diff --git a/libc/cmake/modules/LLVMLibCHeaderRules.cmake b/libc/cmake/modules/LLVMLibCHeaderRules.cmake
index 0de5e14359cfb..a95c13389012f 100644
--- a/libc/cmake/modules/LLVMLibCHeaderRules.cmake
+++ b/libc/cmake/modules/LLVMLibCHeaderRules.cmake
@@ -75,7 +75,7 @@ function(add_gen_header target_name)
   cmake_parse_arguments(
     "ADD_GEN_HDR"
     "PUBLIC" # No optional arguments
-    "YAML_FILE;DEF_FILE;GEN_HDR" # Single value arguments
+    "YAML_FILE;GEN_HDR" # Single value arguments
     "DEPENDS"     # Multi value arguments
     ${ARGN}
   )
@@ -84,9 +84,6 @@ function(add_gen_header target_name)
     add_library(${fq_target_name} INTERFACE)
     return()
   endif()
-  if(NOT ADD_GEN_HDR_DEF_FILE)
-    message(FATAL_ERROR "`add_gen_hdr` rule requires DEF_FILE to be specified.")
-  endif()
   if(NOT ADD_GEN_HDR_GEN_HDR)
     message(FATAL_ERROR "`add_gen_hdr` rule requires GEN_HDR to be specified.")
   endif()
@@ -97,8 +94,8 @@ function(add_gen_header target_name)
   set(absolute_path ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_GEN_HDR_GEN_HDR})
   file(RELATIVE_PATH relative_path ${LIBC_INCLUDE_SOURCE_DIR} ${absolute_path})
   set(out_file ${LIBC_INCLUDE_DIR}/${relative_path})
+  set(dep_file "${out_file}.d")
   set(yaml_file ${CMAKE_SOURCE_DIR}/${ADD_GEN_HDR_YAML_FILE})
-  set(def_file ${CMAKE_CURRENT_SOURCE_DIR}/${ADD_GEN_HDR_DEF_FILE})
 
   set(fq_data_files "")
   if(ADD_GEN_HDR_DATA_FILES)
@@ -108,18 +105,19 @@ function(add_gen_header target_name)
   endif()
 
   set(entry_points "${TARGET_ENTRYPOINT_NAME_LIST}")
-  list(TRANSFORM entry_points PREPEND "--e=")
+  list(TRANSFORM entry_points PREPEND "--entry-point=")
 
-  set(LIBC_HDRGEN "${LIBC_SOURCE_DIR}/utils/hdrgen/yaml_to_classes.py")
   add_custom_command(
     OUTPUT ${out_file}
-    COMMAND ${Python3_EXECUTABLE} ${LIBC_HDRGEN}
-            ${yaml_file}
-            --h_def_file ${def_file}
+    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+    COMMAND ${Python3_EXECUTABLE} "${LIBC_SOURCE_DIR}/utils/hdrgen/main.py"
+            --output ${out_file}
+            --depfile ${dep_file}
             ${entry_points}
-            --output_dir ${out_file}
-    DEPENDS ${yaml_file} ${def_file} ${fq_data_files}
-    COMMENT "Generating header ${ADD_GEN_HDR_GEN_HDR} from ${yaml_file} and ${def_file}"
+            ${yaml_file}
+    DEPENDS ${yaml_file} ${fq_data_files}
+    DEPFILE ${dep_file}
+    COMMENT "Generating header ${ADD_GEN_HDR_GEN_HDR} from ${yaml_file}"
   )
   if(LIBC_TARGET_OS_IS_GPU)
     file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/llvm-libc-decls)
@@ -127,7 +125,7 @@ function(add_gen_header target_name)
     set(decl_out_file ${LIBC_INCLUDE_DIR}/llvm-libc-decls/${relative_path})
     add_custom_command(
       OUTPUT ${decl_out_file}
-      COMMAND ${Python3_EXECUTABLE} ${LIBC_HDRGEN}
+      COMMAND ${Python3_EXECUTABLE} "${LIBC_SOURCE_DIR}/utils/hdrgen/yaml_to_classes.py"
               ${yaml_file}
               --export-decls
               ${entry_points}
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
index eb407183c99f5..568bb05d92302 100644
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -19,11 +19,10 @@ add_header(
 
