select（）和具有C上动态缓冲区的非阻塞recv

我试图弄清楚为什么recv在下面的代码中被阻止，如果我通过telnet并发送“ GET / HTTP / 1.1”，则recv会一直在等待数据并阻止另一个telnet连接。但是，如果我只是使用固定的缓冲区而不是，它就可以正常工作并且不会阻塞do{} while，即，

char buffer[1024];
nbytes = recv(i, buffer, sizeof buffer, 0);

到目前为止，我所了解的select()处于准备就绪的状态。我需要设置recv为O_NONBLOCK吗？

char *buffer = NULL;
unsigned long LEN = 200;
unsigned long bytes_received = 0;
unsigned long cur_size = 0;
int status = 0;

FD_SET(listener, &master);
fdmax = listener;

for(;;){
    read_fds = master; // copy it
    if(select(fdmax+1, &read_fds, NULL, NULL, NULL) == -1){
        exit(4);
    }   

    for(i = 0; i <= fdmax; i++){
        if(FD_ISSET(i, &read_fds)){
            if(i == listener){
                // handle new connections
                addrlen = sizeof remoteaddr;
                newfd = accept(listener, (struct sockaddr *)&remoteaddr, &addrlen);

                if(newfd == -1){
                    perror("accept");
                }else{
                    FD_SET(newfd, &master);
                    if(newfd > fdmax) { 
                        fdmax = newfd;
                    }
                }
            }else{
                do{
                    if(bytes_received >= cur_size){
                        char * tmp;
                        cur_size += LEN;
                        tmp = realloc(buffer, cur_size);
                        buffer = tmp;
                    }
                    status = recv(i, buffer + bytes_received, LEN, 0); 
                    if(status == 0){ 
                        printf("Done\n");
                    }
                    else if(status > 0){ 
                        bytes_received += status;
                    }
                    else{
                        fprintf(stderr, "socket error\n");
                    }
                } while (status > 0); 
                if(send(i, buffer, strlen(buffer), 0) == -1){
                    perror("send");
                }   
            }   
        }   
    }
}