diff --git a/open_mode/open_mode.c b/open_mode/open_mode.c
new file mode 100644
index 0000000..68573b1
--- /dev/null
+++ b/open_mode/open_mode.c
@@ -0,0 +1,58 @@
+#define _GNU_SOURCE 1
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+struct {
+    int mode;
+    char *name;
+} modes[] = {
+    { O_APPEND, "O_APPEND" },
+    { O_ASYNC, "O_ASYNC" },
+    { O_CLOEXEC, "O_CLOEXEC" },
+    { O_CREAT, "O_CREAT" },
+    { O_DIRECT, "O_DIRECT" },
+    { O_DIRECTORY, "O_DIRECTORY" },
+    { O_EXCL, "O_EXCL" },
+    { O_LARGEFILE, "O_LARGEFILE" },
+    { O_NOATIME, "O_NOATIME" },
+    { O_NOCTTY, "O_NOCTTY" },
+    { O_NOFOLLOW, "O_NOFOLLOW" },
+    { O_NONBLOCK, "O_NONBLOCK" },
+    { O_NDELAY, "O_NDELAY" },
+    { O_SYNC, "O_SYNC" },
+    { O_TRUNC, "O_TRUNC" },
+};
+void dump_mode(int mode) {
+    if ((mode & 3) == O_RDONLY) {
+	printf("%7o %s\n", O_RDONLY, "O_RDONLY");
+    } else
+    if ((mode & 3) == O_WRONLY) {
+	printf("%7o %s\n", O_WRONLY, "O_WRONLY");
+    } else
+    if ((mode & 3) == O_RDWR) {
+	printf("%7o %s\n", O_RDWR, "O_RDWR");
+    } else {
+	printf("%7o %s\n", (mode & 3), "none");
+    }
+
+    for (int i = 0; i < sizeof(modes)/sizeof(modes[0]); i++) {
+	if (modes[i].mode & mode) {
+	    printf("%7o %s\n", modes[i].mode, modes[i].name);
+	}
+    }
+    printf("\n");
+}
+
+int main(int argc, char **argv) {
+    if (argc == 1) {
+	dump_mode(-1);
+    } else {
+	for (char **arg = argv+1; *arg; arg++) {
+	    int mode = strtoul(*arg, NULL, 0);
+	    dump_mode(mode);
+	}
+    }
+}