 # TODO: Can we simplify this macro expansion?
 # https://github.com/llvm/llvm-project/issues/117254
-macro(add_header_macro TARGET_NAME YAML_FILE DEF_FILE GEN_HDR DEPENDS)
+macro(add_header_macro TARGET_NAME YAML_FILE GEN_HDR DEPENDS)
   add_gen_header(
     ${TARGET_NAME}
     YAML_FILE ${YAML_FILE}
-    DEF_FILE ${DEF_FILE}
     GEN_HDR ${GEN_HDR}
     ${DEPENDS}
     ${ARGN}
@@ -33,7 +32,6 @@ endmacro()
 add_header_macro(
   ctype
   ../libc/include/ctype.yaml
-  ctype.h.def
   ctype.h
   DEPENDS
     .llvm_libc_common_h
@@ -43,7 +41,6 @@ add_header_macro(
 add_header_macro(
   dirent
   ../libc/include/dirent.yaml
-  dirent.h.def
   dirent.h
   DEPENDS
     .llvm_libc_common_h
@@ -55,7 +52,6 @@ add_header_macro(
 add_header_macro(
   fcntl
   ../libc/include/fcntl.yaml
-  fcntl.h.def
   fcntl.h
   DEPENDS
     .llvm-libc-macros.fcntl_macros
@@ -71,7 +67,6 @@ add_header_macro(
 add_header_macro(
   dlfcn
   ../libc/include/dlfcn.yaml
-  dlfcn.h.def
   dlfcn.h
   DEPENDS
     .llvm-libc-macros.dlfcn_macros
@@ -81,7 +76,6 @@ add_header_macro(
 add_header_macro(
   features
   ../libc/include/features.yaml
-  features.h.def
   features.h
   DEPENDS
     .llvm_libc_common_h
@@ -91,7 +85,6 @@ add_header_macro(
 add_header_macro(
   fenv
   ../libc/include/fenv.yaml
-  fenv.h.def
   fenv.h
   DEPENDS
     .llvm_libc_common_h
@@ -103,7 +96,6 @@ add_header_macro(
 add_header_macro(
   inttypes
   ../libc/include/inttypes.yaml
-  inttypes.h.def
   inttypes.h
   DEPENDS
     .llvm_libc_common_h
@@ -114,7 +106,6 @@ add_header_macro(
 add_header_macro(
   float
   ../libc/include/float.yaml
-  float.h.def
   float.h
   DEPENDS
     .llvm-libc-macros.float_macros
@@ -123,7 +114,6 @@ add_header_macro(
 add_header_macro(
   stdint
   ../libc/include/stdint.yaml
-  stdint.h.def
   stdint.h
   DEPENDS
     .llvm-libc-macros.stdint_macros
@@ -132,7 +122,6 @@ add_header_macro(
 add_header_macro(
   limits
   ../libc/include/limits.yaml
-  limits.h.def
   limits.h
   DEPENDS
     .llvm-libc-macros.limits_macros
@@ -141,7 +130,6 @@ add_header_macro(
 add_header_macro(
   malloc
   ../libc/include/malloc.yaml
-  malloc.h.def
   malloc.h
   DEPENDS
     .llvm_libc_common_h
@@ -151,7 +139,6 @@ add_header_macro(
 add_header_macro(
   math
   ../libc/include/math.yaml
-  math.h.def
   math.h
   DEPENDS
     .llvm_libc_common_h
@@ -166,7 +153,6 @@ add_header_macro(
 add_header_macro(
   stdfix
   ../libc/include/stdfix.yaml
-  stdfix.h.def
   stdfix.h
   DEPENDS
     .llvm-libc-macros.stdfix_macros
@@ -179,7 +165,6 @@ file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/arpa)
 add_header_macro(
   arpa_inet
   ../libc/include/arpa/inet.yaml
-  arpa/inet.h.def
   arpa/inet.h
   DEPENDS
     .llvm_libc_common_h
@@ -188,7 +173,6 @@ add_header_macro(
 add_header_macro(
   assert
   ../libc/include/assert.yaml
-  assert.h.def
   assert.h
   DEPENDS
     .llvm_libc_common_h
@@ -198,7 +182,6 @@ add_header_macro(
 add_header_macro(
   complex
   ../libc/include/complex.yaml
-  complex.h.def
   complex.h
   DEPENDS
     .llvm_libc_common_h
@@ -208,7 +191,6 @@ add_header_macro(
 add_header_macro(
   setjmp
   ../libc/include/setjmp.yaml
-  setjmp.h.def
   setjmp.h
   DEPENDS
     .llvm_libc_common_h
@@ -218,7 +200,6 @@ add_header_macro(
 add_header_macro(
   string
   ../libc/include/string.yaml
-  string.h.def
   string.h
   DEPENDS
     .llvm_libc_common_h
@@ -229,7 +210,6 @@ add_header_macro(
 add_header_macro(
   strings
   ../libc/include/strings.yaml
-  strings.h.def
   strings.h
   DEPENDS
     .llvm_libc_common_h
@@ -239,7 +219,6 @@ add_header_macro(
 add_header_macro(
   search
   ../libc/include/search.yaml
-  search.h.def
   search.h
   DEPENDS
     .llvm_libc_common_h
@@ -253,7 +232,6 @@ add_header_macro(
 add_header_macro(
   time
   ../libc/include/time.yaml
-  time.h.def
   time.h
   DEPENDS
     .llvm_libc_common_h
@@ -269,7 +247,6 @@ add_header_macro(
 add_header_macro(
   threads
   ../libc/include/threads.yaml
-  threads.h.def
   threads.h
   DEPENDS
     .llvm_libc_common_h
@@ -286,7 +263,6 @@ add_header_macro(
 add_header_macro(
   errno
   ../libc/include/errno.yaml
-  errno.h.def
   errno.h
   DEPENDS
     .llvm-libc-macros.generic_error_number_macros
@@ -296,7 +272,6 @@ add_header_macro(
 add_header_macro(
   signal
   ../libc/include/signal.yaml
-  signal.h.def
   signal.h
   DEPENDS
     .llvm-libc-macros.signal_macros
@@ -312,7 +287,6 @@ add_header_macro(
 add_header_macro(
   stdbit
   ../libc/include/stdbit.yaml
-  stdbit.h.def
   stdbit.h
   DEPENDS
     .llvm_libc_common_h
@@ -322,7 +296,6 @@ add_header_macro(
 add_header_macro(
   stdckdint
   ../libc/include/stdckdint.yaml
-  stdckdint.h.def
   stdckdint.h
   DEPENDS
     .llvm_libc_common_h
@@ -332,7 +305,6 @@ add_header_macro(
 add_header_macro(
   stdio
   ../libc/include/stdio.yaml
-  stdio.h.def
   stdio.h
   DEPENDS
     .llvm-libc-macros.file_seek_macros
@@ -348,7 +320,6 @@ add_header_macro(
 add_header_macro(
   stdlib
   ../libc/include/stdlib.yaml
-  stdlib.h.def
   stdlib.h
   DEPENDS
     .llvm_libc_common_h
@@ -367,7 +338,6 @@ add_header_macro(
 add_header_macro(
