os: Add read & write functions.

os/os.py

@@ -5,6 +5,8 @@ libc = ffi.open("libc.so.6")
 
 errno = libc.var("i", "errno")
 mkdir_ = libc.func("i", "mkdir", "si")
+read_ = libc.func("i", "read", "iPi")
+write_ = libc.func("i", "write", "iPi")
 
 
 def check_error(ret):
@@ -15,3 +17,14 @@ def check_error(ret):
 def mkdir(name, mode=0o777):
     e = mkdir_(name, mode)
     check_error(e)
+
+def read(fd, n):
+    buf = bytearray(n)
+    r = read_(fd, buf, n)
+    check_error(r)
+    return buf[:r]
+
+def write(fd, buf):
+    r = write_(fd, buf, len(buf))
+    check_error(r)
+    return r