writeFd像readFd一样封装一下，把fd的写细节也封装到outputBuffer_里边。

muduo/net/Buffer.cc

@@ -13,6 +13,7 @@
 
 #include <errno.h>
 #include <sys/uio.h>
+#include <unistd.h>
 
 using namespace muduo;
 using namespace muduo::net;
@@ -56,3 +57,12 @@ ssize_t Buffer::readFd(int fd, int* savedErrno)
   return n;
 }
 
+ssize_t Buffer::writeFd(int fd, int* savedErrno)
+{
+  ssize_t n = ::write(fd, peek(), readableBytes());
+  if (n < 0)
+  {
+    *savedErrno = errno;
+  }
+  return n;
+}

muduo/net/Buffer.h

@@ -379,6 +379,9 @@ class Buffer : public muduo::copyable
   /// @return result of read(2), @c errno is saved
   ssize_t readFd(int fd, int* savedErrno);
 
+  // Send data by fd.
+  ssize_t writeFd(int fd, int* savedErrno);
+
  private:
 
   char* begin()

muduo/net/TcpConnection.cc

@@ -370,9 +370,8 @@ void TcpConnection::handleWrite()
   loop_->assertInLoopThread();
   if (channel_->isWriting())
   {
-    ssize_t n = sockets::write(channel_->fd(),
-                               outputBuffer_.peek(),
-                               outputBuffer_.readableBytes());
+    int savedErrno = 0;
+    ssize_t n = outputBuffer_.writeFd(channel_->fd(), &savedErrno);
     if (n > 0)
     {
       outputBuffer_.retrieve(n);