environ: link to from readme

README.adoc

@@ -17822,6 +17822,10 @@ These links provide a clear overview of what POSIX is:
 * https://stackoverflow.com/questions/1780599/what-is-the-meaning-of-posix/31865755#31865755
 * https://unix.stackexchange.com/questions/11983/what-exactly-is-posix/220877#220877
 
+==== Environment variables
+
+POSIX C example that prints all environment variables: link:userland/posix/environ.c[]
+
 ==== unistd.h
 
 * link:userland/posix/count.c[] illustrates `sleep()`

userland/posix/environ.c

@@ -1,4 +1,4 @@
-/* Print all environment variables. */
+/* https://cirosantilli.com/linux-kernel-module-cheat#environment-variables */
 
 #include <stdio.h>
 #include <stdlib.h>