;
+    if (result.has_error())
+      errno = result.error;
+
     if (r != 1)
       break;
     str[i] = c;
diff --git a/libc/src/stdio/fopen.cpp b/libc/src/stdio/fopen.cpp
--- a/libc/src/stdio/fopen.cpp
+++ b/libc/src/stdio/fopen.cpp
@@ -9,13 +9,19 @@
 #include "src/stdio/fopen.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(::FILE *, fopen,
                    (const char *__restrict name, const char *__restrict mode)) {
-  return reinterpret_cast<::FILE *>(__llvm_libc::openfile(name, mode));
+  auto result = __llvm_libc::openfile(name, mode);
+  if (!result.has_value()) {
+    errno = result.error();
+    return nullptr;
+  }
+  return reinterpret_cast<::FILE *>(result.value());
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fopencookie.cpp b/libc/src/stdio/fopencookie.cpp
--- a/libc/src/stdio/fopencookie.cpp
+++ b/libc/src/stdio/fopencookie.cpp
@@ -23,7 +23,7 @@
   cookie_io_functions_t ops;
 };
 
-size_t write_func(File *f, const void *data, size_t size) {
+FileIOResult write_func(File *f, const void *data, size_t size) {
   auto cookie_file = reinterpret_cast<CookieFile *>(f);
   if (cookie_file->ops.write == nullptr)
     return 0;
@@ -31,7 +31,7 @@
                                 reinterpret_cast<const char *>(data), size);
 }
 
-size_t read_func(File *f, void *data, size_t size) {
+FileIOResult read_func(File *f, void *data, size_t size) {
   auto cookie_file = reinterpret_cast<CookieFile *>(f);
   if (cookie_file->ops.read == nullptr)
     return 0;
@@ -39,11 +39,10 @@
                                reinterpret_cast<char *>(data), size);
 }
 
-long seek_func(File *f, long offset, int whence) {
+ErrorOr<long> seek_func(File *f, long offset, int whence) {
   auto cookie_file = reinterpret_cast<CookieFile *>(f);
   if (cookie_file->ops.seek == nullptr) {
-    errno = EINVAL;
-    return -1;
+    return Error(EINVAL);
   }
   off64_t offset64 = offset;
   int result = cookie_file->ops.seek(cookie_file->cookie, &offset64, whence);
diff --git a/libc/src/stdio/fputc.cpp b/libc/src/stdio/fputc.cpp
--- a/libc/src/stdio/fputc.cpp
+++ b/libc/src/stdio/fputc.cpp
@@ -9,13 +9,19 @@
 #include "src/stdio/fputc.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fputc, (int c, ::FILE *stream)) {
   unsigned char uc = static_cast<unsigned char>(c);
-  size_t written = reinterpret_cast<__llvm_libc::File *>(stream)->write(&uc, 1);
+
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->write(&uc, 1);
+  if (result.has_error())
+    errno = result.error;
+  size_t written = result.value;
+
   if (1 != written) {
     // The stream should be in an error state in this case.
     return EOF;
diff --git a/libc/src/stdio/fputs.cpp b/libc/src/stdio/fputs.cpp
--- a/libc/src/stdio/fputs.cpp
+++ b/libc/src/stdio/fputs.cpp
@@ -10,6 +10,7 @@
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
@@ -18,8 +19,12 @@
                    (const char *__restrict str, ::FILE *__restrict stream)) {
   cpp::string_view str_view(str);
 
