Add sync() syscall and a /bin/sync.

It walks all the live Inode objects and flushes pending metadata changes
wherever needed.

This could be optimized by keeping a separate list of dirty Inodes,
but let's not get ahead of ourselves.

Userland/.gitignore

@@ -22,3 +22,4 @@ strsignal
 fgrep
 mkdir
 touch
+sync

Userland/Makefile

@@ -44,7 +44,8 @@ APPS = \
        fgrep \
        tty \
        mkdir \
-       touch
+       touch \
+       sync
 
 ARCH_FLAGS =
 STANDARD_FLAGS = -std=c++17 -nostdinc++ -nostdlib -nostdinc
@@ -130,6 +131,9 @@ mkdir: mkdir.o
 touch: touch.o
 	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
 
+sync: sync.o
+	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
+
 .cpp.o:
 	@echo "CXX $<"; $(CXX) $(CXXFLAGS) -o $@ -c $<
 

Userland/sync.cpp

@@ -0,0 +1,8 @@
+#include <stdio.h>
+#include <unistd.h>
+
+int main(int, char**)
+{
+    sync();
+    return 0;
+}