#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <stdio.h>#include <unistd.h>#include <fcntl.h>#include <cstdint>#include <stdio.h>#include <string>#include <assert.h>#include <arpa/inet.h>#include <string.h>#include "netStream.h"int SetNonBlocking(int nSockFd){ int nOldOption = fcntl(nSockFd, F_GETFL); if(nOldOption == -1) { assert(false); return -1; } int nNewOption = nOldOption | O_NONBLOCK; if(fcntl(nSockFd, F_SETFL, nNewOption) == -1) { assert(false); return -1; } return 0;}int main(){ struct sockaddr_in stServerAddress; bzero(&stServerAddress, sizeof(stServerAddress)); stServerAddress.sin_family = AF_INET; inet_pton(AF_INET, "127.0.0.1", &stServerAddress.sin_addr); stServerAddress.sin_port = htons(12345); int fd = socket(PF_INET, SOCK_STREAM, 0); assert(fd >= 0); SetNonBlocking(fd); int ret = connect(fd, (struct sockaddr*)&stServerAddress, sizeof(stServerAddress)); printf("ret = %d, errno is %d, error:%s\n", ret, errno, strerror(errno)); ret = connect(fd, (struct sockaddr*)&stServerAddress, sizeof(stServerAddress)); printf("ret = %d, errno is %d, error:%s\n", ret, errno, strerror(errno)); ret = connect(fd, (struct sockaddr*)&stServerAddress, sizeof(stServerAddress)); printf("ret = %d, errno is %d, error:%s\n", ret, errno, strerror(errno));}我的执行结果是：ret = -1, errno is 115, error:Operation now in progressret = -1, errno is 111, error:Connection refusedret = -1, errno is 115, error:Operation now in progress为什么第二个connect才是ECONNREFUSED，第一个connect其实就连不上。现在困惑的是哪些情况下返回ECONNREFUSED，哪些情况下返回EINPROGRESS，这两个错误码有什么区别呢？