Possible select read fd set fix + code passing current grademe.fr

[jwikiera]

Hello, I have been compiling this repo's solution and variations of it, since some time, on various linuxes and compilers. Consistently, I get the situation where after a couple compiles and program starts, netcat disconnects as soon as I try to send a message with it. I heard (but can't find a source for it), that it's bad practice to pass in write ready descriptors to select if we actually don't want to write to them, especially in a loop. In my implementation, I add the current fds to write_set only at the beginning of the send_to_allfunction, before rerunning a select on them. At function exit, I FD_ZEROthe write set.

Here is the full code which currently passes the grademe.fr test exam:

Code

```c #include #include #include #include #include #include int serverfd; typedef struct s_client { int id; char msg[300000]; } t_client; t_client clients[1024]; fd_set read_set, write_set, current; int maxfd = 0, gid = 0; char send_buffer[300000], recv_buffer[300000]; void err(char *msg) { if (msg) write(2, msg, strlen(msg)); else write(2, "Fatal error", 11); write(2, "\n", 1); exit(1); } void send_to_all(int except) { write_set = current; if (select(maxfd + 1, &read_set, &write_set, 0, 0) == -1) { FD_ZERO(&write_set); bzero(send_buffer, sizeof(send_buffer)); return; } for (int fd=0; fd <= maxfd; fd ++) { if (fd != except && FD_ISSET(fd, &write_set) && fd != serverfd) { if (send(fd, send_buffer, strlen(send_buffer), 0) == -1) { err(NULL); } } } FD_ZERO(&write_set); bzero(send_buffer, sizeof(send_buffer)); } int main(int ac, char **av) { if (ac != 2) err("Wrong number of arguments"); struct sockaddr_in serveraddr; socklen_t len; int serverfd = socket(AF_INET, SOCK_STREAM, 0); if (serverfd == -1) err(NULL); maxfd = serverfd; FD_ZERO(¤t); FD_ZERO(&write_set); FD_SET(serverfd, ¤t); bzero(clients, sizeof(clients)); bzero(&serveraddr, sizeof(serveraddr)); bzero(recv_buffer, sizeof(recv_buffer)); bzero(send_buffer, sizeof(send_buffer)); serveraddr.sin_family = AF_INET; serveraddr.sin_addr.s_addr = htonl(INADDR_ANY); serveraddr.sin_port = htons(atoi(av[1])); if (bind(serverfd, (const struct sockaddr *)&serveraddr, sizeof(serveraddr)) == -1 || listen(serverfd, 100) == -1) err(NULL); while (1) { read_set = current; if (select(maxfd + 1, &read_set, &write_set, 0, 0) == -1) continue; for (int fd = 0; fd <= maxfd; fd++) { if (FD_ISSET(fd, &read_set)) { if (fd == serverfd) { int clientfd = accept(serverfd, (struct sockaddr *)&serveraddr, &len); if (clientfd == -1) continue; if (clientfd > maxfd) maxfd = clientfd; clients[clientfd].id = gid++; FD_SET(clientfd, ¤t); sprintf(send_buffer, "server: client %d just arrived\n", clients[clientfd].id); send_to_all(clientfd); } else { int ret = recv(fd, recv_buffer, sizeof(recv_buffer), 0); if (ret <= 0) { sprintf(send_buffer, "server: client %d just left\n", clients[fd].id); send_to_all(fd); FD_CLR(fd, ¤t); close(fd); bzero(clients[fd].msg, strlen(clients[fd].msg)); } else { for (int i = 0, j = strlen(clients[fd].msg); i < ret; i++, j++) { clients[fd].msg[j] = recv_buffer[i]; if (clients[fd].msg[j] == '\n') { clients[fd].msg[j] = '\0'; sprintf(send_buffer, "client %d: %s\n", clients[fd].id, clients[fd].msg); send_to_all(fd); bzero(clients[fd].msg, strlen(clients[fd].msg)); j = -1; } } } } break; } } } return (0); } ```

[jwikiera]

Oh well, it sometimes passes grademe on linux sometimes not. I had the chance to try on mac and select seems to behave differently, the solution of the repo works.

[CarloCattano]

just by changing if (fd != except && FD_ISSET(fd, &write_set) && fd != serverfd) { this if statement with the serverfd global check worked for me , thanks for the idea. Despite the int declared twice global int serverfd, and int serverfd in main, which I removed.

I also didn't add the select on the send_to_all and It passed the grademe, not sure about the actual exam. the current solution in this repo does not pass grademe for me but it does with this changes

[pasqualerossi]

create a pull request and I’ll update the code.