diff --git a/libc/src/unistd/CMakeLists.txt b/libc/src/unistd/CMakeLists.txt
@@ -333,3 +333,13 @@ add_entrypoint_external(
 add_entrypoint_external(
   opterr
 )
+
+add_entrypoint_object(
+  gettid
+  SRCS
+    gettid.cpp
+  HDRS
+    gettid.h
+  DEPENDS
+    libc.src.__support.threads.tid
+)
diff --git a/libc/src/unistd/getpid.h b/libc/src/unistd/getpid.h
@@ -9,12 +9,12 @@
 #ifndef LLVM_LIBC_SRC_UNISTD_GETPID_H
 #define LLVM_LIBC_SRC_UNISTD_GETPID_H
 
+#include "hdr/types/pid_t.h"
 #include "src/__support/macros/config.h"
-#include <unistd.h>
 
 namespace LIBC_NAMESPACE_DECL {
 
-pid_t getpid();
+pid_t getpid(void);
 
 } // namespace LIBC_NAMESPACE_DECL
 

diff --git a/libc/src/unistd/gettid.cpp b/libc/src/unistd/gettid.cpp
@@ -0,0 +1,17 @@
+//===-- Implementation file for gettid --------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/gettid.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/tid.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(pid_t, gettid, (void)) { return gettid_inline(); }
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/unistd/gettid.h b/libc/src/unistd/gettid.h
@@ -0,0 +1,21 @@
+//===-- Implementation header for gettid ------------------------*- 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_GETTID_H
+#define LLVM_LIBC_SRC_UNISTD_GETTID_H
+
+#include "hdr/types/pid_t.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+pid_t gettid(void);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_UNISTD_GETTID_H
diff --git a/libc/src/unistd/linux/CMakeLists.txt b/libc/src/unistd/linux/CMakeLists.txt
@@ -101,6 +101,7 @@ add_entrypoint_object(
     libc.include.sys_syscall
     libc.src.__support.threads.fork_callbacks
     libc.src.__support.OSUtil.osutil
+    libc.src.__support.OSUtil.pid
     libc.src.__support.threads.thread
     libc.src.errno.errno
 )
@@ -204,8 +205,7 @@ add_entrypoint_object(
     ../getpid.h
   DEPENDS
     libc.include.unistd
-    libc.include.sys_syscall
-    libc.src.__support.OSUtil.osutil
+    libc.src.__support.OSUtil.pid
 )
 
 add_entrypoint_object(

diff --git a/libc/src/unistd/linux/fork.cpp b/libc/src/unistd/linux/fork.cpp
@@ -8,13 +8,14 @@
 
 #include "src/unistd/fork.h"
 
+#include "src/__support/OSUtil/pid.h"
 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
 #include "src/__support/common.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/threads/fork_callbacks.h"
 #include "src/__support/threads/thread.h" // For thread self object
-
 #include "src/errno/libc_errno.h"
+
 #include <signal.h>      // For SIGCHLD
 #include <sys/syscall.h> // For syscall numbers.
 
@@ -25,29 +26,40 @@ namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(pid_t, fork, (void)) {
   invoke_prepare_callbacks();
+
+  // Invalidate tid/pid cache before fork to avoid post fork signal handler from
+  // getting wrong values. gettid() is not async-signal-safe, but let's provide
+  // our best efforts here.
+  pid_t parent_tid = self.get_tid();
+  self.invalidate_tid();
+  ProcessIdentity::start_fork();
+
 #ifdef SYS_fork
   pid_t ret = LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_fork);
 #elif defined(SYS_clone)
   pid_t ret = LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_clone, SIGCHLD, 0);
 #else
 #error "fork and clone syscalls not available."
 #endif
-  if (ret == 0) {
-    // Return value is 0 in the child process.
-    // The child is created with a single thread whose self object will be a
-    // copy of parent process' thread which called fork. So, we have to fix up
-    // the child process' self object with the new process' tid.
-    self.attrib->tid = LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_gettid);
-    invoke_child_callbacks();
-    return 0;
-  }
 
   if (ret < 0) {
     // Error case, a child process was not created.
     libc_errno = static_cast<int>(-ret);
     return -1;
   }
 
+  // Child process
+  if (ret == 0) {
+    self.refresh_tid();
+    ProcessIdentity::refresh_cache();
+    ProcessIdentity::end_fork();
+    invoke_child_callbacks();
+    return 0;
+  }
+
+  // Parent process
+  self.refresh_tid(parent_tid);
+  ProcessIdentity::end_fork();
   invoke_parent_callbacks();
   return ret;
 }

diff --git a/libc/src/unistd/linux/getpid.cpp b/libc/src/unistd/linux/getpid.cpp
@@ -7,17 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/unistd/getpid.h"
-
-#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/OSUtil/pid.h"
 #include "src/__support/common.h"
-#include "src/__support/macros/config.h"
-
-#include <sys/syscall.h> // For syscall numbers.
-
 namespace LIBC_NAMESPACE_DECL {
 
