c - 为什么我的recv调用会阻塞？

Question

我有一个简单的 Web 服务，可以通过 HTTP 访问。我有一个使用此服务的 C 程序。由于我无法理解的原因，我的 recv 调用阻塞了近 5 秒。响应的大小小于 600 字节，API 调用时间小于 100 毫秒(从浏览器调用时)。

什么可能导致此行为？我尝试过使用 TCP_NODELAY 选项，这似乎没有什么区别。发送命令(大小相似，并且紧接在接收之前发生)相对较快。

编辑:代码，根据要求 - 整个函数很大，但这是有问题的代码:

connect(tcpSocket, (struct sockaddr *) &serveraddr, sizeof(serveraddr));

char* args = cJSON_PrintUnformatted(root);
sprintf(request_string, "GET %s?json=%s HTTP/1.1\r\nHost: %s\r\n\r\n\r\n", API_PAGE, curl_easy_escape(NULL, args, strlen(args)), API_HOST);
send(tcpSocket, request_string, strlen(request_string), 0);
recv(tcpSocket, request_string, 4095,0);

编辑2:tcpdump输出

20:03:03.976311 IP localhost.48732 > localhost.http: Flags [S], seq 3254094788, win 43690, options [mss 65495,sackOK,TS val 83001890 ecr 0,nop,wscale 7], length 0
20:03:03.976324 IP localhost.http > localhost.48732: Flags [S.], seq 3772034334, ack 3254094789, win 43690, options [mss 65495,sackOK,TS val 83001890 ecr 83001890,nop,wscale 7], length 0
20:03:03.976335 IP localhost.48732 > localhost.http: Flags [.], ack 1, win 342, options [nop,nop,TS val 83001890 ecr 83001890], length 0
20:03:03.976367 IP localhost.48732 > localhost.http: Flags [P.], seq 1:380, ack 1, win 342, options [nop,nop,TS val 83001890 ecr 83001890], length 379
20:03:03.976382 IP localhost.http > localhost.48732: Flags [.], ack 380, win 350, options [nop,nop,TS val 83001890 ecr 83001890], length 0
20:03:09.055255 IP localhost.http > localhost.48732: Flags [P.], seq 1:605, ack 380, win 350, options [nop,nop,TS val 83003159 ecr 83001890], length 604
20:03:09.055293 IP localhost.48732 > localhost.http: Flags [.], ack 605, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055310 IP localhost.48732 > localhost.http: Flags [F.], seq 380, ack 605, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055314 IP localhost.http > localhost.48732: Flags [F.], seq 605, ack 380, win 350, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055320 IP localhost.48732 > localhost.http: Flags [.], ack 606, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055320 IP localhost.http > localhost.48732: Flags [.], ack 381, win 350, options [nop,nop,TS val 83003159 ecr 83003159], length 0

Answer 1

让我们回顾一下 tcpdump:

一、TCP连接建立:

20:03:03.976311 IP localhost.48732 > localhost.http: Flags [S], seq 3254094788, win 43690, options [mss 65495,sackOK,TS val 83001890 ecr 0,nop,wscale 7], length 0
20:03:03.976324 IP localhost.http > localhost.48732: Flags [S.], seq 3772034334, ack 3254094789, win 43690, options [mss 65495,sackOK,TS val 83001890 ecr 83001890,nop,wscale 7], length 0
20:03:03.976335 IP localhost.48732 > localhost.http: Flags [.], ack 1, win 342, options [nop,nop,TS val 83001890 ecr 83001890], length 0

接下来，数据从客户端发送到服务器并收到 ACK:

20:03:03.976367 IP localhost.48732 > localhost.http: Flags [P.], seq 1:380, ack 1, win 342, options [nop,nop,TS val 83001890 ecr 83001890], length 379
20:03:03.976382 IP localhost.http > localhost.48732: Flags [.], ack 380, win 350, options [nop,nop,TS val 83001890 ecr 83001890], length 0

并且，5秒后，来自服务器的答复:

20:03:09.055255 IP localhost.http > localhost.48732: Flags [P.], seq 1:605, ack 380, win 350, options [nop,nop,TS val 83003159 ecr 83001890], length 604
20:03:09.055293 IP localhost.48732 > localhost.http: Flags [.], ack 605, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0

最后，关闭连接:

20:03:09.055310 IP localhost.48732 > localhost.http: Flags [F.], seq 380, ack 605, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055314 IP localhost.http > localhost.48732: Flags [F.], seq 605, ack 380, win 350, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055320 IP localhost.48732 > localhost.http: Flags [.], ack 606, win 351, options [nop,nop,TS val 83003159 ecr 83003159], length 0
20:03:09.055320 IP localhost.http > localhost.48732: Flags [.], ack 381, win 350, options [nop,nop,TS val 83003159 ecr 83003159], length 0

总而言之，服务器导致了延迟。