-  size_t written = reinterpret_cast<__llvm_libc::File *>(stream)->write(
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->write(
       str, str_view.size());
+  if (result.has_error())
+    errno = result.error;
+  size_t written = result.value;
+
   if (str_view.size() != written) {
     // The stream should be in an error state in this case.
     return EOF;
diff --git a/libc/src/stdio/fread.cpp b/libc/src/stdio/fread.cpp
--- a/libc/src/stdio/fread.cpp
+++ b/libc/src/stdio/fread.cpp
@@ -9,6 +9,7 @@
 #include "src/stdio/fread.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
@@ -18,9 +19,11 @@
                     ::FILE *stream)) {
   if (size == 0 || nmemb == 0)
     return 0;
-  return reinterpret_cast<__llvm_libc::File *>(stream)->read(buffer,
-                                                             size * nmemb) /
-         size;
+  auto result =
+      reinterpret_cast<__llvm_libc::File *>(stream)->read(buffer, size * nmemb);
+  if (result.has_error())
+    errno = result.error;
+  return result.value / size;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fread_unlocked.cpp b/libc/src/stdio/fread_unlocked.cpp
--- a/libc/src/stdio/fread_unlocked.cpp
+++ b/libc/src/stdio/fread_unlocked.cpp
@@ -9,6 +9,7 @@
 #include "src/stdio/fread_unlocked.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
@@ -16,8 +17,13 @@
 LLVM_LIBC_FUNCTION(size_t, fread_unlocked,
                    (void *__restrict buffer, size_t size, size_t nmemb,
                     ::FILE *stream)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->read_unlocked(
+  if (size == 0 || nmemb == 0)
+    return 0;
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->read_unlocked(
       buffer, size * nmemb);
+  if (result.has_error())
+    errno = result.error;
+  return result.value / size;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fseek.cpp b/libc/src/stdio/fseek.cpp
--- a/libc/src/stdio/fseek.cpp
+++ b/libc/src/stdio/fseek.cpp
@@ -9,12 +9,19 @@
 #include "src/stdio/fseek.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, fseek, (::FILE * stream, long offset, int whence)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->seek(offset, whence);
+  auto result =
+      reinterpret_cast<__llvm_libc::File *>(stream)->seek(offset, whence);
+  if (!result.has_value()) {
+    errno = result.error();
+    return -1;
+  }
+  return 0;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/ftell.cpp b/libc/src/stdio/ftell.cpp
--- a/libc/src/stdio/ftell.cpp
+++ b/libc/src/stdio/ftell.cpp
@@ -9,12 +9,18 @@
 #include "src/stdio/ftell.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(long, ftell, (::FILE * stream)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->tell();
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->tell();
+  if (!result.has_value()) {
+    errno = result.error();
+    return -1;
+  }
+  return result.value();
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fwrite.cpp b/libc/src/stdio/fwrite.cpp
--- a/libc/src/stdio/fwrite.cpp
+++ b/libc/src/stdio/fwrite.cpp
@@ -9,6 +9,7 @@
 #include "src/stdio/fwrite.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
@@ -18,9 +19,12 @@
                     ::FILE *stream)) {
   if (size == 0 || nmemb == 0)
     return 0;
-  return reinterpret_cast<__llvm_libc::File *>(stream)->write(buffer,
-                                                              size * nmemb) /
-         size;
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->write(
+      buffer, size * nmemb);
+  if (result.has_error())
+    errno = result.error;
+
+  return result.value / size;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/fwrite_unlocked.cpp b/libc/src/stdio/fwrite_unlocked.cpp
--- a/libc/src/stdio/fwrite_unlocked.cpp
+++ b/libc/src/stdio/fwrite_unlocked.cpp
@@ -9,6 +9,7 @@
 #include "src/stdio/fwrite_unlocked.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
@@ -16,8 +17,15 @@
 LLVM_LIBC_FUNCTION(size_t, fwrite_unlocked,
                    (const void *__restrict buffer, size_t size, size_t nmemb,
                     ::FILE *stream)) {
-  return reinterpret_cast<__llvm_libc::File *>(stream)->write_unlocked(
+
+  if (size == 0 || nmemb == 0)
+    return 0;
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->write_unlocked(
       buffer, size * nmemb);
+  if (result.has_error())
+    errno = result.error;
+
+  return result.value / size;
 }
 
 } // namespace __llvm_libc
diff --git a/libc/src/stdio/getc.cpp b/libc/src/stdio/getc.cpp
--- a/libc/src/stdio/getc.cpp
+++ b/libc/src/stdio/getc.cpp
@@ -9,13 +9,18 @@
 #include "src/stdio/getc.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, getc, (::FILE * stream)) {
   unsigned char c;
-  size_t r = reinterpret_cast<__llvm_libc::File *>(stream)->read(&c, 1);
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->read(&c, 1);
+  size_t r = result.value;
+  if (result.has_error())
+    errno = result.error;
+
   if (r != 1)
     return EOF;
   return c;
diff --git a/libc/src/stdio/getc_unlocked.cpp b/libc/src/stdio/getc_unlocked.cpp
--- a/libc/src/stdio/getc_unlocked.cpp
+++ b/libc/src/stdio/getc_unlocked.cpp
@@ -9,14 +9,19 @@
 #include "src/stdio/getc_unlocked.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, getc_unlocked, (::FILE * stream)) {
   unsigned char c;
-  size_t r =
+  auto result =
       reinterpret_cast<__llvm_libc::File *>(stream)->read_unlocked(&c, 1);
+  size_t r = result.value;
+  if (result.has_error())
+    errno = result.error;
+
   if (r != 1)
     return EOF;
   return c;
diff --git a/libc/src/stdio/printf_core/file_writer.cpp b/libc/src/stdio/printf_core/file_writer.cpp
--- a/libc/src/stdio/printf_core/file_writer.cpp
+++ b/libc/src/stdio/printf_core/file_writer.cpp
@@ -16,8 +16,9 @@
 namespace printf_core {
 
