diff --git a/_demo/fcntl/fcntl.go b/_demo/fcntl/fcntl.go
new file mode 100644
index 00000000..8e312cea
--- /dev/null
+++ b/_demo/fcntl/fcntl.go
@@ -0,0 +1,77 @@
+package main
+
+import (
+	"github.com/goplus/llgo/c"
+	"github.com/goplus/llgo/c/os"
+	"unsafe"
+)
+
+func main() {
+
+	filename := c.Str("testfile.txt")
+	data := c.Str("Hello, os!")
+	var buffer [20]c.Char
+
+	// Open a file, O_CREAT|O_WRONLY|O_TRUNC means create, write only, or clear the file
+	fd := os.Open(filename, os.O_CREAT|os.O_WRONLY|os.O_TRUNC, 0644)
+	if fd == -1 {
+		c.Printf(c.Str("open error\n"))
+		return
+	}
+
+	// Writing data to a file
+	bytesWritten := os.Write(fd, c.Pointer(data), c.Strlen(data))
+	if bytesWritten == -1 {
+		c.Printf(c.Str("write error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("Written %ld bytes to %s\n"), bytesWritten, filename)
+
+	// Get file status flags
+	flags := os.Fcntl(fd, os.F_GETFL)
+	if flags == -1 {
+		c.Printf(c.Str("os error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("File flags: %d\n"), flags)
+
+	// Set the file status flag to non-blocking mode
+	if os.Fcntl(fd, os.F_SETFL, flags|os.O_NONBLOCK) == -1 {
+		c.Printf(c.Str("os error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("set file status successfully\n"))
+
+
+
+	c.Printf(c.Str("111"))
+	// Close file
+	os.Close(fd)
+
+	// Reopen the file, O_RDONLY means read-only
+	fd = os.Open(filename, os.O_RDONLY)
+	if fd == -1 {
+		c.Printf(c.Str("open error\n"))
+		return
+	}
+
+	// Reading data from a file
+	// &buffer[:][0]
+	// unsafe.SliceData(buffer[:])
+	bytesRead := os.Read(fd, c.Pointer(unsafe.SliceData(buffer[:])), unsafe.Sizeof(buffer)-1)
+	if bytesRead == -1 {
+		c.Printf(c.Str("read error\n"))
+		os.Close(fd)
+		return
+	}
+
+	// Ensure that the buffer is null-terminated
+	buffer[bytesRead] = c.Char(0)
+	c.Printf(c.Str("Read %ld bytes: %s\n"), bytesRead, &buffer[0])
+
+	// Close file
+	os.Close(fd)
+}
diff --git a/c/os/os.go b/c/os/os.go
index 04ce435b..583cf631 100644
--- a/c/os/os.go
+++ b/c/os/os.go
@@ -35,6 +35,29 @@ const (
 	PATH_MAX = C.PATH_MAX
 )
 
+const (
+	/* get file status flags */
+	F_GETFL = 3
+	/* set file status flags */
+	F_SETFL = 4
+
+	/* open for reading only */
+	O_RDONLY = 0x0000
+	/* open for writing only */
+	O_WRONLY = 0x0001
+	/* open for reading and writing */
+	O_RDWR = 0x0002
+	/* mask for above modes */
+	O_ACCMODE = 0x0003
+
+	/* no delay */
+	O_NONBLOCK = 0x00000004
+	/* create if nonexistant */
+	O_CREAT = 0x00000200
+	/* truncate to zero length */
+	O_TRUNC = 0x00000400
+)
+
 type (
 	ModeT C.mode_t
 	UidT  C.uid_t
@@ -146,7 +169,7 @@ func Fstatat(dirfd c.Int, path *c.Char, buf *StatT, flags c.Int) c.Int
 // -----------------------------------------------------------------------------
 
 //go:linkname Open C.open
-func Open(path *c.Char, flags c.Int, mode ModeT) c.Int
+func Open(path *c.Char, flags c.Int, __llgo_va_list ...any) c.Int
 
 //go:linkname Openat C.openat
 func Openat(dirfd c.Int, path *c.Char, flags c.Int, mode ModeT) c.Int
@@ -154,6 +177,9 @@ func Openat(dirfd c.Int, path *c.Char, flags c.Int, mode ModeT) c.Int
 //go:linkname Creat C.creat
 func Creat(path *c.Char, mode ModeT) c.Int
 
+//go:linkname Fcntl C.fcntl
+func Fcntl(a c.Int, b c.Int, __llgo_va_list ...any) c.Int
+
 //go:linkname Dup C.dup
 func Dup(fd c.Int) c.Int