diff --git a/Makefile b/Makefile
index ef0190e..d9e51f7 100644
--- a/Makefile
+++ b/Makefile
@@ -25,10 +25,11 @@ endif
 clean:
 	-rm -f rosella *.o *.gcda *.gcno
 
-rosella: rosella.o gc.o builtin.o interp.o reader.o
+rosella: rosella.o gc.o builtin.o interp.o reader.o io_builtin.o
 
 rosella.o: rosella.c gc.h builtin.h interp.h
 gc.o: gc.c gc.h
-builtin.o: builtin.c builtin.h gc.h
+builtin.o: builtin.c builtin.h interp.h gc.h
 interp.o: interp.c interp.h gc.h builtin.h
 reader.o: reader.c reader.h gc.h builtin.h
+io_builtin.o: io_builtin.c io_builtin.h builtin.h interp.h gc.h
diff --git a/gc.h b/gc.h
index 534d381..f8e0fa2 100644
--- a/gc.h
+++ b/gc.h
@@ -259,6 +259,11 @@ static inline fixnum_t _get_fixnum(value_t n)
   return ((fixnum_t)n) >> 1;
 }
 
+static inline bool is_valid_fixnum(fixnum_t n)
+{
+  return _get_fixnum(fixnum_value(n)) == n;
+}
+
 static inline value_t object_value(void *obj)
 {
   assert((uintptr_t)obj >= 4096);
diff --git a/io_builtin.c b/io_builtin.c
new file mode 100644
index 0000000..3de740d
--- /dev/null
+++ b/io_builtin.c
@@ -0,0 +1,219 @@
+#include <assert.h>
+#include <inttypes.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <math.h>
+#include <errno.h>
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "gc.h"
+#include "builtin.h"
+#include "interp.h"
+#include "io_builtin.h"
+
+static void bi_posix_open(interp_state_t *state);
+//static void bi_posix_openat(interp_state_t *state);
+static void bi_posix_dup(interp_state_t *state);
+static void bi_posix_dup2(interp_state_t *state);
+static void bi_posix_read(interp_state_t *state);
+static void bi_posix_write(interp_state_t *state);
+static void bi_posix_lseek(interp_state_t *state);
+static void bi_posix_close(interp_state_t *state);
+
+void io_builtin_init(void)
+{
+  register_builtin(BI_IO_POSIX_OPEN, make_builtin_fn(bi_posix_open));
+  //register_builtin(BI_IO_POSIX_OPENAT, make_builtin_fn(bi_posix_openat));
+  register_builtin(BI_IO_POSIX_OPENAT, UNDEFINED);
+
+  register_builtin(BI_IO_POSIX_DUP, make_builtin_fn(bi_posix_dup));
+  register_builtin(BI_IO_POSIX_DUP2, make_builtin_fn(bi_posix_dup2));
+
+  register_builtin(BI_IO_POSIX_READ, make_builtin_fn(bi_posix_read));
+  register_builtin(BI_IO_POSIX_WRITE, make_builtin_fn(bi_posix_write));
+  register_builtin(BI_IO_POSIX_LSEEK, make_builtin_fn(bi_posix_lseek));
+
+  register_builtin(BI_IO_POSIX_CLOSE, make_builtin_fn(bi_posix_close));
+}
+
+static void simple_return(interp_state_t *state, value_t rval)
+{
+  state->lambda.value = state->k.value;
+  state->argv.value   = rval;
+  state->k.value      = FALSE_VALUE;
+  state->ctx.value    = FALSE_VALUE;
+}
+
+static void simple_return2(interp_state_t *state, value_t v1, value_t v2)
+{
+  simple_return(state, cons(v1, cons(v2, NIL)));
+}
+
+static void bi_posix_open(interp_state_t *state)
+{
+  char *pathname;
+  int flags;
+  mode_t mode;
+  int fd;
+  int saved_errno;
+
+  release_assert(is_nil(CDDR(state->argv.value)) || is_nil(CDR(_CDDR(state->argv.value))));
+
+  pathname = value_to_string(CAR(state->argv.value));
+  flags = get_fixnum(CAR(_CDR(state->argv.value)));
+
+  if (!is_nil(_CDDR(state->argv.value)))
+  {
+    mode = get_fixnum(CAR(_CDDR(state->argv.value)));
+  }
+  else
+  {
+    release_assert((flags & O_CREAT) == 0);
+    mode = 0;
+  }
+
+  errno = 0;
+  fd = open(pathname, flags, mode);
+  saved_errno = errno;
+  free(pathname);
+
+  release_assert(is_valid_fixnum(fd));
+  release_assert(is_valid_fixnum(saved_errno));
+
+  simple_return2(state, (fd >= 0) ? fixnum_value(fd) : FALSE_VALUE,
+                        fixnum_value(saved_errno));
+}
+
+//static void bi_posix_openat(interp_state_t *state) {}
+
+static void bi_posix_dup(interp_state_t *state)
+{
+  int oldfd = get_fixnum(CAR(state->argv.value));
+  int newfd;
+  int saved_errno;
+
+  release_assert(is_nil(_CDR(state->argv.value)));
+
+  errno = 0;
+  newfd = dup(oldfd);
+  saved_errno = errno;
+
+  release_assert(is_valid_fixnum(newfd));
+  release_assert(is_valid_fixnum(saved_errno));
+
+  simple_return2(state, (newfd >= 0) ? fixnum_value(newfd) : FALSE_VALUE,
+                        fixnum_value(saved_errno));
+}
+
+static void bi_posix_dup2(interp_state_t *state)
+{
+  int oldfd = get_fixnum(CAR(state->argv.value));