 int FileWriter::write(const char *__restrict to_write, size_t len) {
-  int written = file->write_unlocked(to_write, len);
-  if (written != static_cast<int>(len))
+  auto result = file->write_unlocked(to_write, len);
+  int written = result.value;
+  if (written != static_cast<int>(len) || result.has_error())
     written = FILE_WRITE_ERROR;
   if (file->error_unlocked())
     written = FILE_STATUS_ERROR;
diff --git a/libc/src/stdio/putc.cpp b/libc/src/stdio/putc.cpp
--- a/libc/src/stdio/putc.cpp
+++ b/libc/src/stdio/putc.cpp
@@ -9,13 +9,19 @@
 #include "src/stdio/putc.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, putc, (int c, ::FILE *stream)) {
   unsigned char uc = static_cast<unsigned char>(c);
-  size_t written = reinterpret_cast<__llvm_libc::File *>(stream)->write(&uc, 1);
+
+  auto result = reinterpret_cast<__llvm_libc::File *>(stream)->write(&uc, 1);
+  if (result.has_error())
+    errno = result.error;
+  size_t written = result.value;
+
   if (1 != written) {
     // The stream should be in an error state in this case.
     return EOF;
diff --git a/libc/src/stdio/putchar.cpp b/libc/src/stdio/putchar.cpp
--- a/libc/src/stdio/putchar.cpp
+++ b/libc/src/stdio/putchar.cpp
@@ -9,13 +9,19 @@
 #include "src/stdio/putchar.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, putchar, (int c)) {
   unsigned char uc = static_cast<unsigned char>(c);
-  size_t written = __llvm_libc::stdout->write(&uc, 1);
+
+  auto result = __llvm_libc::stdout->write(&uc, 1);
+  if (result.has_error())
+    errno = result.error;
+  size_t written = result.value;
+
   if (1 != written) {
     // The stream should be in an error state in this case.
     return EOF;
diff --git a/libc/src/stdio/puts.cpp b/libc/src/stdio/puts.cpp
--- a/libc/src/stdio/puts.cpp
+++ b/libc/src/stdio/puts.cpp
@@ -10,18 +10,25 @@
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/File/file.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 namespace __llvm_libc {
 
 LLVM_LIBC_FUNCTION(int, puts, (const char *__restrict str)) {
   cpp::string_view str_view(str);
-  size_t written = __llvm_libc::stdout->write(str, str_view.size());
+  auto result = __llvm_libc::stdout->write(str, str_view.size());
+  if (result.has_error())
+    errno = result.error;
+  size_t written = result.value;
   if (str_view.size() != written) {
     // The stream should be in an error state in this case.
     return EOF;
   }
-  written = __llvm_libc::stdout->write("\n", 1);
+  result = __llvm_libc::stdout->write("\n", 1);
+  if (result.has_error())
+    errno = result.error;
+  written = result.value;
   if (1 != written) {
     // The stream should be in an error state in this case.
     return EOF;
diff --git a/libc/src/stdio/scanf_core/file_reader.cpp b/libc/src/stdio/scanf_core/file_reader.cpp
--- a/libc/src/stdio/scanf_core/file_reader.cpp
+++ b/libc/src/stdio/scanf_core/file_reader.cpp
@@ -15,7 +15,8 @@
 
 char FileReader::get_char() {
   char tiny_buff = 0;
-  if (file->read_unlocked(&tiny_buff, 1) != 1)
+  auto result = file->read_unlocked(&tiny_buff, 1);
+  if (result.value != 1 || result.has_error())
     return 0;
   return tiny_buff;
 }
diff --git a/libc/test/src/__support/File/file_test.cpp b/libc/test/src/__support/File/file_test.cpp
--- a/libc/test/src/__support/File/file_test.cpp
+++ b/libc/test/src/__support/File/file_test.cpp
@@ -7,15 +7,17 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/__support/File/file.h"
+#include "src/__support/error_or.h"
 #include "utils/UnitTest/MemoryMatcher.h"
 #include "utils/UnitTest/Test.h"
 
-#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 
 using ModeFlags = __llvm_libc::File::ModeFlags;
 using MemoryView = __llvm_libc::memory::testing::MemoryView;
+using __llvm_libc::ErrorOr;
+using __llvm_libc::FileIOResult;
 
 class StringFile : public __llvm_libc::File {
   static constexpr size_t SIZE = 512;
@@ -24,9 +26,10 @@
   size_t eof_marker;
   bool write_append;
 
-  static size_t str_read(__llvm_libc::File *f, void *data, size_t len);
-  static size_t str_write(__llvm_libc::File *f, const void *data, size_t len);
-  static long str_seek(__llvm_libc::File *f, long offset, int whence);
+  static FileIOResult str_read(__llvm_libc::File *f, void *data, size_t len);
+  static FileIOResult str_write(__llvm_libc::File *f, const void *data,
+                                size_t len);
+  static ErrorOr<long> str_seek(__llvm_libc::File *f, long offset, int whence);
   static int str_close(__llvm_libc::File *f) { return 0; }
   static int str_flush(__llvm_libc::File *f) { return 0; }
 
@@ -67,7 +70,8 @@
   }
 };
 
-size_t StringFile::str_read(__llvm_libc::File *f, void *data, size_t len) {
+FileIOResult StringFile::str_read(__llvm_libc::File *f, void *data,
+                                  size_t len) {
   StringFile *sf = static_cast<StringFile *>(f);
   if (sf->pos >= sf->eof_marker)
     return 0;
@@ -78,8 +82,8 @@
   return i;
 }
 
-size_t StringFile::str_write(__llvm_libc::File *f, const void *data,
-                             size_t len) {
+FileIOResult StringFile::str_write(__llvm_libc::File *f, const void *data,
+                                   size_t len) {
   StringFile *sf = static_cast<StringFile *>(f);
   if (sf->write_append)
     sf->pos = sf->eof_marker;
@@ -94,7 +98,8 @@
   return i;
 }
 
