send函数返回值

1有关SIGPIPE信号

在Unix系统下，如果send 、 recv 、 write在等待协议传送数据时 ， socket 被 shutdown，调用send的进程会接收到一个SIGPIPE信号，进程对该信号的默认处理是进程终止。 此种情况 应用就很难查 出 处理进程为什么退出。

 

SIGPIPE 信号：

对一个已经收到FIN包的socket调用read方法, 如果接收缓冲已空, 则返回0, 这就是常说的表示连接关闭. 但第一次对其调用write方法时, 如果发送缓冲没问题, 会返回正确写入(发送). 但发送的报文会导致对端发送RST报文, 因为对端的socket已经调用了close, 完全关闭, 既不发送, 也不接收数据. 所以, 第二次调用write方法(假设在收到RST之后), 会生成SIGPIPE信号, 导致进程退出 。如果对 SIGPIPE 进行忽略处理， 二次调用write方法时, 会返回-1, 同时errno置为SIGPIPE.

处理方法：

在初始化时调用 signal(SIGPIPE,SIG_IGN) 忽略该信号（只需一次） ， SIGPIPE交给了系统处理。 此时send 、 recv 或 write 函数将返回-1，errno为EPIPE，可视情况关闭socket或其他处理    

SIGPIPE 被忽略的情况下，如果 服务器采用了fork的话，要收集垃圾进程，防止僵尸进程的产生，可以这样处理：  signal(SIGCHLD,SIG_IGN);　交给系统init去回收。 这样 子进程就不会产生僵尸进程了。

 

ACE中发送和接收超时都是基于select的

 

 ssize_t
 ACE::send_n_i (ACE_HANDLE handle,
                const void *buf,
                size_t len,
                int flags,
                const ACE_Time_Value *timeout,
                size_t *bt)
 {
   size_t temp;
   size_t &bytes_transferred = bt == 0 ? temp : *bt;
   ssize_t n;
   ssize_t result = 0;
   int error = 0;
   int val = 0;
   ACE::record_and_set_non_blocking_mode (handle, val);
   for (bytes_transferred = 0;
        bytes_transferred < len;
        bytes_transferred += n)
     {
       // Try to transfer as much of the remaining data as possible.
       // Since the socket is in non-blocking mode, this call will not
       // block.
       n = ACE_OS::send (handle,
                         (char *) buf + bytes_transferred,
                         len - bytes_transferred,
                         flags);
       // Check for errors.
       if (n == 0 ||
           n == -1)
         {
           // Check for possible blocking.
           if (n == -1 &&
               (errno == EWOULDBLOCK || errno == ENOBUFS))
             {
               // Wait upto  for the blocking to subside.
               int rtn = ACE::handle_write_ready (handle,
                                                  timeout);
               // Did select() succeed?
               if (rtn != -1)
                 {
                   // Blocking subsided in  period.  Continue
                   // data transfer.
                   n = 0;
                   continue;
                 }
             }
           // Wait in select() timed out or other data transfer or
           // select() failures.
           error = 1;
           result = n;
           break;
         }
     }
   ACE::restore_non_blocking_mode (handle, val);
   if (error)
     {
       return result;
     }
   else
     {
       return ACE_Utils::truncate_cast (bytes_transferred);
     }
 }
 int
 ACE::handle_ready (ACE_HANDLE handle,
                    const ACE_Time_Value *timeout,
                    int read_ready,
                    int write_ready,
                    int exception_ready)
 {
 #if defined (ACE_HAS_POLL) && defined (ACE_HAS_LIMITED_SELECT)
   ACE_UNUSED_ARG (write_ready);
   ACE_UNUSED_ARG (exception_ready);
   struct pollfd fds;
   fds.fd = handle;
   fds.events = read_ready ? POLLIN : POLLOUT;
   fds.revents = 0;
   int result = ACE_OS::poll (&fds, 1, timeout);
 #else
   ACE_Handle_Set handle_set;
   handle_set.set_bit (handle);
   // Wait for data or for the timeout to elapse.
   int select_width;
 #  if defined (ACE_WIN32)
   // This arg is ignored on Windows and causes pointer truncation
   // warnings on 64-bit compiles.
   select_width = 0;
 #  else
   select_width = int (handle) + 1;
 #  endif /* ACE_WIN64 */
   int result = ACE_OS::select (select_width,
                                read_ready ? handle_set.fdset () : 0, // read_fds.
                                write_ready ? handle_set.fdset () : 0, // write_fds.
                                exception_ready ? handle_set.fdset () : 0, // exception_fds.
                                timeout);
 #endif /* ACE_HAS_POLL && ACE_HAS_LIMITED_SELECT */
   switch (result)
     {
     case 0:  // Timer expired.
       errno = ETIME;
       /* FALLTHRU */
     case -1: // we got here directly - select() returned -1.
       return -1;
     case 1: // Handle has data.
       /* FALLTHRU */
     default: // default is case result > 0; return a
       // ACE_ASSERT (result == 1);
       return result;
     }