   unistd
   ../libc/include/unistd.yaml
-  unistd.h.def
   unistd.h
   DEPENDS
     .llvm_libc_common_h
@@ -386,7 +356,6 @@ add_header_macro(
 add_header_macro(
   pthread
   ../libc/include/pthread.yaml
-  pthread.h.def
   pthread.h
   DEPENDS
     .llvm-libc-macros.pthread_macros
@@ -410,7 +379,6 @@ add_header_macro(
 add_header_macro(
   sched
   ../libc/include/sched.yaml
-  sched.h.def
   sched.h
   DEPENDS
     .llvm_libc_common_h
@@ -427,7 +395,6 @@ add_header_macro(
 add_header_macro(
   spawn
   ../libc/include/spawn.yaml
-  spawn.h.def
   spawn.h
   DEPENDS
     .llvm_libc_common_h
@@ -440,7 +407,6 @@ add_header_macro(
 add_header_macro(
   link
   ../libc/include/link.yaml
-  link.h.def
   link.h
   DEPENDS
     .llvm_libc_common_h
@@ -450,7 +416,6 @@ add_header_macro(
 add_header_macro(
   elf
   ../libc/include/elf.yaml
-  elf.h.def
   elf.h
   DEPENDS
     .llvm-libc-macros.elf_macros
@@ -464,7 +429,6 @@ file(MAKE_DIRECTORY ${LIBC_INCLUDE_DIR}/sys)
 add_header_macro(
   sys_auxv
   ../libc/include/sys/auxv.yaml
-  sys/auxv.h.def
   sys/auxv.h
   DEPENDS
     .llvm_libc_common_h
@@ -474,7 +438,6 @@ add_header_macro(
 add_header_macro(
   sys_epoll
   ../libc/include/sys/epoll.yaml
-  sys/epoll.h.def
   sys/epoll.h
   DEPENDS
     .llvm_libc_common_h
@@ -487,7 +450,6 @@ add_header_macro(
 add_header_macro(
   sys_ioctl
   ../libc/include/sys/ioctl.yaml
-  sys/ioctl.h.def
   sys/ioctl.h
   DEPENDS
     .llvm_libc_common_h
@@ -497,7 +459,6 @@ add_header_macro(
 add_header_macro(
   sys_mman
   ../libc/include/sys/mman.yaml
-  sys/mman.h.def
   sys/mman.h
   DEPENDS
     .llvm_libc_common_h
@@ -510,7 +471,6 @@ add_header_macro(
 add_header_macro(
   sys_prctl
   ../libc/include/sys/prctl.yaml
-  sys/prctl.h.def
   sys/prctl.h
   DEPENDS
     .llvm_libc_common_h
@@ -527,7 +487,6 @@ add_header(
 add_header_macro(
   sys_random
   ../libc/include/sys/random.yaml
-  sys/random.h.def
   sys/random.h
   DEPENDS
     .llvm_libc_common_h
@@ -539,7 +498,6 @@ add_header_macro(
 add_header_macro(
   sys_resource
   ../libc/include/sys/resource.yaml
-  sys/resource.h.def
   sys/resource.h
   DEPENDS
     .llvm_libc_common_h
@@ -551,7 +509,6 @@ add_header_macro(
 add_header_macro(
   sys_stat
   ../libc/include/sys/stat.yaml
-  sys/stat.h.def
   sys/stat.h
   DEPENDS
     .llvm_libc_common_h
@@ -573,7 +530,6 @@ add_header_macro(
 add_header_macro(
   sys_select
   ../libc/include/sys/select.yaml
-  sys/select.h.def
   sys/select.h
   DEPENDS
     .llvm_libc_common_h
@@ -589,7 +545,6 @@ add_header_macro(
 add_header_macro(
   sys_sendfile
   ../libc/include/sys/sendfile.yaml
-  sys/sendfile.h.def
   sys/sendfile.h
   DEPENDS
     .llvm_libc_common_h
@@ -601,7 +556,6 @@ add_header_macro(
 add_header_macro(
   sys_socket
   ../libc/include/sys/socket.yaml
-  sys/socket.h.def
   sys/socket.h
   DEPENDS
     .llvm_libc_common_h
@@ -617,7 +571,6 @@ add_header_macro(
 add_header_macro(
   sys_statvfs
   ../libc/include/sys/statvfs.yaml
-  sys/statvfs.h.def
   sys/statvfs.h
   DEPENDS
     .llvm_libc_common_h
@@ -627,7 +580,6 @@ add_header_macro(
 add_header_macro(
   sys_syscall
   ../libc/include/sys/syscall.yaml
-  sys/syscall.h.def
   sys/syscall.h
   DEPENDS
 )
@@ -635,7 +587,6 @@ add_header_macro(
 add_header_macro(
   sys_time
   ../libc/include/sys/time.yaml
-  sys/time.h.def
   sys/time.h
   DEPENDS
     .llvm_libc_common_h
@@ -646,7 +597,6 @@ add_header_macro(
 add_header_macro(
   sys_types
   ../libc/include/sys/types.yaml
-  sys/types.h.def
   sys/types.h
   DEPENDS
     .llvm_libc_common_h
@@ -676,7 +626,6 @@ add_header_macro(
 add_header_macro(
   sys_utsname
   ../libc/include/sys/utsname.yaml
-  sys/utsname.h.def
   sys/utsname.h
   DEPENDS
     .llvm_libc_common_h
@@ -686,7 +635,6 @@ add_header_macro(
 add_header_macro(
   sys_wait
   ../libc/include/sys/wait.yaml
-  sys/wait.h.def
   sys/wait.h
   DEPENDS
     .llvm_libc_common_h
@@ -699,7 +647,6 @@ add_header_macro(
 add_header_macro(
   termios
   ../libc/include/termios.yaml
-  termios.h.def
   termios.h
   DEPENDS
     .llvm_libc_common_h
@@ -714,7 +661,6 @@ add_header_macro(
 add_header_macro(
   uchar
   ../libc/include/uchar.yaml
-  uchar.h.def
   uchar.h
   DEPENDS
     .llvm_libc_common_h
@@ -727,7 +673,6 @@ add_header_macro(
 add_header_macro(
   wchar
   ../libc/include/wchar.yaml
-  wchar.h.def
   wchar.h
   DEPENDS
     .llvm_libc_common_h
@@ -741,7 +686,6 @@ add_header_macro(
 add_header_macro(
   locale
   ../libc/include/locale.yaml
-  locale.h.def
   locale.h
   DEPENDS
     .llvm_libc_common_h
diff --git a/libc/include/arpa/inet.yaml b/libc/include/arpa/inet.yaml
index cb366e0f5d694..10cd56d6ce786 100644
--- a/libc/include/arpa/inet.yaml
+++ b/libc/include/arpa/inet.yaml
@@ -1,4 +1,5 @@
-header: arpa-inet.h
+header: arpa/inet.h
+header_template: inet.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/assert.yaml b/libc/include/assert.yaml
index f740554488ed5..1a3bdeda7e542 100644
--- a/libc/include/assert.yaml
+++ b/libc/include/assert.yaml
@@ -1,4 +1,5 @@
 header: assert.h
+header_template: assert.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/complex.yaml b/libc/include/complex.yaml
