diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -110,6 +110,24 @@
   list(APPEND TARGET_ENTRYPOINT_NAME_LIST ${entrypoint_name})
 endforeach()
 
+# External entrypoints are only included in TARGET_ENTRYPOINT_NAME_LIST, 
+# not TARGET_LLVMLIBC_ENTRYPOINTS.
+# TARGET_ENTRYPOINT_NAME_LIST just defines which function prototypes are
+# included in the generated headers, while TARGET_LLVMLIBC_ENTRYPOINTS is used
+# for determining which functions will be built.
+if(TARGET_LIBC_EXTERNAL_ENTRYPOINTS)
+  foreach(entrypoint IN LISTS TARGET_LIBC_EXTERNAL_ENTRYPOINTS)
+    string(FIND ${entrypoint} "." last_dot_loc REVERSE)
+    if(${last_dot_loc} EQUAL -1)
+      message(FATAL "Invalid entrypoint target name ${entrypoint}; Expected a '.' "
+                    "(dot) in the name.")
+    endif()
+    math(EXPR name_loc "${last_dot_loc} + 1")
+    string(SUBSTRING ${entrypoint} ${name_loc} -1 entrypoint_name)
+    list(APPEND TARGET_ENTRYPOINT_NAME_LIST ${entrypoint_name})
+  endforeach()
+endif()
+
 if(LLVM_LIBC_FULL_BUILD)
   # We need to set up hdrgen first since other targets depend on it.
   add_subdirectory(utils/LibcTableGenUtil)
diff --git a/libc/cmake/modules/LLVMLibCObjectRules.cmake b/libc/cmake/modules/LLVMLibCObjectRules.cmake
--- a/libc/cmake/modules/LLVMLibCObjectRules.cmake
+++ b/libc/cmake/modules/LLVMLibCObjectRules.cmake
@@ -258,6 +258,29 @@
 
 endfunction(add_entrypoint_object)
 
+# A rule for external entrypoint targets.
+# Usage:
+#     add_entrypoint_external(
+#       <target_name>
+#     )
+function(add_entrypoint_external target_name)
+
+  get_fq_target_name(${target_name} fq_target_name)
+  set(entrypoint_name ${target_name})
+
+  add_custom_target(${fq_target_name})
+  set_target_properties(
+    ${fq_target_name}
+    PROPERTIES
+      "ENTRYPOINT_NAME" ${entrypoint_name}
+      "TARGET_TYPE" ${ENTRYPOINT_OBJ_TARGET_TYPE}
+      "OBJECT_FILE" ""
+      "OBJECT_FILE_RAW" ""
+      "DEPS" ""
+  )
+
+endfunction(add_entrypoint_external)
+
 # Rule build a redirector object file.
 function(add_redirector_object target_name)
   cmake_parse_arguments(
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
@@ -36,6 +36,7 @@
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
+    libc.src.string.strdup
     libc.src.string.strlen
     libc.src.string.strncat
     libc.src.string.strncmp
@@ -228,6 +229,14 @@
   )
 endif()
 
+set(TARGET_LIBC_EXTERNAL_ENTRYPOINTS
+    # stdlib.h external entrypoints
+    libc.src.stdlib.malloc
+    libc.src.stdlib.calloc
+    libc.src.stdlib.realloc
+    libc.src.stdlib.free
+)
+
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
   ${TARGET_LIBM_ENTRYPOINTS}
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -279,6 +279,11 @@
               RetValSpec<SizeTType>,
               [ArgSpec<ConstCharPtr>, ArgSpec<ConstCharPtr>]
           >,
+          FunctionSpec<
+              "strdup",
+              RetValSpec<CharPtr>,
+              [ArgSpec<ConstCharPtr>]
+          >,
           FunctionSpec<
               "strpbrk",
               RetValSpec<CharPtr>,
@@ -513,6 +518,11 @@
           FunctionSpec<"strtoul", RetValSpec<UnsignedLongType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>, ArgSpec<IntType>]>,
           FunctionSpec<"strtoull", RetValSpec<UnsignedLongLongType>, [ArgSpec<ConstCharRestrictedPtr>, ArgSpec<CharRestrictedPtrPtr>, ArgSpec<IntType>]>,
 
