diff --git a/libc/config/linux/api.td b/libc/config/linux/api.td
--- a/libc/config/linux/api.td
+++ b/libc/config/linux/api.td
@@ -157,7 +157,7 @@
     SimpleMacroDef<"_IONBF", "2">,
     SimpleMacroDef<"EOF", "-1">,
   ];
-  let Types = ["size_t", "FILE", "cookie_io_functions_t"];
+  let Types = ["off_t", "size_t", "FILE", "cookie_io_functions_t"];
 }
 
 def StdlibAPI : PublicAPI<"stdlib.h"> {
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -385,6 +385,8 @@
     libc.src.stdio.fread
     libc.src.stdio.fread_unlocked
     libc.src.stdio.fseek
+    libc.src.stdio.ftell
+    libc.src.stdio.ftello
     libc.src.stdio.funlockfile
     libc.src.stdio.fwrite
     libc.src.stdio.fwrite_unlocked
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -162,6 +162,7 @@
     .llvm-libc-macros.stdio_macros
     .llvm-libc-types.cookie_io_functions_t
     .llvm-libc-types.FILE
+    .llvm-libc-types.off_t
     .llvm-libc-types.size_t
 )
 
diff --git a/libc/spec/posix.td b/libc/spec/posix.td
--- a/libc/spec/posix.td
+++ b/libc/spec/posix.td
@@ -998,7 +998,7 @@
   HeaderSpec StdIO = HeaderSpec<
       "stdio.h",
       [], // Macros
-      [], // Types
+      [OffTType], // Types
       [], // Enumerations
       [
           FunctionSpec<
@@ -1011,6 +1011,11 @@
               RetValSpec<VoidType>,
               [ArgSpec<FILEPtr>]
           >,
+          FunctionSpec<
+              "ftello",
+              RetValSpec<OffTType>,
+              [ArgSpec<FILEPtr>]
+          >,
       ]
   >;
 
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -563,6 +563,11 @@
               [ArgSpec<IntType>,
                ArgSpec<FILEPtr>]
           >,
+          FunctionSpec<
+              "ftell",
+              RetValSpec<LongType>,
+              [ArgSpec<FILEPtr>]
+          >,
           FunctionSpec<
               "putc",
               RetValSpec<IntType>,
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
@@ -283,6 +283,10 @@
 // library.
 File *openfile(const char *path, const char *mode);
 
+// The platform_file library should implement it if it relevant for that
+// platform.
+int get_fileno(File *f);
+
 extern File *stdin;
 extern File *stdout;
 extern File *stderr;
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
@@ -164,6 +164,11 @@
   return file;
 }
 
+int get_fileno(File *f) {
+  auto *lf = reinterpret_cast<LinuxFile *>(f);
+  return lf->get_fd();
+}
+
 constexpr size_t STDIN_BUFFER_SIZE = 512;
 char stdin_buffer[STDIN_BUFFER_SIZE];
 static LinuxFile StdIn(0, stdin_buffer, STDIN_BUFFER_SIZE, _IOFBF, false,
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
@@ -416,6 +416,20 @@
     libc.src.stdio.printf_core.vfprintf_internal
 )
 