index cd81de7dd9e20..05318480a02f1 100644
--- a/libc/include/complex.yaml
+++ b/libc/include/complex.yaml
@@ -1,4 +1,5 @@
 header: complex.h
+header_template: complex.h.def
 macros: []
 types:
   - type_name: cfloat16
diff --git a/libc/include/ctype.yaml b/libc/include/ctype.yaml
index b4823c3e53234..6238f1b889986 100644
--- a/libc/include/ctype.yaml
+++ b/libc/include/ctype.yaml
@@ -1,4 +1,5 @@
 header: ctype.h
+header_template: ctype.h.def
 macros: []
 types:
   - type_name: locale_t
diff --git a/libc/include/dirent.yaml b/libc/include/dirent.yaml
index cdccf6a0c7f29..3fc522fda80e4 100644
--- a/libc/include/dirent.yaml
+++ b/libc/include/dirent.yaml
@@ -1,4 +1,5 @@
 header: dirent.h
+header_template: dirent.h.def
 macros: []
 types:
   - type_name: struct_dirent
diff --git a/libc/include/dlfcn.yaml b/libc/include/dlfcn.yaml
index 725ee705714a7..9e8803cb5fa78 100644
--- a/libc/include/dlfcn.yaml
+++ b/libc/include/dlfcn.yaml
@@ -1,4 +1,5 @@
 header: dlfcn.h
+header_template: dlfcn.h.def
 macros:
   - macro_name: RTLD_LAZY
     macro_value: null
diff --git a/libc/include/elf.yaml b/libc/include/elf.yaml
index 2e9db329e2297..f78ae82c77850 100644
--- a/libc/include/elf.yaml
+++ b/libc/include/elf.yaml
@@ -1,4 +1,5 @@
 header: elf.h
+header_template: elf.h.def
 standards:
   - Linux
 macros: []
diff --git a/libc/include/errno.yaml b/libc/include/errno.yaml
index a894063a1ee2c..188a9fa1211a1 100644
--- a/libc/include/errno.yaml
+++ b/libc/include/errno.yaml
@@ -1,4 +1,5 @@
 header: errno.h
+header_template: errno.h.def
 standards:
   - stdc
   - Linux
diff --git a/libc/include/fcntl.yaml b/libc/include/fcntl.yaml
index 71c0df3b0fada..78f93533b84d3 100644
--- a/libc/include/fcntl.yaml
+++ b/libc/include/fcntl.yaml
@@ -1,4 +1,5 @@
 header: fcntl.h
+header_template: fcntl.h.def
 macros: []
 types:
   - type_name: off_t
diff --git a/libc/include/features.yaml b/libc/include/features.yaml
index a18af22edb743..726320a40881d 100644
--- a/libc/include/features.yaml
+++ b/libc/include/features.yaml
@@ -1,4 +1,5 @@
 header: features.h
+header_template: features.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/fenv.yaml b/libc/include/fenv.yaml
index 1010efc6402c1..1ecaf63085504 100644
--- a/libc/include/fenv.yaml
+++ b/libc/include/fenv.yaml
@@ -1,4 +1,5 @@
 header: fenv.h
+header_template: fenv.h.def
 macros: []
 types:
   - type_name: fenv_t
diff --git a/libc/include/float.yaml b/libc/include/float.yaml
index 63639a6e8ed13..21df6513e77e4 100644
--- a/libc/include/float.yaml
+++ b/libc/include/float.yaml
@@ -1,4 +1,5 @@
 header: float.h
+header_template: float.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/inttypes.yaml b/libc/include/inttypes.yaml
index ad636cc5121a1..d5dec5b465ba4 100644
--- a/libc/include/inttypes.yaml
+++ b/libc/include/inttypes.yaml
@@ -1,4 +1,5 @@
 header: inttypes.h
+header_template: inttypes.h.def
 macros: []
 types:
   - type_name: imaxdiv_t
diff --git a/libc/include/limits.yaml b/libc/include/limits.yaml
index bf33ed24e7a8d..b664041bb56c2 100644
--- a/libc/include/limits.yaml
+++ b/libc/include/limits.yaml
@@ -1,4 +1,5 @@
 header: limits.h
+header_template: limits.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/link.yaml b/libc/include/link.yaml
index d1963a86813af..1cd609e292b53 100644
--- a/libc/include/link.yaml
+++ b/libc/include/link.yaml
@@ -1,4 +1,5 @@
 header: link.h
+header_template: link.h.def
 standards:
   - Linux
 macros: []
diff --git a/libc/include/locale.yaml b/libc/include/locale.yaml
index 7da7966ea730f..9ff53c16398a5 100644
--- a/libc/include/locale.yaml
+++ b/libc/include/locale.yaml
@@ -1,4 +1,5 @@
 header: locale.h
+header_template: locale.h.def
 functions:
   - name: localeconv
     standards:
diff --git a/libc/include/malloc.yaml b/libc/include/malloc.yaml
index 8db4f3aebb9b3..ec73c9090f729 100644
--- a/libc/include/malloc.yaml
+++ b/libc/include/malloc.yaml
@@ -1,4 +1,5 @@
 header: malloc.h
+header_template: malloc.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/math.yaml b/libc/include/math.yaml
index 3b8caec66bbfd..831d045745677 100644
--- a/libc/include/math.yaml
+++ b/libc/include/math.yaml
@@ -1,4 +1,5 @@
 header: math.h
+header_template: math.h.def
 macros: []
 types:
   - type_name: float_t
diff --git a/libc/include/pthread.yaml b/libc/include/pthread.yaml
index b9068c3f17657..4f386bdd11cfd 100644
--- a/libc/include/pthread.yaml
+++ b/libc/include/pthread.yaml
@@ -1,4 +1,5 @@
 header: pthread.h
+header_template: pthread.h.def
 macros: []
 types:
   - type_name: pthread_t
diff --git a/libc/include/sched.yaml b/libc/include/sched.yaml
index 2d4876b722ab2..57871f524bf11 100644
--- a/libc/include/sched.yaml
+++ b/libc/include/sched.yaml
@@ -1,4 +1,5 @@
 header: sched.h
+header_template: sched.h.def
 macros: []
 types:
   - type_name: struct_timespec
diff --git a/libc/include/search.yaml b/libc/include/search.yaml
index a0c73bc679d81..b7ce06d48e704 100644
--- a/libc/include/search.yaml
+++ b/libc/include/search.yaml
@@ -1,4 +1,5 @@
 header: search.h
+header_template: search.h.def
 macros: []
 types:
   - type_name: struct_hsearch_data