-long StringFile::str_seek(__llvm_libc::File *f, long offset, int whence) {
+ErrorOr<long> StringFile::str_seek(__llvm_libc::File *f, long offset,
+                                   int whence) {
   StringFile *sf = static_cast<StringFile *>(f);
   if (whence == SEEK_SET)
     sf->pos = offset;
@@ -119,7 +124,7 @@
   StringFile *f =
       new_string_file(file_buffer, FILE_BUFFER_SIZE, _IOFBF, false, "w");
 
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_EQ(f->get_pos(), size_t(0)); // Data is buffered in the file stream
   ASSERT_EQ(f->flush(), 0);
   EXPECT_EQ(f->get_pos(), sizeof(data)); // Data should now be available
@@ -127,10 +132,10 @@
 
   f->reset();
   ASSERT_EQ(f->get_pos(), size_t(0));
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_EQ(f->get_pos(), size_t(0)); // Data is buffered in the file stream
   // The second write should trigger a buffer flush.
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_GE(f->get_pos(), size_t(0));
   ASSERT_EQ(f->flush(), 0);
   EXPECT_EQ(f->get_pos(), 2 * sizeof(data));
@@ -139,11 +144,13 @@
   EXPECT_MEM_EQ(src1, dst1);
 
   char read_data[sizeof(data)];
-  // This is not a readable file.
-  EXPECT_EQ(f->read(read_data, sizeof(data)), size_t(0));
-  EXPECT_TRUE(f->error());
-  EXPECT_NE(errno, 0);
-  errno = 0;
+  {
+    // This is not a readable file.
+    auto result = f->read(read_data, sizeof(data));
+    EXPECT_EQ(result.value, size_t(0));
+    EXPECT_TRUE(f->error());
+    EXPECT_TRUE(result.has_error());
+  }
 
   ASSERT_EQ(f->close(), 0);
 }
@@ -162,8 +169,8 @@
   StringFile *f_full =
       new_string_file(file_buffer_full, FILE_BUFFER_SIZE, _IOFBF, false, "w");
 
-  ASSERT_EQ(sizeof(data), f_line->write(data, sizeof(data)));
-  ASSERT_EQ(sizeof(data), f_full->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f_line->write(data, sizeof(data)).value);
+  ASSERT_EQ(sizeof(data), f_full->write(data, sizeof(data)).value);
 
   EXPECT_EQ(f_line->get_pos(), size_t(6)); // buffer after the newline
   EXPECT_EQ(f_full->get_pos(), size_t(0)); // buffer all of data
@@ -175,12 +182,12 @@
 
   const char data2[] = "longer for an \n overflow";
 
-  ASSERT_EQ(sizeof(data2), f_line->write(data2, sizeof(data2)));
+  ASSERT_EQ(sizeof(data2), f_line->write(data2, sizeof(data2)).value);
   // The line buffer's initial contents should be " file\0"
   // Writing data2 should write up until the newline, even though that doesn't
   // all fit in the buffer.
 
-  ASSERT_EQ(sizeof(data2), f_full->write(data2, sizeof(data2)));
+  ASSERT_EQ(sizeof(data2), f_full->write(data2, sizeof(data2)).value);
   // The full buffer's initial contents should be "hello\n file\0"
   // Writing data2 should cause a flush of the buffer, as well as the remainder
   // to be written directly since it doesn't fit in the buffer.
@@ -215,7 +222,7 @@
   StringFile *f =
       new_string_file(file_buffer, FILE_BUFFER_SIZE, _IONBF, false, "w");
 
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_EQ(f->get_pos(),
             sizeof(data)); // no buffering means this is written immediately.
   EXPECT_STREQ(f->get_str(), data);
@@ -233,7 +240,7 @@
 
   constexpr size_t READ_SIZE = sizeof(initial_content) / 2;
   char read_data[READ_SIZE];
-  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE).value);
   EXPECT_FALSE(f->iseof());
   // Reading less than file buffer worth will still read one
   // full buffer worth of data.
@@ -246,14 +253,14 @@
 
   // Reading another buffer worth should read out everything in
   // the file.
-  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE).value);
   EXPECT_FALSE(f->iseof());
   MemoryView src2(initial_content + READ_SIZE, READ_SIZE),
       dst2(read_data, READ_SIZE);
   EXPECT_MEM_EQ(src2, dst2);
 
   // Another read should trigger an EOF.
-  ASSERT_GT(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_GT(READ_SIZE, f->read(read_data, READ_SIZE).value);
   EXPECT_TRUE(f->iseof());
 
   // Reset the pos to the beginning of the file which should allow
@@ -261,15 +268,17 @@
   for (size_t i = 0; i < READ_SIZE; ++i)
     read_data[i] = 0;
   f->seek(0, SEEK_SET);
-  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE).value);
   MemoryView src3(initial_content, READ_SIZE), dst3(read_data, READ_SIZE);
   EXPECT_MEM_EQ(src3, dst3);
 
-  // This is not a writable file.
-  EXPECT_EQ(f->write(initial_content, sizeof(initial_content)), size_t(0));
-  EXPECT_TRUE(f->error());
-  EXPECT_NE(errno, 0);
-  errno = 0;
+  {
+    // This is not a writable file.
+    auto result = f->write(initial_content, sizeof(initial_content));
+    EXPECT_EQ(result.value, size_t(0));
+    EXPECT_TRUE(f->error());
+    EXPECT_TRUE(result.has_error());
+  }
 
