解决:阻塞socket连接成功情况下recv返回值为-1

原创

李兆龙的博客 2022-07-01 00:19:31 博主文章分类：网络编程 ©著作权

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引言

直入正题在前几天进行Web服务器的编写时遇到了这么一个奇怪的问题就是在socket连接成功且在epoll触发可读事件以后recv却没有收到任何东西返回-1 在解决后遂记录一篇博客

首先在刚开始我使用了ET非阻塞，所以在出现recv返回值为-1 且 errno等于EAGAIN时当然首先想到的就是一次没有收到这个包循环接收即可但当我改为循环接收时却发现每一次都没有收到这个包即陷入了死循环这就很奇怪了，这个时候鸭鸭学长告诉要不改成LT 但是不recv看看包是否发到，结果发现包确实是接收到了，但当我加上recv时心想应该出现阻塞或接收成功时却出现了死循环的情况，这是阻塞啊，奇怪奇怪，怎么会出现这种情况呢，这个时候突然想起recv的第四个参数有一个是可以使本次recv成为非阻塞的，难道是参数的问题，一看socket类的构造函数，顺手把第四个参数的默认参数写成了-1，这个时候问题的答案也就水落石出了，-1的补码为1111 1111 1111 1111，下面是recv的第四个参数

/* Bits in the FLAGS argument to `send', `recv', et al.  */
enum
  {
    MSG_OOB   = 0x01, /* Process out-of-band data.  */
#define MSG_OOB   MSG_OOB
    MSG_PEEK    = 0x02, /* Peek at incoming messages.  */
#define MSG_PEEK  MSG_PEEK
    MSG_DONTROUTE = 0x04, /* Don't use local routing.  */
#define MSG_DONTROUTE MSG_DONTROUTE
#ifdef __USE_GNU
    /* DECnet uses a different name.  */
    MSG_TRYHARD   = MSG_DONTROUTE,
# define MSG_TRYHARD  MSG_DONTROUTE
#endif
    MSG_CTRUNC    = 0x08, /* Control data lost before delivery.  */
#define MSG_CTRUNC  MSG_CTRUNC
    MSG_PROXY   = 0x10, /* Supply or ask second address.  */
#define MSG_PROXY MSG_PROXY
    MSG_TRUNC   = 0x20,
#define MSG_TRUNC MSG_TRUNC
    MSG_DONTWAIT  = 0x40, /* Nonblocking IO.  */
#define MSG_DONTWAIT  MSG_DONTWAIT
    MSG_EOR   = 0x80, /* End of record.  */
#define MSG_EOR   MSG_EOR
    MSG_WAITALL   = 0x100, /* Wait for a full request.  */
#define MSG_WAITALL MSG_WAITALL
    MSG_FIN   = 0x200,
#define MSG_FIN   MSG_FIN
    MSG_SYN   = 0x400,
#define MSG_SYN   MSG_SYN
    MSG_CONFIRM   = 0x800, /* Confirm path validity.  */
#define MSG_CONFIRM MSG_CONFIRM
    MSG_RST   = 0x1000,
#define MSG_RST   MSG_RST
    MSG_ERRQUEUE  = 0x2000, /* Fetch message from error queue.  */
#define MSG_ERRQUEUE  MSG_ERRQUEUE
    MSG_NOSIGNAL  = 0x4000, /* Do not generate SIGPIPE.  */
#define MSG_NOSIGNAL  MSG_NOSIGNAL
    MSG_MORE    = 0x8000,  /* Sender will send more.  */
#define MSG_MORE  MSG_MORE
    MSG_WAITFORONE  = 0x10000, /* Wait for at least one packet to return.*/
#define MSG_WAITFORONE  MSG_WAITFORONE
    MSG_BATCH   = 0x40000, /* sendmmsg: more messages coming.  */
#define MSG_BATCH MSG_BATCH
    MSG_ZEROCOPY  = 0x4000000, /* Use user data in kernel path.  */
#define MSG_ZEROCOPY  MSG_ZEROCOPY
    MSG_FASTOPEN  = 0x20000000, /* Send data in TCP SYN.  */
#define MSG_FASTOPEN  MSG_FASTOPEN

    MSG_CMSG_CLOEXEC  = 0x40000000  /* Set close_on_exit for file
             descriptor received through
             SCM_RIGHTS.  */
#define MSG_CMSG_CLOEXEC MSG_CMSG_CLOEXEC
  };