-LLVM_LIBC_FUNCTION(pid_t, getpid, ()) {
-  return LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_getpid);
-}
+LLVM_LIBC_FUNCTION(pid_t, getpid, (void)) { return ProcessIdentity::get(); }
 
 } // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/startup/linux/CMakeLists.txt b/libc/startup/linux/CMakeLists.txt
@@ -101,6 +101,7 @@ add_object_library(
     libc.include.llvm-libc-macros.link_macros
     libc.src.__support.threads.thread
     libc.src.__support.OSUtil.osutil
+    libc.src.__support.OSUtil.pid
     libc.src.stdlib.exit
     libc.src.stdlib.atexit
     libc.src.unistd.environ

diff --git a/libc/startup/linux/do_start.cpp b/libc/startup/linux/do_start.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 #include "startup/linux/do_start.h"
 #include "include/llvm-libc-macros/link-macros.h"
+#include "src/__support/OSUtil/pid.h"
 #include "src/__support/OSUtil/syscall.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/threads/thread.h"
@@ -127,6 +128,10 @@ static ThreadAttributes main_thread_attrib;
   if (tls.size != 0 && !set_thread_ptr(tls.tp))
     syscall_impl<long>(SYS_exit, 1);
 
+  // Validate process identity cache (TLS needed).
+  ProcessIdentity::refresh_cache();
+  ProcessIdentity::end_fork();
+
   self.attrib = &main_thread_attrib;
   main_thread_attrib.atexit_callback_mgr =
       internal::get_thread_atexit_callback_mgr();

diff --git a/libc/test/integration/src/unistd/CMakeLists.txt b/libc/test/integration/src/unistd/CMakeLists.txt
@@ -31,6 +31,10 @@ add_integration_test(
     libc.src.sys.wait.wait4
     libc.src.sys.wait.waitpid
     libc.src.unistd.fork
+    libc.src.unistd.getpid
+    libc.src.unistd.gettid
+    libc.src.stdlib.exit
+    libc.include.sys_syscall
 )
 
 if((${LIBC_TARGET_OS} STREQUAL "linux") AND (${LIBC_TARGET_ARCHITECTURE_IS_X86}))

diff --git a/libc/test/integration/src/unistd/fork_test.cpp b/libc/test/integration/src/unistd/fork_test.cpp
@@ -6,17 +6,21 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/OSUtil/syscall.h"
 #include "src/pthread/pthread_atfork.h"
 #include "src/signal/raise.h"
+#include "src/stdlib/exit.h"
 #include "src/sys/wait/wait.h"
 #include "src/sys/wait/wait4.h"
 #include "src/sys/wait/waitpid.h"
 #include "src/unistd/fork.h"
-
+#include "src/unistd/getpid.h"
+#include "src/unistd/gettid.h"
 #include "test/IntegrationTest/test.h"
 
 #include <errno.h>
 #include <signal.h>
+#include <sys/syscall.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
@@ -140,7 +144,25 @@ void fork_with_atfork_callbacks() {
   ASSERT_NE(child, DONE);
 }
 
+void fork_pid_tid_test() {
+  pid_t pid = fork();
+  ASSERT_TRUE(pid >= 0);
+  ASSERT_EQ(LIBC_NAMESPACE::gettid(),
+            LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_gettid));
+  ASSERT_EQ(LIBC_NAMESPACE::getpid(),
+            LIBC_NAMESPACE::syscall_impl<pid_t>(SYS_getpid));
+
+  if (pid == 0) {
+    LIBC_NAMESPACE::exit(0);
+  } else {
+    int status;
+    LIBC_NAMESPACE::waitpid(pid, &status, 0);
+    ASSERT_EQ(status, 0);
+  }
+}
+
 TEST_MAIN(int argc, char **argv, char **envp) {
+  fork_pid_tid_test();
   fork_and_wait_normal_exit();
   fork_and_wait4_normal_exit();
   fork_and_waitpid_normal_exit();

diff --git a/libc/test/src/unistd/CMakeLists.txt b/libc/test/src/unistd/CMakeLists.txt
@@ -378,6 +378,16 @@ add_libc_unittest(
     libc.src.unistd.getpid
 )
 
+add_libc_unittest(
+  gettid_test
+  SUITE
+    libc_unistd_unittests
+  SRCS
+    gettid_test.cpp
+  DEPENDS
+    libc.src.unistd.gettid
+)
+
 add_libc_unittest(
   getppid_test
   SUITE

diff --git a/libc/test/src/unistd/gettid_test.cpp b/libc/test/src/unistd/gettid_test.cpp
@@ -0,0 +1,15 @@
+//===-- Unittests for gettid ----------------------------------------------===//
+//
+// 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/gettid.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcGetTidTest, SmokeTest) {
+  // gettid always succeeds. So, we just call it as a smoke test.
+  ASSERT_GT(LIBC_NAMESPACE::gettid(), 0);
+}