   ASSERT_EQ(f->close(), 0);
 }
@@ -285,13 +294,13 @@
   constexpr size_t READ_SIZE = 5;
   char data[READ_SIZE];
   data[READ_SIZE - 1] = '\0';
-  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(f->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "1234");
-  ASSERT_EQ(f->seek(5, SEEK_CUR), 0);
-  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(f->seek(5, SEEK_CUR).value(), 0);
+  ASSERT_EQ(f->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "0987");
-  ASSERT_EQ(f->seek(-5, SEEK_CUR), 0);
-  ASSERT_EQ(f->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(f->seek(-5, SEEK_CUR).value(), 0);
+  ASSERT_EQ(f->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "9098");
   ASSERT_EQ(f->close(), 0);
 }
@@ -307,14 +316,17 @@
 
   constexpr size_t READ_SIZE = 5;
   char read_data[READ_SIZE];
-  // This is not a readable file.
-  ASSERT_EQ(f->read(read_data, READ_SIZE), size_t(0));
-  EXPECT_TRUE(f->error());
-  EXPECT_NE(errno, 0);
-  errno = 0;
+
+  {
+    // This is not a readable file.
+    auto result = f->read(read_data, READ_SIZE);
+    EXPECT_EQ(result.value, size_t(0));
+    EXPECT_TRUE(f->error());
+    EXPECT_TRUE(result.has_error());
+  }
 
   // Write should succeed but will be buffered in the file stream.
-  ASSERT_EQ(f->write(write_data, sizeof(write_data)), sizeof(write_data));
+  ASSERT_EQ(f->write(write_data, sizeof(write_data)).value, sizeof(write_data));
   EXPECT_EQ(f->get_pos(), size_t(0));
   // Flushing will write to the file.
   EXPECT_EQ(f->flush(), int(0));
@@ -330,14 +342,14 @@
   StringFile *f =
       new_string_file(file_buffer, FILE_BUFFER_SIZE, _IOFBF, false, "w+");
 
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_EQ(f->get_pos(), size_t(0)); // Data is buffered in the file stream
 
-  ASSERT_EQ(f->seek(0, SEEK_SET), 0);
+  ASSERT_EQ(f->seek(0, SEEK_SET).value(), 0);
 
   // Seek flushes the stream buffer so we can read the previously written data.
   char read_data[sizeof(data)];
-  ASSERT_EQ(f->read(read_data, sizeof(data)), sizeof(data));
+  ASSERT_EQ(f->read(read_data, sizeof(data)).value, sizeof(data));
   EXPECT_STREQ(read_data, data);
 
   ASSERT_EQ(f->close(), 0);
@@ -353,7 +365,7 @@
 
   constexpr size_t READ_SIZE = sizeof(initial_content) / 2;
   char read_data[READ_SIZE];
-  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE).value);
   EXPECT_FALSE(f->iseof());
   // Reading less than file buffer worth will still read one
   // full buffer worth of data.
@@ -364,9 +376,9 @@
   MemoryView src1(initial_content, READ_SIZE), dst1(read_data, READ_SIZE);
   EXPECT_MEM_EQ(src1, dst1);
 
-  ASSERT_EQ(f->seek(0, SEEK_SET), 0);
+  ASSERT_EQ(f->seek(0, SEEK_SET).value(), 0);
   const char write_data[] = "hello, file";
-  ASSERT_EQ(sizeof(write_data), f->write(write_data, sizeof(write_data)));
+  ASSERT_EQ(sizeof(write_data), f->write(write_data, sizeof(write_data)).value);
   EXPECT_STREQ(file_buffer, write_data);
   ASSERT_EQ(f->flush(), 0);
   MemoryView dst2(f->get_str(), sizeof(write_data)),
@@ -385,16 +397,16 @@
       new_string_file(file_buffer, FILE_BUFFER_SIZE, _IOFBF, false, "a+");
   f->reset_and_fill(initial_content, sizeof(initial_content));
 
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   EXPECT_EQ(f->get_pos(), size_t(0)); // Data is buffered in the file stream
   ASSERT_EQ(f->flush(), 0);
   // The flush should write |data| to the endof the file.
   EXPECT_EQ(f->get_pos(), sizeof(data) + sizeof(initial_content));
 
-  ASSERT_EQ(f->seek(0, SEEK_SET), 0);
+  ASSERT_EQ(f->seek(0, SEEK_SET).value(), 0);
   // Seeking to the beginning of the file should not affect the place
   // where write happens.
-  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)));
+  ASSERT_EQ(sizeof(data), f->write(data, sizeof(data)).value);
   ASSERT_EQ(f->flush(), 0);
   EXPECT_EQ(f->get_pos(), sizeof(data) * 2 + sizeof(initial_content));
   MemoryView src1(initial_content, sizeof(initial_content)),
@@ -408,10 +420,10 @@
   EXPECT_MEM_EQ(src3, dst3);
 