其它lighttpd：

 // if  (len == 0 || (len < 0 && errno != EAGAIN && errno != EINTR) ) {
 case -1:
             if (errno == EWOULDBLOCK || errno == EAGAIN || errno == EINPROGRESS)
             {
                 return 0;
             }
 if (-1 == (cnt = accept(srv_socket->fd, (struct sockaddr *) &cnt_addr, &cnt_len))) {
         switch (errno) {
         case EAGAIN:
 #if EWOULDBLOCK != EAGAIN
         case EWOULDBLOCK:
 #endif
         case EINTR:
             /* we were stopped _before_ we had a connection */
         case ECONNABORTED: /* this is a FreeBSD thingy */
             /* we were stopped _after_ we had a connection */
             break;
         case EMFILE:
             /* out of fds */
             break;
         default:
             log_error_write(srv, __FILE__, __LINE__, "ssd", "accept failed:", strerror(errno), errno);
         }
         return NULL;
     }
 fcgi
 if (-1 == connect(fcgi_fd, fcgi_addr, servlen)) {
         if (errno == EINPROGRESS ||
             errno == EALREADY ||
             errno == EINTR) {
             if (hctx->conf.debug > 2) {
                 log_error_write(srv, __FILE__, __LINE__, "sb",
                     "connect delayed; will continue later:", proc->connection_name);
             }
             return CONNECTION_DELAYED;
         } else if (errno == EAGAIN) {
             if (hctx->conf.debug) {
                 log_error_write(srv, __FILE__, __LINE__, "sbsd",
                     "This means that you have more incoming requests than your FastCGI backend can handle in parallel."
                     "It might help to spawn more FastCGI backends or PHP children; if not, decrease server.max-connections."
                     "The load for this FastCGI backend", proc->connection_name, "is", proc->load);
             }
             return CONNECTION_OVERLOADED;
         } else {
             log_error_write(srv, __FILE__, __LINE__, "sssb",
                     "connect failed:",
                     strerror(errno), "on",
                     proc->connection_name);
             return CONNECTION_DEAD;
         }
     }

 2有关SEND,RECV函数的返回值及错误码

2.1EINTR

指操作被中断唤醒，需要重新读/写
2.2EAGAIN
在Linux环境下开发经常会碰到很多错误(设置errno)，其中EAGAIN是其中比较常见的一个错误(比如用在非阻塞操作中)。从字面上来看，是提示再试一次。这个错误经常出现在当应用程序进行一些非阻塞(non-blocking)操作(对文件或socket)的时候。例如，以 O_NONBLOCK的标志打开文件/socket/FIFO，如果你连续做read操作而没有数据可读。此时程序不会阻塞起来等待数据准备就绪返 回，read函数会返回一个错误EAGAIN，提示你的应用程序现在没有数据可读请稍后再试。又例如，当一个系统调用(比如fork)因为没有足够的资源(比如虚拟内存)而执行失败，返回EAGAIN提示其再调用一次(也许下次就能成功)。

2.3EAGIN处理
当客户通过Socket提供的send函数发送大的数据包时，就可能返回一个EGGAIN的错误。该错误产生的原因是由于send 

函数 中的size变量大小超过了tcp_sendspace的值。tcp_sendspace定义了应用在调用send之前能够在kernel中缓存的数据 量。当应用程序在socket中设置了O_NDELAY或者O_NONBLOCK属性后，如果发送缓存被占满，send就会返回EAGAIN的错误。 
为了消除该错误，有三种方法可以选择： 
1.调大tcp_sendspace，使之大于send中的size参数 
---no -p -o tcp_sendspace=65536 

2.在调用send前，在setsockopt函数中为SNDBUF设置更大的值