diff --git a/libc/include/setjmp.yaml b/libc/include/setjmp.yaml
index 68e3ff046e4b8..2c4f7fb6dfcc7 100644
--- a/libc/include/setjmp.yaml
+++ b/libc/include/setjmp.yaml
@@ -1,4 +1,5 @@
 header: setjmp.h
+header_template: setjmp.h.def
 macros: []
 types:
   - type_name: jmp_buf
diff --git a/libc/include/signal.yaml b/libc/include/signal.yaml
index c66abb1a87441..576e77576ac74 100644
--- a/libc/include/signal.yaml
+++ b/libc/include/signal.yaml
@@ -1,4 +1,5 @@
 header: signal.h
+header_template: signal.h.def
 macros: []
 types:
   - type_name: pid_t
diff --git a/libc/include/spawn.yaml b/libc/include/spawn.yaml
index be3f4e99d27fc..e725ab9719eda 100644
--- a/libc/include/spawn.yaml
+++ b/libc/include/spawn.yaml
@@ -1,4 +1,5 @@
 header: spawn.h
+header_template: spawn.h.def
 macros: []
 types:
   - type_name: posix_spawn_file_actions_t
diff --git a/libc/include/stdbit.yaml b/libc/include/stdbit.yaml
index 25d2d326c30eb..e9bd6b3918e78 100644
--- a/libc/include/stdbit.yaml
+++ b/libc/include/stdbit.yaml
@@ -1,4 +1,5 @@
 header: stdbit.h
+header_template: stdbit.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/stdckdint.yaml b/libc/include/stdckdint.yaml
index ea8fc47625b03..e8b2e80ee029f 100644
--- a/libc/include/stdckdint.yaml
+++ b/libc/include/stdckdint.yaml
@@ -1,4 +1,5 @@
 header: stdckdint.h
+header_template: stdckdint.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/stdfix.yaml b/libc/include/stdfix.yaml
index 9787eaba45e4e..7b3bdba082dd5 100644
--- a/libc/include/stdfix.yaml
+++ b/libc/include/stdfix.yaml
@@ -1,4 +1,5 @@
 header: stdfix.h
+header_template: stdfix.h.def
 macros: []
 types: 
   - type_name: stdfix-types
diff --git a/libc/include/stdint.yaml b/libc/include/stdint.yaml
index 8887f596bc8aa..d583a104af374 100644
--- a/libc/include/stdint.yaml
+++ b/libc/include/stdint.yaml
@@ -1,4 +1,5 @@
 header: stdint.h
+header_template: stdint.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/stdio.yaml b/libc/include/stdio.yaml
index fd116bbc00895..2619984cca264 100644
--- a/libc/include/stdio.yaml
+++ b/libc/include/stdio.yaml
@@ -1,4 +1,5 @@
 header: stdio.h
+header_template: stdio.h.def
 macros:
   - macro_name: stdout
     macro_value: stdout
diff --git a/libc/include/stdlib.yaml b/libc/include/stdlib.yaml
index c6c95e421cee3..4b68f272613b1 100644
--- a/libc/include/stdlib.yaml
+++ b/libc/include/stdlib.yaml
@@ -1,4 +1,5 @@
 header: stdlib.h
+header_template: stdlib.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/string.yaml b/libc/include/string.yaml
index af1750e91243e..deded309abc2c 100644
--- a/libc/include/string.yaml
+++ b/libc/include/string.yaml
@@ -1,4 +1,5 @@
 header: string.h
+header_template: string.h.def
 macros: []
 types:
   - type_name: size_t
diff --git a/libc/include/strings.yaml b/libc/include/strings.yaml
index ca91b626740c1..e672dca6a94dd 100644
--- a/libc/include/strings.yaml
+++ b/libc/include/strings.yaml
@@ -1,4 +1,5 @@
 header: strings.h
+header_template: strings.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/sys/auxv.yaml b/libc/include/sys/auxv.yaml
index 9d546b3588243..82ecee75c40a1 100644
--- a/libc/include/sys/auxv.yaml
+++ b/libc/include/sys/auxv.yaml
@@ -1,4 +1,5 @@
-header: sys-auxv.h
+header: sys/auxv.h
+header_template: auxv.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/sys/epoll.yaml b/libc/include/sys/epoll.yaml
index ee188c17fedc6..996eb785bc715 100644
--- a/libc/include/sys/epoll.yaml
+++ b/libc/include/sys/epoll.yaml
@@ -1,4 +1,5 @@
-header: sys-epoll.h
+header: sys/epoll.h
+header_template: epoll.h.def
 macros: []
 types:
   - type_name: struct_epoll_event
diff --git a/libc/include/sys/ioctl.yaml b/libc/include/sys/ioctl.yaml
index ffe73a84d51b9..5f7b7f333191e 100644
--- a/libc/include/sys/ioctl.yaml
+++ b/libc/include/sys/ioctl.yaml
@@ -1,4 +1,5 @@
-header: sys-ioctl.h
+header: sys/ioctl.h
+header_template: ioctl.h.def
 standards: POSIX
 macros: []
 types: []
diff --git a/libc/include/sys/mman.yaml b/libc/include/sys/mman.yaml
index 962ca3591917f..8c207552f9805 100644
--- a/libc/include/sys/mman.yaml
+++ b/libc/include/sys/mman.yaml
@@ -1,4 +1,5 @@
-header: sys-mman.h
+header: sys/mman.h
+header_template: mman.h.def
 macros: []
 types:
   - type_name: mode_t
diff --git a/libc/include/sys/prctl.yaml b/libc/include/sys/prctl.yaml
index 82374be87d5d0..53f57645b18e2 100644
--- a/libc/include/sys/prctl.yaml
+++ b/libc/include/sys/prctl.yaml
@@ -1,4 +1,5 @@
-header: sys-prctl.h
+header: sys/prctl.h
+header_template: prctl.h.def
 macros: []
 types: []
 enums: []
diff --git a/libc/include/sys/random.yaml b/libc/include/sys/random.yaml
index 228bb50d5db99..4efb2fbb44733 100644
--- a/libc/include/sys/random.yaml
+++ b/libc/include/sys/random.yaml
@@ -1,4 +1,5 @@
-header: sys-random.h
+header: sys/random.h
+header_template: random.h.def
 macros: []
 types:
   - type_name: ssize_t
diff --git a/libc/include/sys/resource.yaml b/libc/include/sys/resource.yaml
index 85ea1ad12f19f..3652d6d490a49 100644
--- a/libc/include/sys/resource.yaml
+++ b/libc/include/sys/resource.yaml
@@ -1,4 +1,5 @@
-header: sys-resource.h
+header: sys/resource.h
+header_template: resource.h.def
