Select Back

• to support multiplexing
• multiplex means to use one channel to transmit several signals
  • WDM (波分複用): 按照頻率來區分各路信號
  • TDM (時分複用): 按照時間片來區分各路信號
• select() is to check several socket descriptors, and return when one is ready for multiplexing
• 可用於監聽多個進程(多個文件描述符)

1. select()

method

/* select */
int select(int maxfd, fd_set* rdset, fd_set* wrset, fd_set* exset, struct timeval* timeout)

/* timeval */
struct timval
{
    long tv_sec;    //second
    long tv_usec;    //millisecond
}

parameters

• maxfd: 需要測試的描述符集合中的最大值 + 1, 實際是測試[0, maxfd-1]
• rdset: 需要測試是否可讀的描述符集合(包括處於listen狀態的socket接收到鏈接請求)
• wrset: 需要測試是否可寫的描述符集合(包括以非阻塞方式調用connect是否成功)
• exset: 需要測試是否異常的描述符集合(包括接受帶外數據的socket有带外数据到達)
• timeout: 指定測試時間閾值上界
  • NULL: forever
  • >0: time
  • 0: return immediately after check, without waiting

return value

• num: the number of ready descriptors
• 0: timeout but none is ready
• -1: failure
• errno: wrong code

2. Operations with sets of socket descriptors

• FD_ZERO(fd_set* fdset)清空描述符集合
• FD_SET(int fd, fd_set* fdset)添加一個描述符到描述符集合
• FD_CLR(int fd, fd_set* fdset)從描述符集合刪除一个描述符
• FD_ISSET(int fd, fd_set* fdset)檢測一個描述符是否存在於描述符集合

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