fdatasync() if fd is writable

Makefile

 default:
 	gcc -Wall -fPIC -c -o nocache.o nocache.c
-	gcc -Wall -fPIC -c -o fadv_dontneed.o fadv_dontneed.c
-	gcc -Wall -shared -Wl,-soname,nocache.so.1 -ldl -o nocache.so nocache.o fadv_dontneed.o
+	gcc -Wall -fPIC -c -o fcntl_helpers.o fcntl_helpers.c
+	gcc -Wall -shared -Wl,-soname,nocache.so.1 -ldl -o nocache.so nocache.o fcntl_helpers.o

fadv_dontneed.c → fcntl_helpers.c

 #include <fcntl.h>
+#include <unistd.h>
 
 /* Since open() and close() are re-defined in nocache.c, it's not
  * possible to include <fcntl.h> there. So we do it here. */
@@ -7,3 +8,12 @@ int fadv_dontneed(int fd, off_t offset, off_t len)
 {
         return posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
 }
+
+void sync_if_writable(int fd)
+{
+    int r;
+    if((r = fcntl(fd, F_GETFL, 0)) == -1)
+        return;
+    if(!(r & O_RDONLY))
+        fdatasync(fd);
+}

nocache.c

@@ -18,6 +18,7 @@ int close(int fd);
 static void store_pageinfo(int fd);
 static void free_unclaimed_pages(int fd);
 extern int fadv_dontneed(int fd, off_t offset, off_t len);
+extern void sync_if_writable(int fd);
 
 #define _MAX_FDS 1024
 
@@ -141,6 +142,8 @@ static void free_unclaimed_pages(int fd)
     if(i == _MAX_FDS)
         return; /* not found */
 
+    sync_if_writable(fd);
+
     for(j = 0; j < fds[i].nr_pages; j++) {
         if(!(fds[i].info[j] & 1)) {
             fadv_dontneed(fd, j*PAGESIZE, PAGESIZE);