 macros: []
 types:
   - type_name: struct_rlimit
diff --git a/libc/include/sys/select.yaml b/libc/include/sys/select.yaml
index c6806122aa816..6066fd341f077 100644
--- a/libc/include/sys/select.yaml
+++ b/libc/include/sys/select.yaml
@@ -1,4 +1,5 @@
-header: sys-select.h
+header: sys/select.h
+header_template: select.h.def
 macros: []
 types:
   - type_name: struct_timeval
diff --git a/libc/include/sys/sendfile.yaml b/libc/include/sys/sendfile.yaml
index 7e45e40e171da..259ab83dff54b 100644
--- a/libc/include/sys/sendfile.yaml
+++ b/libc/include/sys/sendfile.yaml
@@ -1,4 +1,5 @@
-header: sys-sendfile.h
+header: sys/sendfile.h
+header_template: sendfile.h.def
 macros: []
 types:
   - type_name: ssize_t
diff --git a/libc/include/sys/socket.yaml b/libc/include/sys/socket.yaml
index 47d835fa5f4a1..00d5de6af8a80 100644
--- a/libc/include/sys/socket.yaml
+++ b/libc/include/sys/socket.yaml
@@ -1,4 +1,5 @@
-header: sys-socket.h
+header: sys/socket.h
+header_template: socket.h.def
 macros: []
 types:
   - type_name: struct_sockaddr_un
diff --git a/libc/include/sys/stat.yaml b/libc/include/sys/stat.yaml
index ed500f832f90e..7f013420818ab 100644
--- a/libc/include/sys/stat.yaml
+++ b/libc/include/sys/stat.yaml
@@ -1,4 +1,5 @@
-header: sys-stat.h
+header: sys/stat.h
+header_template: stat.h.def
 macros: []
 types:
   - type_name: blkcnt_t
diff --git a/libc/include/sys/statvfs.yaml b/libc/include/sys/statvfs.yaml
index 22e0ef22c4139..8c1d254add37f 100644
--- a/libc/include/sys/statvfs.yaml
+++ b/libc/include/sys/statvfs.yaml
@@ -1,4 +1,5 @@
-header: sys-statvfs.h
+header: sys/statvfs.h
+header_template: statvfs.h.def
 macros: []
 types:
   - type_name: struct_statvfs
diff --git a/libc/include/sys/syscall.yaml b/libc/include/sys/syscall.yaml
index c0a64338b6f7e..879d95c2ea39c 100644
--- a/libc/include/sys/syscall.yaml
+++ b/libc/include/sys/syscall.yaml
@@ -1,4 +1,5 @@
-header: sys-syscall.h
+header: sys/syscall.h
+header_template: syscall.h.def
 standards: Linux
 macros: []
 types: []
diff --git a/libc/include/sys/time.yaml b/libc/include/sys/time.yaml
index eb3dd548389b3..687c1f83028d0 100644
--- a/libc/include/sys/time.yaml
+++ b/libc/include/sys/time.yaml
@@ -1,4 +1,5 @@
-header: sys-time.h
+header: sys/time.h
+header_template: time.h.def
 standards: Linux
 macros: []
 types: []
diff --git a/libc/include/sys/types.yaml b/libc/include/sys/types.yaml
index 15eaf107f6915..6fa0b448fcd38 100644
--- a/libc/include/sys/types.yaml
+++ b/libc/include/sys/types.yaml
@@ -1,4 +1,5 @@
-header: sys-types.h
+header: sys/types.h
+header_template: types.h.def
 standards: POSIX
 macros: []
 types:
diff --git a/libc/include/sys/utsname.yaml b/libc/include/sys/utsname.yaml
index eecd55b180829..6c7cb71f9a34f 100644
--- a/libc/include/sys/utsname.yaml
+++ b/libc/include/sys/utsname.yaml
@@ -1,4 +1,5 @@
-header: sys-utsname.h
+header: sys/utsname.h
+header_template: utsname.h.def
 macros: []
 types:
   - type_name: struct_utsname
diff --git a/libc/include/sys/wait.yaml b/libc/include/sys/wait.yaml
index 4f0c69baee2c4..6257e34b9e08e 100644
--- a/libc/include/sys/wait.yaml
+++ b/libc/include/sys/wait.yaml
@@ -1,4 +1,5 @@
-header: sys-wait.h
+header: sys/wait.h
+header_template: wait.h.def
 macros: []
 types:
   - type_name: siginfo_t
diff --git a/libc/include/termios.yaml b/libc/include/termios.yaml
index e9c4cd375d0b9..8815097264f95 100644
--- a/libc/include/termios.yaml
+++ b/libc/include/termios.yaml
@@ -1,4 +1,5 @@
 header: termios.h
+header_template: termios.h.def
 macros: []
 types:
   - type_name: tcflag_t
diff --git a/libc/include/threads.yaml b/libc/include/threads.yaml
index aadcaf5f66e06..7014822f9251d 100644
--- a/libc/include/threads.yaml
+++ b/libc/include/threads.yaml
@@ -1,4 +1,5 @@
 header: threads.h
+header_template: threads.h.def
 macros:
   - macro_name: ONCE_FLAG_INIT
     macro_value: '{0}'
diff --git a/libc/include/time.yaml b/libc/include/time.yaml
index 3f745e5ee3386..b71b9ab72075b 100644
--- a/libc/include/time.yaml
+++ b/libc/include/time.yaml
@@ -1,4 +1,5 @@
 header: time.h
+header_template: time.h.def
 macros: []
 types:
   - type_name: struct_timeval
diff --git a/libc/include/uchar.yaml b/libc/include/uchar.yaml
index 18ca840612e07..713919796762d 100644
--- a/libc/include/uchar.yaml
+++ b/libc/include/uchar.yaml
@@ -1,4 +1,5 @@
 header: uchar.h
+header_template: uchar.h.def
 standards:
   - stdc
 macros: []
diff --git a/libc/include/unistd.yaml b/libc/include/unistd.yaml
index c6441c04ce3a3..fada365e0103d 100644
--- a/libc/include/unistd.yaml
+++ b/libc/include/unistd.yaml
@@ -1,4 +1,5 @@
 header: unistd.h
+header_template: unistd.h.def
 macros: []
 types:
   - type_name: uid_t
diff --git a/libc/include/wchar.yaml b/libc/include/wchar.yaml
index bc824b21d8be1..27a5926b57455 100644
--- a/libc/include/wchar.yaml
+++ b/libc/include/wchar.yaml
@@ -1,4 +1,5 @@
 header: wchar.h
+header_template: wchar.h.def
 macros: []
 types:
   - type_name: size_t
diff --git a/libc/utils/hdrgen/header.py b/libc/utils/hdrgen/header.py
index df8ce613bd0f9..9339acceaf7a9 100644
--- a/libc/utils/hdrgen/header.py
+++ b/libc/utils/hdrgen/header.py
@@ -9,6 +9,7 @@
 
