diff --git a/sortix/include/sortix/fcntl.h b/sortix/include/sortix/fcntl.h
index c787882e..91251250 100644
--- a/sortix/include/sortix/fcntl.h
+++ b/sortix/include/sortix/fcntl.h
@@ -88,6 +88,12 @@ __BEGIN_DECLS
 #define F_GETFL_NUM 3
 #define F_GETFL F_ENCODE_CMD(F_GETFL_NUM, F_TYPE_VOID)
 
+/* Duplicates the file descriptor with at least the given index. */
+#define F_DUPFD_NUM 4
+#define F_DUPFD F_ENCODE_CMD(F_ENCODE(F_DUPFD_NUM, 0), F_TYPE_INT)
+#define F_DUPFD_CLOEXEC F_ENCODE_CMD(F_ENCODE(F_DUPFD_NUM, FD_CLOEXEC), F_TYPE_INT)
+#define F_DUPFD_CLOFORK F_ENCODE_CMD(F_ENCODE(F_DUPFD_NUM, FD_CLOFORK), F_TYPE_INT)
+
 #define AT_FDCWD (-100)
 #define AT_REMOVEDIR (1<<0)
 #define AT_EACCESS (1<<1)
diff --git a/sortix/io.cpp b/sortix/io.cpp
index 51deec31..d9a72bdf 100644
--- a/sortix/io.cpp
+++ b/sortix/io.cpp
@@ -328,6 +328,11 @@ static int sys_fcntl(int fd, int cmd, uintptr_t arg)
 		return dtable->GetFlags(fd);
 
 	// Operations on the file description.
+	if ( F_DECODE_CMD(F_DECODE_CMD_RAW(cmd)) == F_DUPFD_NUM )
+	{
+		int fd_flags = F_DECODE_FLAGS(F_DECODE_CMD_RAW(cmd));
+		return dtable->Allocate(fd, fd_flags, (int) arg);
+	}
 	Ref<Descriptor> desc = dtable->Get(fd);
 	if ( !desc )
 		return -1;