+add_entrypoint_object(
+  ftell
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.ftell
+)
+
+add_entrypoint_object(
+  ftello
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.ftello
+)
+
 add_entrypoint_object(
   remove
   ALIAS
diff --git a/libc/src/stdio/ftell.h b/libc/src/stdio/ftell.h
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/ftell.h
@@ -0,0 +1,20 @@
+//===-- Implementation header of ftell --------------------------*- 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_STDIO_FTELL_H
+#define LLVM_LIBC_SRC_STDIO_FTELL_H
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+long ftell(::FILE *f);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FTELL_H
diff --git a/libc/src/stdio/ftello.h b/libc/src/stdio/ftello.h
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/ftello.h
@@ -0,0 +1,20 @@
+//===-- Implementation header of ftello -------------------------*- 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_STDIO_FTELLO_H
+#define LLVM_LIBC_SRC_STDIO_FTELLO_H
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+off_t ftello(::FILE *f);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FTELLO_H
diff --git a/libc/src/stdio/linux/CMakeLists.txt b/libc/src/stdio/linux/CMakeLists.txt
--- a/libc/src/stdio/linux/CMakeLists.txt
+++ b/libc/src/stdio/linux/CMakeLists.txt
@@ -11,3 +11,40 @@
     libc.src.__support.OSUtil.osutil
     libc.src.errno.errno
 )
+
+add_header_library(
+  ftell_operation
+  HDRS
+    ftell_operation.h
+  DEPENDS
+    libc.include.stdio
+    libc.include.sys_syscall
+    libc.src.__support.File.file
+    libc.src.errno.errno
+)
+
+add_entrypoint_object(
+  ftell
+  SRCS
+    ftell.cpp
+  HDRS
+    ../ftell.h
+  DEPENDS
+    libc.include.stdio
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
+
+add_entrypoint_object(
+  ftello
+  SRCS
+    ftello.cpp
+  HDRS
+    ../ftello.h
+  DEPENDS
+    libc.include.stdio
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
diff --git a/libc/src/stdio/linux/ftell.cpp b/libc/src/stdio/linux/ftell.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/linux/ftell.cpp
@@ -0,0 +1,20 @@
+//===-- Implementation of ftell -------------------------------------------===//
+//
+// 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/stdio/ftell.h"
+#include "ftell_operation.h"
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(long, ftell, (::FILE * stream)) {
+  return ftell_operation(stream);
+}
+
+} // namespace __llvm_libc
diff --git a/libc/src/stdio/linux/ftell_operation.h b/libc/src/stdio/linux/ftell_operation.h
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/linux/ftell_operation.h
@@ -0,0 +1,42 @@
+//===-- Implementation of linux ftell operation ---------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/File/file.h"
+
+#include <errno.h>
+#include <stdio.h>
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace __llvm_libc {
+
+// An implementation of ftell to be used by the C standard ftell and POSIX
+// ftello functions.
+inline long ftell_operation(FILE *stream) {
+  File *file = reinterpret_cast<__llvm_libc::File *>(stream);
+  int fd = __llvm_libc::get_fileno(file);
+  int whence = SEEK_CUR;
+  if (file->iseof())
+    whence = SEEK_END;
+  long result;
+#ifdef SYS_lseek
+  long ret = __llvm_libc::syscall_impl(SYS_lseek, fd, 0, whence);
+  result = ret;
+#elif defined(SYS__llseek)
+  long ret = __llvm_libc::syscall_impl(SYS__llseek, fd, 0, 0, &result, whence);
+#else
+#error "lseek and _llseek syscalls not available."
+#endif
+
+  if (ret < 0) {
+    errno = -ret;
+    return -1;
+  }
+  return result;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/src/stdio/linux/ftello.cpp b/libc/src/stdio/linux/ftello.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/linux/ftello.cpp
@@ -0,0 +1,20 @@
+//===-- Implementation of ftello ------------------------------------------===//
+//
+// 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/stdio/ftello.h"
+#include "ftell_operation.h"
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(long, ftello, (::FILE * stream)) {
+  return ftell_operation(stream);
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdio/CMakeLists.txt b/libc/test/src/stdio/CMakeLists.txt
--- a/libc/test/src/stdio/CMakeLists.txt
+++ b/libc/test/src/stdio/CMakeLists.txt
@@ -235,6 +235,40 @@
     libc.src.stdio.fwrite
 )
 
