diff --git a/src/header/stdio/mod.rs b/src/header/stdio/mod.rs
--- a/src/header/stdio/mod.rs
+++ b/src/header/stdio/mod.rs
@@ -46,4 +46,7 @@
 pub use self::getdelim::*;
 mod getdelim;
 
+pub use self::open_memstream::*;
+mod open_memstream;
+
 mod ext;
diff --git a/src/header/stdio/open_memstream.rs b/src/header/stdio/open_memstream.rs
new file mode 100644
--- /dev/null
+++ b/src/header/stdio/open_memstream.rs
@@ -0,0 +1,124 @@
+use alloc::{boxed::Box, vec, vec::Vec};
+use core::ptr;
+
+use super::{
+    Buffer, FILE,
+    constants::{BUFSIZ, F_NORD},
+};
+use crate::{
+    error::{Errno, ResultExtPtrMut},
+    fs::File,
+    header::{
+        errno::{EFAULT, ENOMEM},
+        fcntl, pthread, stdlib, unistd,
+    },
+    io::{self, BufWriter, Write},
+    platform::{
+        ERRNO,
+        types::{c_char, size_t},
+    },
+};
+
+struct MemstreamWriter {
+    bufp: *mut *mut c_char,
+    sizep: *mut size_t,
+    current: *mut c_char,
+    buffer: Vec<u8>,
+}
+
+unsafe impl Send for MemstreamWriter {}
+
+impl MemstreamWriter {
+    fn new(bufp: *mut *mut c_char, sizep: *mut size_t) -> Self {
+        Self {
+            bufp,
+            sizep,
+            current: ptr::null_mut(),
+            buffer: Vec::new(),
+        }
+    }
+
+    fn sync_output(&mut self) -> io::Result<()> {
+        let size = self.buffer.len();
+        let alloc_size = size
+            .checked_add(1)
+            .ok_or_else(|| io::Error::from_raw_os_error(ENOMEM))?;
+
+        let raw = if self.current.is_null() {
+            unsafe { stdlib::malloc(alloc_size) }
+        } else {
+            unsafe { stdlib::realloc(self.current.cast(), alloc_size) }
+        };
+        if raw.is_null() {
+            return Err(io::Error::from_raw_os_error(ENOMEM));
+        }
+
+        let raw = raw.cast::<c_char>();
+        if size != 0 {
+            unsafe { ptr::copy_nonoverlapping(self.buffer.as_ptr(), raw.cast::<u8>(), size) };
+        }
+        unsafe {
+            *raw.add(size) = 0;
+            *self.bufp = raw;
+            *self.sizep = size;
+        }
+        self.current = raw;
+        Ok(())
+    }
+}
+
+impl Write for MemstreamWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.buffer
+            .try_reserve(buf.len())
+            .map_err(|_| io::Error::from_raw_os_error(ENOMEM))?;
+        self.buffer.extend_from_slice(buf);
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.sync_output()
+    }
+}
+
+fn create_memstream(bufp: *mut *mut c_char, sizep: *mut size_t) -> Result<Box<FILE>, Errno> {
+    if bufp.is_null() || sizep.is_null() {
+        return Err(Errno(EFAULT));
+    }
+
+    unsafe {
+        *bufp = ptr::null_mut();
+        *sizep = 0;
+    }
+
+    let mut fds = [0; 2];
+    if unsafe { unistd::pipe2(fds.as_mut_ptr(), fcntl::O_CLOEXEC) } != 0 {
+        return Err(Errno(ERRNO.get()));
+    }
+    let _ = unistd::close(fds[0]);
+
+    let file = File::new(fds[1]);
+    let writer = Box::new(BufWriter::new(MemstreamWriter::new(bufp, sizep)));
+    let mutex_attr = pthread::RlctMutexAttr {
+        ty: pthread::PTHREAD_MUTEX_RECURSIVE,
+        ..Default::default()
+    };
+
+    Ok(Box::new(FILE {
+        lock: pthread::RlctMutex::new(&mutex_attr).unwrap(),
+        file,
+        flags: F_NORD,
+        read_buf: Buffer::Owned(vec![0; BUFSIZ as usize]),
+        read_pos: 0,
+        read_size: 0,
+        unget: Vec::new(),
+        writer,
+        pid: None,
+        orientation: 0,
+    }))
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn open_memstream(bufp: *mut *mut c_char, sizep: *mut size_t) -> *mut FILE {
+    create_memstream(bufp, sizep).or_errno_null_mut()
+}