+          FunctionSpec<"malloc", RetValSpec<VoidPtr>, [ArgSpec<SizeTType>]>,
+          FunctionSpec<"calloc", RetValSpec<VoidPtr>, [ArgSpec<SizeTType>, ArgSpec<SizeTType>]>,
+          FunctionSpec<"realloc", RetValSpec<VoidPtr>, [ArgSpec<VoidPtr>, ArgSpec<SizeTType>]>,
+          FunctionSpec<"free", RetValSpec<VoidType>, [ArgSpec<VoidPtr>]>,
+
           FunctionSpec<"_Exit", RetValSpec<NoReturn>, [ArgSpec<IntType>]>,
       ]
   >;
diff --git a/libc/src/stdlib/CMakeLists.txt b/libc/src/stdlib/CMakeLists.txt
--- a/libc/src/stdlib/CMakeLists.txt
+++ b/libc/src/stdlib/CMakeLists.txt
@@ -181,6 +181,19 @@
     libc.include.stdlib
 )
 
+add_entrypoint_external(
+  malloc
+)
+add_entrypoint_external(
+  calloc
+)
+add_entrypoint_external(
+  realloc
+)
+add_entrypoint_external(
+  free
+)
+
 if(NOT LLVM_LIBC_FULL_BUILD)
   return()
 endif()
diff --git a/libc/src/string/CMakeLists.txt b/libc/src/string/CMakeLists.txt
--- a/libc/src/string/CMakeLists.txt
+++ b/libc/src/string/CMakeLists.txt
@@ -104,6 +104,18 @@
     .string_utils
 )
 
+add_entrypoint_object(
+  strdup
+  SRCS
+    strdup.cpp
+  HDRS
+    strdup.h
+  DEPENDS
+    .memcpy
+    .string_utils
+    libc.include.stdlib
+)
+
 add_entrypoint_object(
   strlen
   SRCS
diff --git a/libc/src/string/strdup.h b/libc/src/string/strdup.h
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strdup.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for strdup ------------------------*- 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_STRING_STRDUP_H
+#define LLVM_LIBC_SRC_STRING_STRDUP_H
+
+#include <string.h>
+
+namespace __llvm_libc {
+
+char *strdup(const char *src);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRDUP_H
diff --git a/libc/src/string/strdup.cpp b/libc/src/string/strdup.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/string/strdup.cpp
@@ -0,0 +1,32 @@
+//===-- Implementation of strdup ------------------------------------------===//
+//
+// 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/string/strdup.h"
+#include "src/string/memcpy.h"
+#include "src/string/string_utils.h"
+
+#include "src/__support/common.h"
+
+#include <stdlib.h>
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(char *, strdup, (const char *src)) {
+  if (src == nullptr) {
+    return nullptr;
+  }
+  size_t len = internal::string_length(src) + 1;
+  char *dest = reinterpret_cast<char *>(::malloc(len)); // NOLINT
+  if (dest == nullptr) {
+    return nullptr;
+  }
+  char *result = reinterpret_cast<char *>(__llvm_libc::memcpy(dest, src, len));
+  return result;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -92,6 +92,17 @@
     libc.src.string.strcspn
 )
 
+add_libc_unittest(
+  strdup_test
+  SUITE
+    libc_string_unittests
+  SRCS
+  strdup_test.cpp
+  DEPENDS
+    libc.include.stdlib
+    libc.src.string.strdup
+)
+
 add_libc_unittest(
   strlen_test
   SUITE
diff --git a/libc/test/src/string/strdup_test.cpp b/libc/test/src/string/strdup_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/string/strdup_test.cpp
@@ -0,0 +1,39 @@
+//===-- Unittests for strdup ----------------------------------------------===//
+//
+// 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/string/strdup.h"
+#include "utils/UnitTest/Test.h"
+#include <stdlib.h>
+
+TEST(LlvmLibcStrDupTest, EmptyString) {
+  const char *empty = "";
+
+  char *result = __llvm_libc::strdup(empty);
+  ASSERT_NE(result, static_cast<char *>(nullptr));
+  ASSERT_NE(empty, const_cast<const char *>(result));
+  ASSERT_STREQ(empty, result);
+  ::free(result);
+}
+
+TEST(LlvmLibcStrDupTest, AnyString) {
+  const char *abc = "abc";
+
+  char *result = __llvm_libc::strdup(abc);
+
+  ASSERT_NE(result, static_cast<char *>(nullptr));
+  ASSERT_NE(abc, const_cast<const char *>(result));
+  ASSERT_STREQ(abc, result);
+  ::free(result);
+}
+
+TEST(LlvmLibcStrDupTest, NullPtr) {
+
+  char *result = __llvm_libc::strdup(nullptr);
+
+  ASSERT_EQ(result, static_cast<char *>(nullptr));
+}