+  int newfd = get_fixnum(CAR(_CDR(state->argv.value)));
+  int saved_errno;
+
+  release_assert(is_nil(_CDDR(state->argv.value)));
+
+  errno = 0;
+  newfd = dup2(oldfd, newfd);
+  saved_errno = errno;
+
+  release_assert(is_valid_fixnum(newfd));
+  release_assert(is_valid_fixnum(saved_errno));
+
+  simple_return2(state, (newfd >= 0) ? fixnum_value(newfd) : FALSE_VALUE,
+                        fixnum_value(saved_errno));
+}
+
+static void bi_posix_read(interp_state_t *state)
+{
+  int fd = get_fixnum(CAR(state->argv.value));
+  value_t str = CAR(_CDR(state->argv.value));
+  fixnum_t count = get_fixnum(CAR(_CDDR(state->argv.value)));
+  ssize_t result;
+  int saved_errno;
+
+  release_assert(is_byte_string(str));
+  release_assert(is_nil(_CDR(_CDDR(state->argv.value))));
+  release_assert((0 <= count) && (count <= _get_byte_string(str)->size));
+
+  errno = 0;
+  result = read(fd, _get_byte_string(str)->bytes, count);
+  saved_errno = errno;
+
+  release_assert(is_valid_fixnum(result));
+  release_assert(is_valid_fixnum(saved_errno));
+
+  simple_return2(state, (result >= 0) ? fixnum_value(result) : FALSE_VALUE,
+                        fixnum_value(saved_errno));
+}
+
+static void bi_posix_write(interp_state_t *state)
+{
+  int fd = get_fixnum(CAR(state->argv.value));
+  value_t str = CAR(_CDR(state->argv.value));
+  fixnum_t count = get_fixnum(CAR(_CDDR(state->argv.value)));
+  ssize_t result;
+  int saved_errno;
+
+  release_assert(is_byte_string(str));
+  release_assert(is_nil(_CDR(_CDDR(state->argv.value))));
+  release_assert((0 <= count) && (count <= _get_byte_string(str)->size));
+
+  errno = 0;
+  result = write(fd, _get_byte_string(str)->bytes, count);
+  saved_errno = errno;
+
+  release_assert(is_valid_fixnum(result));
+  release_assert(is_valid_fixnum(saved_errno));
+
+  simple_return2(state, (result >= 0) ? fixnum_value(result) : FALSE_VALUE,
+                        fixnum_value(saved_errno));
+}
+
+static void bi_posix_lseek(interp_state_t *state)
+{
+  int fd = get_fixnum(CAR(state->argv.value));
+  fixnum_t off = get_fixnum(CAR(_CDR(state->argv.value)));
+  fixnum_t whence = get_fixnum(CAR(_CDDR(state->argv.value)));
+  off_t result;
+  int saved_errno;
+
+  release_assert(is_nil(_CDR(_CDDR(state->argv.value))));
+
+  errno = 0;
+  result = lseek(fd, off, whence);
+  saved_errno = errno;
+
+  release_assert(is_valid_fixnum(saved_errno));
+
+  if (result == (off_t)(-1))
+  {
+    simple_return2(state, FALSE_VALUE, fixnum_value(saved_errno));
+  }
+  else
+  {
+    release_assert((result >= 0) && (result <= FIXNUM_MAX));
+    simple_return2(state, fixnum_value(result), fixnum_value(saved_errno));
+  }
+}
+
+static void bi_posix_close(interp_state_t *state)
+{
+  int fd = get_fixnum(CAR(state->argv.value));
+  ssize_t result;
+  int saved_errno;
+
+  release_assert(is_nil(_CDR(state->argv.value)));
+
+  errno = 0;
+  result = close(fd);
+  saved_errno = errno;
+
+  simple_return2(state, boolean_value(!result), fixnum_value(saved_errno));
+}
+
+/* vim:set sw=2 expandtab: */
diff --git a/io_builtin.h b/io_builtin.h
new file mode 100644
index 0000000..16fdd32
--- /dev/null
+++ b/io_builtin.h
@@ -0,0 +1,22 @@
+#ifndef IO_BUILTIN_H_a9dd5b4902aa45e05dcf2b0fdb33ec74
+#define IO_BUILTIN_H_a9dd5b4902aa45e05dcf2b0fdb33ec74
+
+#include <inttypes.h>
+#include <stdbool.h>
+
+#define BI_IO_POSIX_OPEN   "posix-open"
+#define BI_IO_POSIX_OPENAT "posix-openat"
+
+#define BI_IO_POSIX_DUP    "posix-dup"
+#define BI_IO_POSIX_DUP2   "posix-dup2"
+
+#define BI_IO_POSIX_READ   "posix-read"
+#define BI_IO_POSIX_WRITE  "posix-write"
+#define BI_IO_POSIX_LSEEK  "posix-lseek"
+
+#define BI_IO_POSIX_CLOSE  "posix-close"
+
+void io_builtin_init(void);
+
+#endif
+/* vim:set sw=2 expandtab: */
diff --git a/rosella.c b/rosella.c
index bf6bf5e..7a63c9a 100644
--- a/rosella.c
+++ b/rosella.c
@@ -11,6 +11,7 @@
 #include "builtin.h"
 #include "interp.h"
 #include "reader.h"
+#include "io_builtin.h"
 
 static void test_builtins(void);
 static void test_weak_boxes_and_wills(void);
@@ -34,6 +35,7 @@ int main(int argc, char **argv)
   gc_init(256*1024, 1024*1024);
   builtin_init();
   interpreter_init();
+  io_builtin_init();
 
   if (argc < 2 || (strcmp(argv[1], "-k") == 0))
   {