   // Reads can happen from any point.
-  ASSERT_EQ(f->seek(0, SEEK_SET), 0);
+  ASSERT_EQ(f->seek(0, SEEK_SET).value(), 0);
   constexpr size_t READ_SIZE = 10;
   char read_data[READ_SIZE];
-  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE));
+  ASSERT_EQ(READ_SIZE, f->read(read_data, READ_SIZE).value);
   MemoryView src4(initial_content, READ_SIZE), dst4(read_data, READ_SIZE);
   EXPECT_MEM_EQ(src4, dst4);
 
@@ -426,7 +438,7 @@
   StringFile *f =
       new_string_file(file_buffer, FILE_BUFFER_SIZE, _IOFBF, false, "w");
 
-  ASSERT_EQ(WRITE_SIZE, f->write(WRITE_DATA, WRITE_SIZE));
+  ASSERT_EQ(WRITE_SIZE, f->write(WRITE_DATA, WRITE_SIZE).value);
   // Since data much larger than the buffer is being written, all of it should
   // be available in the file without a flush operation.
   EXPECT_EQ(f->get_pos(), sizeof(WRITE_DATA));
@@ -442,9 +454,9 @@
   StringFile *f_lbf = new_string_file(nullptr, 0, _IOLBF, true, "w");
   StringFile *f_nbf = new_string_file(nullptr, 0, _IONBF, true, "w");
 
-  ASSERT_EQ(WRITE_SIZE, f_fbf->write(WRITE_DATA, WRITE_SIZE));
-  ASSERT_EQ(WRITE_SIZE, f_lbf->write(WRITE_DATA, WRITE_SIZE));
-  ASSERT_EQ(WRITE_SIZE, f_nbf->write(WRITE_DATA, WRITE_SIZE));
+  ASSERT_EQ(WRITE_SIZE, f_fbf->write(WRITE_DATA, WRITE_SIZE).value);
+  ASSERT_EQ(WRITE_SIZE, f_lbf->write(WRITE_DATA, WRITE_SIZE).value);
+  ASSERT_EQ(WRITE_SIZE, f_nbf->write(WRITE_DATA, WRITE_SIZE).value);
   // Since there is no buffer space, all of the written data should
   // be available in the file without a flush operation.
   EXPECT_EQ(f_fbf->get_pos(), sizeof(WRITE_DATA));
@@ -473,9 +485,9 @@
   StringFile *f_nbf =
       new_string_file(file_buffer_nbf, FILE_BUFFER_SIZE, _IONBF, false, "w");
 
-  ASSERT_EQ(WRITE_SIZE, f_fbf->write(WRITE_DATA, WRITE_SIZE));
-  ASSERT_EQ(WRITE_SIZE, f_lbf->write(WRITE_DATA, WRITE_SIZE));
-  ASSERT_EQ(WRITE_SIZE, f_nbf->write(WRITE_DATA, WRITE_SIZE));
+  ASSERT_EQ(WRITE_SIZE, f_fbf->write(WRITE_DATA, WRITE_SIZE).value);
+  ASSERT_EQ(WRITE_SIZE, f_lbf->write(WRITE_DATA, WRITE_SIZE).value);
+  ASSERT_EQ(WRITE_SIZE, f_nbf->write(WRITE_DATA, WRITE_SIZE).value);
 
   ASSERT_FALSE(f_fbf->error_unlocked());
   ASSERT_FALSE(f_lbf->error_unlocked());
diff --git a/libc/test/src/__support/File/platform_file_test.cpp b/libc/test/src/__support/File/platform_file_test.cpp
--- a/libc/test/src/__support/File/platform_file_test.cpp
+++ b/libc/test/src/__support/File/platform_file_test.cpp
@@ -19,18 +19,18 @@
   constexpr char FILENAME[] = "testdata/create_write_close_and_readback.test";
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->write(TEXT, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->write(TEXT, TEXT_SIZE).value, TEXT_SIZE);
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "r");
   ASSERT_FALSE(file == nullptr);
   char data[sizeof(TEXT)];
-  ASSERT_EQ(file->read(data, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->read(data, TEXT_SIZE).value, TEXT_SIZE);
   data[TEXT_SIZE] = '\0';
   ASSERT_STREQ(data, TEXT);
 
   // Reading more data should trigger EOF.
-  ASSERT_EQ(file->read(data, TEXT_SIZE), size_t(0));
+  ASSERT_EQ(file->read(data, TEXT_SIZE).value, size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
@@ -40,17 +40,17 @@
   constexpr char FILENAME[] = "testdata/create_write_seek_and_readback.test";
   File *file = __llvm_libc::openfile(FILENAME, "w+");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->write(TEXT, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->write(TEXT, TEXT_SIZE).value, TEXT_SIZE);
 
-  ASSERT_EQ(file->seek(0, SEEK_SET), 0);
+  ASSERT_EQ(file->seek(0, SEEK_SET).value(), 0);
 
   char data[sizeof(TEXT)];
-  ASSERT_EQ(file->read(data, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->read(data, TEXT_SIZE).value, TEXT_SIZE);
   data[TEXT_SIZE] = '\0';
   ASSERT_STREQ(data, TEXT);
 
