userland: move getchar from cpp-cheat

README.adoc

@@ -3785,6 +3785,25 @@ Simulated exit code not 0! Exit code is 1
 
 which we parse in link:run[] and then exit with the correct result ourselves...
 
+==== gem5 syscall emulation mode program stdin
+
+gem5 shows its own stdout to terminal, and does not allow you to type stdin to programs.
+
+Instead, you must pass stdin non-interactively with the through a file with the `--se.py --input` option, e.g.:
+
+....
+printf a > f
+./run --emulator gem5 --userland userland/c/getchar.c --static -- --input f
+....
+
+leads to gem5 output:
+
+....
+enter a character: you entered: a
+....
+
+Source: link:userland/c/getchar.c[]
+
 ==== User mode vs full system benchmark
 
 Let's see if user mode runs considerably faster than full system or not.

path_properties.py

@@ -75,6 +75,7 @@ path_properties_tree = PrefixTree({
     'c': PrefixTree({
         'assert_fail.c': PrefixTree(value=PathProperties(exit_status=1)),
         'false.c': PrefixTree(value=PathProperties(exit_status=1)),
+        'getchar.c': PrefixTree(value=PathProperties(interactive=True)),
         'infinite_loop.c': PrefixTree(value=PathProperties(more_than_1s=True)),
     }),
     'kernel_modules': PrefixTree(value=PathProperties(requires_kernel_modules=True)),

userland/c/getchar.c

@@ -0,0 +1,21 @@
+/* Get on character from stdin, and then print it back out.
+ *
+ * Same as getc(stdin).
+ *
+ * You have to press enter for the character to go through:
+ * https://stackoverflow.com/questions/1798511/how-to-avoid-pressing-enter-with-getchar
+ *
+ * Used at:
+ * https://stackoverflow.com/questions/556405/what-do-real-user-and-sys-mean-in-the-output-of-time1/53937376#53937376
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(void) {
+    char c;
+    printf("enter a character: ");
+    c = getchar();
+    printf("you entered: %c\n", c);
+    return EXIT_SUCCESS;
+}