POSIX allows successful system calls to modify errno, so always clear the
error_code if the result indicates success.

asio/include/asio/detail/descriptor_ops.hpp

diff --git a/asio/include/asio/detail/descriptor_ops.hpp b/asio/include/asio/detail/descriptor_ops.hpp
index e886b27..c038386 100644
--- a/asio/include/asio/detail/descriptor_ops.hpp
+++ b/asio/include/asio/detail/descriptor_ops.hpp
@@ -49,13 +49,19 @@
 inline int open(const char* path, int flags, asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::open(path, flags), ec);
+  int result = error_wrapper(::open(path, flags), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 inline int close(int d, asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::close(d), ec);
+  int result = error_wrapper(::close(d), ec);
+  if (result == 0)
+    clear_error(ec);
+  return result;
 }
 
 inline void init_buf_iov_base(void*& base, void* addr)
@@ -87,33 +93,48 @@
     asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::readv(d, bufs, static_cast<int>(count)), ec);
+  int result = error_wrapper(::readv(d, bufs, static_cast<int>(count)), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 inline int gather_write(int d, const buf* bufs, size_t count,
     asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::writev(d, bufs, static_cast<int>(count)), ec);
+  int result = error_wrapper(::writev(d, bufs, static_cast<int>(count)), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 inline int ioctl(int d, long cmd, ioctl_arg_type* arg,
     asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::ioctl(d, cmd, arg), ec);
+  int result = error_wrapper(::ioctl(d, cmd, arg), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 inline int fcntl(int d, long cmd, asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::fcntl(d, cmd), ec);
+  int result = error_wrapper(::fcntl(d, cmd), ec);
+  if (result != -1)
+    clear_error(ec);
+  return result;
 }
 
 inline int fcntl(int d, long cmd, long arg, asio::error_code& ec)
 {
   clear_error(ec);
-  return error_wrapper(::fcntl(d, cmd, arg), ec);
+  int result = error_wrapper(::fcntl(d, cmd, arg), ec);
+  if (result != -1)
+    clear_error(ec);
+  return result;
 }
 
 inline int poll_read(int d, asio::error_code& ec)
@@ -124,7 +145,10 @@
   fds.events = POLLIN;
   fds.revents = 0;
   clear_error(ec);
-  return error_wrapper(::poll(&fds, 1, -1), ec);
+  int result = error_wrapper(::poll(&fds, 1, -1), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 inline int poll_write(int d, asio::error_code& ec)
@@ -135,7 +159,10 @@
   fds.events = POLLOUT;
   fds.revents = 0;
   clear_error(ec);
-  return error_wrapper(::poll(&fds, 1, -1), ec);
+  int result = error_wrapper(::poll(&fds, 1, -1), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 }
 
 } // namespace descriptor_ops

asio/include/asio/detail/socket_ops.hpp

diff --git a/asio/include/asio/detail/socket_ops.hpp b/asio/include/asio/detail/socket_ops.hpp
index 1f00b4c..b08cce9 100644
--- a/asio/include/asio/detail/socket_ops.hpp
+++ b/asio/include/asio/detail/socket_ops.hpp
@@ -102,10 +102,7 @@
   }
 #endif
 
-#if defined(BOOST_WINDOWS)
   clear_error(ec);
-#endif
-
   return new_s;
 }
 
@@ -122,10 +119,8 @@
   clear_error(ec);
   int result = error_wrapper(call_bind(
         &msghdr::msg_namelen, s, addr, addrlen), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -134,22 +129,20 @@
   clear_error(ec);
 #if defined(BOOST_WINDOWS) || defined(__CYGWIN__)
   int result = error_wrapper(::closesocket(s), ec);
+#else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
+  int result = error_wrapper(::close(s), ec);
+#endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
   if (result == 0)
     clear_error(ec);
   return result;
-#else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
-  return error_wrapper(::close(s), ec);
-#endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
 inline int shutdown(socket_type s, int what, asio::error_code& ec)
 {
   clear_error(ec);
   int result = error_wrapper(::shutdown(s, what), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -166,10 +159,8 @@
   clear_error(ec);
   int result = error_wrapper(call_connect(
         &msghdr::msg_namelen, s, addr, addrlen), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -185,7 +176,10 @@
   return -1;
 #else
   clear_error(ec);
-  return error_wrapper(::socketpair(af, type, protocol, sv), ec);
+  int result = error_wrapper(::socketpair(af, type, protocol, sv), ec);
+  if (result == 0)
+    clear_error(ec);
+  return result;
 #endif
 }
 