   // Reading more data should trigger EOF.
-  ASSERT_EQ(file->read(data, TEXT_SIZE), size_t(0));
+  ASSERT_EQ(file->read(data, TEXT_SIZE).value, size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
@@ -60,26 +60,26 @@
   constexpr char FILENAME[] = "testdata/create_append_close_and_readback.test";
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->write(TEXT, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->write(TEXT, TEXT_SIZE).value, TEXT_SIZE);
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "a");
   ASSERT_FALSE(file == nullptr);
   constexpr char APPEND_TEXT[] = " Append Text";
   constexpr size_t APPEND_TEXT_SIZE = sizeof(APPEND_TEXT) - 1;
-  ASSERT_EQ(file->write(APPEND_TEXT, APPEND_TEXT_SIZE), APPEND_TEXT_SIZE);
+  ASSERT_EQ(file->write(APPEND_TEXT, APPEND_TEXT_SIZE).value, APPEND_TEXT_SIZE);
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "r");
   ASSERT_FALSE(file == nullptr);
   constexpr size_t READ_SIZE = TEXT_SIZE + APPEND_TEXT_SIZE;
   char data[READ_SIZE + 1];
-  ASSERT_EQ(file->read(data, READ_SIZE), READ_SIZE);
+  ASSERT_EQ(file->read(data, READ_SIZE).value, READ_SIZE);
   data[READ_SIZE] = '\0';
   ASSERT_STREQ(data, "Hello, File Append Text");
 
   // Reading more data should trigger EOF.
-  ASSERT_EQ(file->read(data, READ_SIZE), size_t(0));
+  ASSERT_EQ(file->read(data, READ_SIZE).value, size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
@@ -89,23 +89,23 @@
   constexpr char FILENAME[] = "testdata/create_append_seek_and_readback.test";
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->write(TEXT, TEXT_SIZE), TEXT_SIZE);
+  ASSERT_EQ(file->write(TEXT, TEXT_SIZE).value, TEXT_SIZE);
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "a+");
   ASSERT_FALSE(file == nullptr);
   constexpr char APPEND_TEXT[] = " Append Text";
   constexpr size_t APPEND_TEXT_SIZE = sizeof(APPEND_TEXT) - 1;
-  ASSERT_EQ(file->write(APPEND_TEXT, APPEND_TEXT_SIZE), APPEND_TEXT_SIZE);
+  ASSERT_EQ(file->write(APPEND_TEXT, APPEND_TEXT_SIZE).value, APPEND_TEXT_SIZE);
 
-  ASSERT_EQ(file->seek(-APPEND_TEXT_SIZE, SEEK_END), 0);
+  ASSERT_EQ(file->seek(-APPEND_TEXT_SIZE, SEEK_END).value(), 0);
   char data[APPEND_TEXT_SIZE + 1];
-  ASSERT_EQ(file->read(data, APPEND_TEXT_SIZE), APPEND_TEXT_SIZE);
+  ASSERT_EQ(file->read(data, APPEND_TEXT_SIZE).value, APPEND_TEXT_SIZE);
   data[APPEND_TEXT_SIZE] = '\0';
   ASSERT_STREQ(data, APPEND_TEXT);
 
   // Reading more data should trigger EOF.
-  ASSERT_EQ(file->read(data, APPEND_TEXT_SIZE), size_t(0));
+  ASSERT_EQ(file->read(data, APPEND_TEXT_SIZE).value, size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
@@ -124,7 +124,7 @@
 
   constexpr int REPEAT = 5;
   for (int i = 0; i < REPEAT; ++i) {
-    ASSERT_EQ(file->write(write_data, DATA_SIZE), DATA_SIZE);
+    ASSERT_EQ(file->write(write_data, DATA_SIZE).value, DATA_SIZE);
   }
   ASSERT_EQ(file->close(), 0);
 
@@ -132,13 +132,13 @@
   ASSERT_FALSE(file == nullptr);
   constexpr size_t READ_SIZE = DATA_SIZE * REPEAT;
   char data[READ_SIZE] = {0};
-  ASSERT_EQ(file->read(data, READ_SIZE), READ_SIZE);
+  ASSERT_EQ(file->read(data, READ_SIZE).value, READ_SIZE);
 
   for (size_t i = 0; i < READ_SIZE; ++i)
     ASSERT_EQ(data[i], BYTE);
 
   // Reading more data should trigger EOF.
-  ASSERT_EQ(file->read(data, 1), size_t(0));
+  ASSERT_EQ(file->read(data, 1).value, size_t(0));
   ASSERT_TRUE(file->iseof());
 
   ASSERT_EQ(file->close(), 0);
@@ -149,7 +149,8 @@
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
   constexpr char CONTENT[] = "1234567890987654321";
-  ASSERT_EQ(sizeof(CONTENT) - 1, file->write(CONTENT, sizeof(CONTENT) - 1));
+  ASSERT_EQ(sizeof(CONTENT) - 1,
+            file->write(CONTENT, sizeof(CONTENT) - 1).value);
   ASSERT_EQ(0, file->close());
 
   file = __llvm_libc::openfile(FILENAME, "r");
@@ -158,13 +159,13 @@
   constexpr size_t READ_SIZE = 5;
   char data[READ_SIZE];
   data[READ_SIZE - 1] = '\0';
-  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(file->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "1234");
-  ASSERT_EQ(file->seek(5, SEEK_CUR), 0);
-  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(file->seek(5, SEEK_CUR).value(), 0);
+  ASSERT_EQ(file->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "0987");
-  ASSERT_EQ(file->seek(-5, SEEK_CUR), 0);
-  ASSERT_EQ(file->read(data, READ_SIZE - 1), READ_SIZE - 1);
+  ASSERT_EQ(file->seek(-5, SEEK_CUR).value(), 0);
+  ASSERT_EQ(file->read(data, READ_SIZE - 1).value, READ_SIZE - 1);
   ASSERT_STREQ(data, "9098");
 