 class HeaderFile:
     def __init__(self, name):
+        self.template_file = None
         self.name = name
         self.macros = []
         self.types = []
@@ -31,7 +32,7 @@ def add_object(self, object):
     def add_function(self, function):
         self.functions.append(function)
 
-    def __str__(self):
+    def public_api(self):
         content = [""]
 
         for macro in self.macros:
diff --git a/libc/utils/hdrgen/main.py b/libc/utils/hdrgen/main.py
new file mode 100755
index 0000000000000..2bba1efa04e42
--- /dev/null
+++ b/libc/utils/hdrgen/main.py
@@ -0,0 +1,81 @@
+#!/usr/bin/env python3
+#
+# ===- Generate headers for libc functions  ------------------*- python -*--==#
+#
+# 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
+#
+# ==------------------------------------------------------------------------==#
+
+import argparse
+import sys
+from pathlib import Path
+
+from header import HeaderFile
+from yaml_to_classes import load_yaml_file, fill_public_api
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Generate header files from YAML")
+    parser.add_argument(
+        "yaml_file",
+        help="Path to the YAML file containing header specification",
+        metavar="FILE",
+        type=Path,
+        nargs=1,
+    )
+    parser.add_argument(
+        "-o",
+        "--output",
+        help="Path to write generated header file",
+        type=Path,
+        required=True,
+    )
+    parser.add_argument(
+        "--depfile",
+        help="Path to write a depfile",
+        type=Path,
+    )
+    parser.add_argument(
+        "-e",
+        "--entry-point",
+        help="Entry point to include; may be given many times",
+        metavar="SYMBOL",
+        action="append",
+    )
+    args = parser.parse_args()
+
+    [yaml_file] = args.yaml_file
+    files_read = {yaml_file}
+
+    def write_depfile():
+        if not args.depfile:
+            return
+        deps = " ".join(str(f) for f in sorted(files_read))
+        args.depfile.parent.mkdir(parents=True, exist_ok=True)
+        with open(args.depfile, "w") as depfile:
+            depfile.write(f"{args.output}: {deps}\n")
+
+    header = load_yaml_file(yaml_file, HeaderFile, args.entry_point)
+
+    if not header.template_file:
+        print(f"{yaml_file}: Missing header_template", sys.stderr)
+        return 2
+
+    # The header_template path is relative to the containing YAML file.
+    template_path = yaml_file.parent / header.template_file
+
+    files_read.add(template_path)
+    with open(template_path) as template:
+        contents = fill_public_api(header.public_api(), template.read())
+
+    write_depfile()
+
+    args.output.parent.mkdir(parents=True, exist_ok=True)
+    with open(args.output, "w") as out:
+        out.write(contents)
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/libc/utils/hdrgen/tests/input/test_small.yaml b/libc/utils/hdrgen/tests/input/test_small.yaml
index 772552faf81dc..1d4b2990a3002 100644
--- a/libc/utils/hdrgen/tests/input/test_small.yaml
+++ b/libc/utils/hdrgen/tests/input/test_small.yaml
@@ -1,4 +1,5 @@
-header: test_header.h
+header: test_small.h
+header_template: test_small.h.def
 macros:
   - macro_name: MACRO_A
     macro_value: 1
@@ -62,5 +63,3 @@ functions:
       - type: float
     standards:
       - stdc
-
-
diff --git a/libc/utils/hdrgen/tests/test_integration.py b/libc/utils/hdrgen/tests/test_integration.py
index ce80026e7bccd..49cb08cd1b339 100644
--- a/libc/utils/hdrgen/tests/test_integration.py
+++ b/libc/utils/hdrgen/tests/test_integration.py
@@ -1,36 +1,27 @@
+import argparse
 import subprocess
+import sys
 import unittest
 from pathlib import Path
-import os
-import argparse
-import sys
 