@@ -193,10 +187,8 @@
 {
   clear_error(ec);
   int result = error_wrapper(::listen(s, backlog), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -280,7 +272,10 @@
   msghdr msg = msghdr();
   msg.msg_iov = bufs;
   msg.msg_iovlen = count;
-  return error_wrapper(::recvmsg(s, &msg, flags), ec);
+  int result = error_wrapper(::recvmsg(s, &msg, flags), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
@@ -310,6 +305,8 @@
   msg.msg_iovlen = count;
   int result = error_wrapper(::recvmsg(s, &msg, flags), ec);
   *addrlen = msg.msg_namelen;
+  if (result >= 0)
+    clear_error(ec);
   return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
@@ -336,7 +333,10 @@
 #if defined(__linux__)
   flags |= MSG_NOSIGNAL;
 #endif // defined(__linux__)
-  return error_wrapper(::sendmsg(s, &msg, flags), ec);
+  int result = error_wrapper(::sendmsg(s, &msg, flags), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
@@ -365,7 +365,10 @@
 #if defined(__linux__)
   flags |= MSG_NOSIGNAL;
 #endif // defined(__linux__)
-  return error_wrapper(::sendmsg(s, &msg, flags), ec);
+  int result = error_wrapper(::sendmsg(s, &msg, flags), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
@@ -408,7 +411,10 @@
 
   return s;
 #else
-  return error_wrapper(::socket(af, type, protocol), ec);
+  int s = error_wrapper(::socket(af, type, protocol), ec);
+  if (s >= 0)
+    clear_error(ec);
+  return s;
 #endif
 }
 
@@ -451,10 +457,8 @@
   clear_error(ec);
   int result = error_wrapper(call_setsockopt(&msghdr::msg_namelen,
         s, level, optname, optval, optlen), ec);
-# if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-# endif
   return result;
 #endif // defined(__BORLANDC__)
 }
@@ -543,6 +547,8 @@
     *static_cast<int*>(optval) /= 2;
   }
 #endif // defined(__linux__)
+  if (result == 0)
+    clear_error(ec);
   return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
@@ -563,10 +569,8 @@
   clear_error(ec);
   int result = error_wrapper(call_getpeername(
         &msghdr::msg_namelen, s, addr, addrlen), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -586,10 +590,8 @@
   clear_error(ec);
   int result = error_wrapper(call_getsockname(
         &msghdr::msg_namelen, s, addr, addrlen), ec);
-#if defined(BOOST_WINDOWS)
   if (result == 0)
     clear_error(ec);
-#endif
   return result;
 }
 
@@ -599,12 +601,12 @@
   clear_error(ec);
 #if defined(BOOST_WINDOWS) || defined(__CYGWIN__)
   int result = error_wrapper(::ioctlsocket(s, cmd, arg), ec);
-  if (result == 0)
+#else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
+  int result = error_wrapper(::ioctl(s, cmd, arg), ec);
+#endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
+  if (result >= 0)
     clear_error(ec);
   return result;
-#else // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
-  return error_wrapper(::ioctl(s, cmd, arg), ec);
-#endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
 inline int select(int nfds, fd_set* readfds, fd_set* writefds,
@@ -642,10 +644,8 @@
 #else
   int result = error_wrapper(::select(nfds, readfds,
         writefds, exceptfds, timeout), ec);
-# if defined(BOOST_WINDOWS)
   if (result >= 0)
     clear_error(ec);
-# endif
   return result;
 #endif
 }
@@ -667,7 +667,10 @@
   fds.events = POLLIN;
   fds.revents = 0;
   clear_error(ec);
-  return error_wrapper(::poll(&fds, 1, -1), ec);
+  int result = error_wrapper(::poll(&fds, 1, -1), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
@@ -688,7 +691,10 @@
   fds.events = POLLOUT;
   fds.revents = 0;
   clear_error(ec);
-  return error_wrapper(::poll(&fds, 1, -1), ec);
+  int result = error_wrapper(::poll(&fds, 1, -1), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }
 
@@ -712,7 +718,10 @@
   fds.events = POLLOUT;
   fds.revents = 0;
   clear_error(ec);
-  return error_wrapper(::poll(&fds, 1, -1), ec);
+  int result = error_wrapper(::poll(&fds, 1, -1), ec);
+  if (result >= 0)
+    clear_error(ec);
+  return result;
 #endif // defined(BOOST_WINDOWS) || defined(__CYGWIN__)
 }