   ASSERT_EQ(file->close(), 0);
@@ -176,21 +177,21 @@
 
   File *file = __llvm_libc::openfile(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->read(data, 1), size_t(0)); // Cannot read
+  ASSERT_EQ(file->read(data, 1).value, size_t(0)); // Cannot read
   ASSERT_FALSE(file->iseof());
   ASSERT_TRUE(file->error());
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "r");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->write(data, 1), size_t(0)); // Cannot write
+  ASSERT_EQ(file->write(data, 1).value, size_t(0)); // Cannot write
   ASSERT_FALSE(file->iseof());
   ASSERT_TRUE(file->error());
   ASSERT_EQ(file->close(), 0);
 
   file = __llvm_libc::openfile(FILENAME, "a");
   ASSERT_FALSE(file == nullptr);
-  ASSERT_EQ(file->read(data, 1), size_t(0)); // Cannot read
+  ASSERT_EQ(file->read(data, 1).value, size_t(0)); // Cannot read
   ASSERT_FALSE(file->iseof());
   ASSERT_TRUE(file->error());
   ASSERT_EQ(file->close(), 0);
diff --git a/libc/test/src/stdio/fileop_test.cpp b/libc/test/src/stdio/fileop_test.cpp
--- a/libc/test/src/stdio/fileop_test.cpp
+++ b/libc/test/src/stdio/fileop_test.cpp
@@ -78,6 +78,11 @@
   __llvm_libc::clearerr(file);
   ASSERT_EQ(__llvm_libc::ferror(file), 0);
 
+  errno = 0;
+  ASSERT_EQ(__llvm_libc::fwrite("nothing", 1, 1, file), size_t(0));
+  ASSERT_NE(errno, 0);
+  errno = 0;
+
   ASSERT_EQ(__llvm_libc::fclose(file), 0);
 
   // Now try puts.
@@ -90,6 +95,12 @@
   __llvm_libc::clearerr(file);
   ASSERT_EQ(__llvm_libc::ferror(file), 0);
 
+  // This is not a readable file.
+  errno = 0;
+  ASSERT_EQ(__llvm_libc::fread(data, 1, 1, file), size_t(0));
+  ASSERT_NE(errno, 0);
+  errno = 0;
+
   ASSERT_EQ(0, __llvm_libc::fclose(file));
 
   file = __llvm_libc::fopen(FILENAME, "r");
@@ -100,6 +111,21 @@
   read_data[sizeof(CONTENT) - 1] = '\0';
   ASSERT_STREQ(read_data, CONTENT);
   ASSERT_EQ(__llvm_libc::fclose(file), 0);
+
+  // Check that the other functions correctly set errno.
+
+  errno = 0;
+  ASSERT_NE(__llvm_libc::fseek(file, 0, SEEK_SET), 0);
+  EXPECT_NE(errno, 0);
+
+  errno = 0;
+  ASSERT_NE(__llvm_libc::fclose(file), 0);
+  EXPECT_NE(errno, 0);
+
+  errno = 0;
+  ASSERT_EQ(__llvm_libc::fopen("INVALID FILE NAME", "r"),
+            static_cast<FILE *>(nullptr));
+  EXPECT_NE(errno, 0);
 }
 
 TEST(LlvmLibcFILETest, FFlush) {
@@ -132,6 +158,7 @@
   constexpr size_t WRITE_NMEMB = sizeof(WRITE_DATA) / sizeof(MyStruct);
   constexpr char FILENAME[] = "testdata/fread_fwrite.test";
 
+  errno = 0;
   FILE *file = __llvm_libc::fopen(FILENAME, "w");
   ASSERT_FALSE(file == nullptr);
   ASSERT_EQ(size_t(0), __llvm_libc::fwrite(WRITE_DATA, 0, 1, file));
diff --git a/libc/test/src/stdio/ftell_test.cpp b/libc/test/src/stdio/ftell_test.cpp
--- a/libc/test/src/stdio/ftell_test.cpp
+++ b/libc/test/src/stdio/ftell_test.cpp
@@ -16,6 +16,7 @@
 #include "src/stdio/setvbuf.h"
 #include "utils/UnitTest/Test.h"
 
+#include <errno.h>
 #include <stdio.h>
 
 class LlvmLibcFTellTest : public __llvm_libc::testing::Test {
@@ -53,6 +54,10 @@
     ASSERT_EQ(size_t(__llvm_libc::ftell(file)), READ_SIZE);
 
     ASSERT_EQ(0, __llvm_libc::fclose(file));
+
+    errno = 0;
+    ASSERT_EQ(__llvm_libc::ftell(file), long(-1));
+    ASSERT_NE(errno, 0);
   }
 };