 
 class TestHeaderGenIntegration(unittest.TestCase):
     def setUp(self):
-        self.output_dir = Path(
-            args.output_dir if args.output_dir else "libc/utils/hdrgen/tests/output"
-        )
-
-        self.maxDiff = None
-
-        self.source_dir = Path(__file__).resolve().parent.parent.parent.parent.parent
+        self.output_dir = TestHeaderGenIntegration.output_dir
+        self.source_dir = Path(__file__).parent
+        self.main_script = self.source_dir.parent / "main.py"
 
-    def run_script(self, yaml_file, h_def_file, output_dir, entry_points):
-        yaml_file = self.source_dir / yaml_file
-        h_def_file = self.source_dir / h_def_file
+    def run_script(self, yaml_file, output_file, entry_points):
         command = [
             "python3",
-            str(self.source_dir / "libc/utils/hdrgen/yaml_to_classes.py"),
+            str(self.main_script),
             str(yaml_file),
-            "--h_def_file",
-            str(h_def_file),
-            "--output_dir",
-            str(output_dir),
+            "--output",
+            str(output_file),
         ]
 
         for entry_point in entry_points:
-            command.extend(["--e", entry_point])
+            command.extend(["--entry-point", entry_point])
 
         result = subprocess.run(
             command,
@@ -51,26 +42,23 @@ def compare_files(self, generated_file, expected_file):
         self.assertEqual(gen_content, exp_content)
 
     def test_generate_header(self):
-        yaml_file = "libc/utils/hdrgen/tests/input/test_small.yaml"
-        h_def_file = "libc/utils/hdrgen/tests/input/test_small.h.def"
-        expected_output_file = (
-            self.source_dir / "libc/utils/hdrgen/tests/expected_output/test_header.h"
-        )
+        yaml_file = self.source_dir / "input/test_small.yaml"
+        expected_output_file = self.source_dir / "expected_output/test_header.h"
         output_file = self.output_dir / "test_small.h"
         entry_points = {"func_b", "func_a", "func_c", "func_d", "func_e"}
 
-        if not self.output_dir.exists():
-            self.output_dir.mkdir(parents=True)
-
-        self.run_script(yaml_file, h_def_file, self.output_dir, entry_points)
+        self.run_script(yaml_file, output_file, entry_points)
 
         self.compare_files(output_file, expected_output_file)
 
 
-if __name__ == "__main__":
+def main():
     parser = argparse.ArgumentParser(description="TestHeaderGenIntegration arguments")
     parser.add_argument(
-        "--output_dir", type=str, help="Output directory for generated headers"
+        "--output_dir",
+        type=Path,
+        help="Output directory for generated headers",
+        required=True,
     )
     args, remaining_argv = parser.parse_known_args()
 
@@ -79,3 +67,7 @@ def test_generate_header(self):
     sys.argv[1:] = remaining_argv
 
     unittest.main()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/libc/utils/hdrgen/yaml_to_classes.py b/libc/utils/hdrgen/yaml_to_classes.py
index ec2441b78aee5..d64feafc260b7 100644
--- a/libc/utils/hdrgen/yaml_to_classes.py
+++ b/libc/utils/hdrgen/yaml_to_classes.py
@@ -35,6 +35,7 @@ def yaml_to_classes(yaml_data, header_class, entry_points=None):
     """
     header_name = yaml_data.get("header")
     header = header_class(header_name)
+    header.template_file = yaml_data.get("header_template")
 
     for macro_data in yaml_data.get("macros", []):
         header.add_macro(Macro(macro_data["macro_name"], macro_data["macro_value"]))
@@ -226,10 +227,6 @@ def main():
         "--output_dir",
         help="Directory to output the generated header file",
     )
-    parser.add_argument(
-        "--h_def_file",
-        help="Path to the .h.def template file (required if not using --export_decls)",
-    )
     parser.add_argument(
         "--add_function",
         nargs=6,
@@ -244,7 +241,10 @@ def main():
         help="Add a function to the YAML file",
     )
     parser.add_argument(
-        "--e", action="append", help="Entry point to include", dest="entry_points"
+        "--entry-point",
+        action="append",
+        help="Entry point to include",
+        dest="entry_points",
     )
     parser.add_argument(
         "--export-decls",
@@ -268,13 +268,7 @@ def main():
     else:
         output_file_path = Path(f"{Path(args.yaml_file).stem}.h")
 
-    if not args.export_decls and args.h_def_file:
-        with open(args.h_def_file, "r") as f:
-            h_def_content = f.read()
-        final_header_content = fill_public_api(header_str, h_def_content)
-        with open(output_file_path, "w") as f:
-            f.write(final_header_content)
-    else:
+    if args.export_decls:
         with open(output_file_path, "w") as f:
             f.write(header_str)