diff --git a/runtime/syntax/c.vim b/runtime/syntax/c.vim
index 50878a7..04b132d 100644
--- a/runtime/syntax/c.vim
+++ b/runtime/syntax/c.vim
@@ -1,7 +1,7 @@
 " Vim syntax file
 " Language:	C
 " Maintainer:	Bram Moolenaar <Bram@vim.org>
-" Last Change:	2022 Oct 05
+" Last Change:	2023 Mar 08
 
 " Quit when a (custom) syntax file was already loaded
 if exists("b:current_syntax")
@@ -348,7 +348,10 @@
   syn keyword cConstant SIGABRT SIGALRM SIGCHLD SIGCONT SIGFPE SIGHUP SIGILL SIGINT SIGKILL SIGPIPE SIGQUIT SIGSEGV
   syn keyword cConstant SIGSTOP SIGTERM SIGTRAP SIGTSTP SIGTTIN SIGTTOU SIGUSR1 SIGUSR2
   syn keyword cConstant _IOFBF _IOLBF _IONBF BUFSIZ EOF WEOF FOPEN_MAX FILENAME_MAX L_tmpnam
-  syn keyword cConstant SEEK_CUR SEEK_END SEEK_SET TMP_MAX stderr stdin stdout EXIT_FAILURE EXIT_SUCCESS RAND_MAX
+  syn keyword cConstant SEEK_CUR SEEK_END SEEK_SET TMP_MAX EXIT_FAILURE EXIT_SUCCESS RAND_MAX
+  syn keyword cConstant stdin stdout stderr
+  " POSIX 2001, in unistd.h
+  syn keyword cConstant STDIN_FILENO STDOUT_FILENO STDERR_FILENO
   " used in assert.h
   syn keyword cConstant NDEBUG
   " POSIX 2001