+add_libc_unittest(
+  ftell_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    ftell_test.cpp
+  DEPENDS
+    libc.include.errno
+    libc.include.stdio
+    libc.src.stdio.fclose
+    libc.src.stdio.fflush
+    libc.src.stdio.fopen
+    libc.src.stdio.fseek
+    libc.src.stdio.ftell
+    libc.src.stdio.fwrite
+)
+
+add_libc_unittest(
+  ftello_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    ftello_test.cpp
+  DEPENDS
+    libc.include.errno
+    libc.include.stdio
+    libc.src.stdio.fclose
+    libc.src.stdio.fflush
+    libc.src.stdio.fopen
+    libc.src.stdio.fseek
+    libc.src.stdio.ftello
+    libc.src.stdio.fwrite
+)
+
 add_subdirectory(printf_core)
 add_subdirectory(scanf_core)
 add_subdirectory(testdata)
diff --git a/libc/test/src/stdio/ftell_test.cpp b/libc/test/src/stdio/ftell_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdio/ftell_test.cpp
@@ -0,0 +1,38 @@
+//===-- Unittests for ftell -----------------------------------------------===//
+//
+// 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/stdio/fclose.h"
+#include "src/stdio/fflush.h"
+#include "src/stdio/fopen.h"
+#include "src/stdio/fseek.h"
+#include "src/stdio/ftell.h"
+#include "src/stdio/fwrite.h"
+#include "utils/UnitTest/Test.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+TEST(LlvmLibcFtellTest, WriteAndTell) {
+  constexpr char FILENAME[] = "testdata/ftell.test";
+  ::FILE *file = __llvm_libc::fopen(FILENAME, "w");
+  ASSERT_FALSE(file == nullptr);
+  constexpr char CONTENT[] = "123456789";
+  constexpr size_t WRITE_SIZE = sizeof(CONTENT) - 1;
+  ASSERT_EQ(WRITE_SIZE, __llvm_libc::fwrite(CONTENT, 1, WRITE_SIZE, file));
+  ASSERT_EQ(0, __llvm_libc::fflush(file));
+
+  ASSERT_EQ(size_t(__llvm_libc::ftell(file)), WRITE_SIZE);
+
+  long offset = 5;
+  ASSERT_EQ(0, __llvm_libc::fseek(file, offset, SEEK_SET));
+  ASSERT_EQ(__llvm_libc::ftell(file), offset);
+  ASSERT_EQ(0, __llvm_libc::fseek(file, -offset, SEEK_END));
+  ASSERT_EQ(size_t(__llvm_libc::ftell(file)), size_t(WRITE_SIZE - offset));
+
+  ASSERT_EQ(0, __llvm_libc::fclose(file));
+}
diff --git a/libc/test/src/stdio/ftello_test.cpp b/libc/test/src/stdio/ftello_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdio/ftello_test.cpp
@@ -0,0 +1,38 @@
+//===-- Unittests for ftello ----------------------------------------------===//
+//
+// 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/stdio/fclose.h"
+#include "src/stdio/fflush.h"
+#include "src/stdio/fopen.h"
+#include "src/stdio/fseek.h"
+#include "src/stdio/ftello.h"
+#include "src/stdio/fwrite.h"
+#include "utils/UnitTest/Test.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+TEST(LlvmLibcFtelloTest, WriteAndTell) {
+  constexpr char FILENAME[] = "testdata/ftello.test";
+  ::FILE *file = __llvm_libc::fopen(FILENAME, "w");
+  ASSERT_FALSE(file == nullptr);
+  constexpr char CONTENT[] = "123456789";
+  constexpr size_t WRITE_SIZE = sizeof(CONTENT) - 1;
+  ASSERT_EQ(WRITE_SIZE, __llvm_libc::fwrite(CONTENT, 1, WRITE_SIZE, file));
+  ASSERT_EQ(0, __llvm_libc::fflush(file));
+
+  ASSERT_EQ(size_t(__llvm_libc::ftello(file)), WRITE_SIZE);
+
+  long offset = 5;
+  ASSERT_EQ(0, __llvm_libc::fseek(file, offset, SEEK_SET));
+  ASSERT_EQ(__llvm_libc::ftello(file), off_t(offset));
+  ASSERT_EQ(0, __llvm_libc::fseek(file, -offset, SEEK_END));
+  ASSERT_EQ(__llvm_libc::ftello(file), off_t(WRITE_SIZE - offset));
+
+  ASSERT_EQ(0, __llvm_libc::fclose(file));
+}