I'm writing user-space application which among other functionality uses netlink sockets to talk to the kernel. I use simple API provided by open source library libmnl.
My application sets certain options over netlink as well as it subscribes to netlink events (notifications), parses it etc. So this second feature (event notifications) is asynchronous, currently I implemented a simple select() based loop:
...
fd_set rfd;
struct timeval tv;
int ret;
while (1) {
tv.tv_sec = 1;
tv.tv_usec = 0;
FD_ZERO(&rfd);
/* fd - is a netlink socket */
FD_SET(fd, &rfd);
ret = select(fd + 1, &rfd, NULL, NULL, &tv);
if (ret < 0) {
perror("select()");
continue;
} else if (ret == 0) {
printf("Timeout on fd %d", fd);
} else if (FD_ISSET(fd, &rfd)) {
/*
count = recv(fd, buf ...)
while (count > 0) {
parse 'buf' for netlink message, validate etc.
count = recv(fd, buf)
}
*/
}
}
So I'm observing now that code inside else if (FD_ISSET(fd, &rfd)) { branch blocks at the second recv() call.
Now I'm trying to understand if I need to set the netlink socket to non-blocking (SOCK_NOBLOCK for example), but then I probably don't need select() at all, I simply can have recv() -> message parse -> recv() loop and it won't block.
... if I need to set the netlink socket to non-blocking ..., but then I probably don't need select() at all ...
Exactly this is the purpose of a non-blocking socket: Instead of doing the if(FD_ISSET(...)) you call recv() and evaluate the return value.
If you use blocking sockets, you must not call recv() more than once after calling select(); then the program is "effectively" non-blocking.
HOWEVER,
... as user "kaylum" already suggested in his comment, you'll have another problem in any case:
It is not guaranteed that one complete "message" is available at the same time. The other end of the socket might send the first part of the message, wait some seconds and then send the second part of the message.
However, select() will tell you that there is at least one byte available; it will not tell you if the complete message is available.
If you want to wait for the complete message in the inner loop (while(count > 0)), you will always have to wait (which means that your program has "effectively" a blocking behavior even if the socket is non-blocking).
If you simply want to process all bytes already available in the inner loop, then the condition count > 0 is wrong. Instead, you should do something like this if you are working with blocking sockets:
else if(FD_ISSET(...))
{
while(FD_ISSET(...))
{
count = recv(...);
if(count > 0)
{
...
select(...);
}
else FD_ZERO(...);
}
}
However, in most cases this will not be necessary and you can simply process the "remaining" data bytes in the next "outer" loop.
Suppose the following series of events occurs:
We set up a listening socket
Thread A blocks waiting for the listening socket to become readable, using EPOLLIN | EPOLLEXCLUSIVE
Thread B also blocks waiting for the listening socket to become readable, also using EPOLLIN | EPOLLEXCLUSIVE
An incoming connection arrives at the listening socket, making the socket readable, and the kernel elects to wake up thread A.
But, before the thread actually wakes up and calls accept, a second incoming connection arrives at the listening socket.
Here, the socket is already readable, so the second connection doesn't change that. This is level-triggered epoll, so according to the normal rules, the second connection can be treated as a no-op, and the second thread doesn't need to be awoken. ...Of course, not waking up the second thread would kind of defeat the whole purpose of EPOLLEXCLUSIVE? But my trust in API designers doing the right thing is not as strong as it once was, and I can't find anything in the documentation to rule this out.
Questions
a) Is the above scenario possible, where two connections arrive but only thread is woken? Or is it guaranteed that every distinct incoming connection on a listening socket will wake another thread?
b) Is there a general rule to predict how EPOLLEXCLUSIVE and level-triggered epoll interact?
b) What about EPOLLIN | EPOLLEXCLUSIVE and EPOLLOUT | EPOLLEXCLUSIVE for byte-stream fds, like a connected TCP socket or a pipe? E.g. what happens if more data arrives while a pipe is already readable?
Edited (original answer is after the code used for testing)
To make sure things are clear, I'll go over EPOLLEXCLUSIVE as it relates to edge triggered events (EPOLLET) as well as level-triggered events, to show how these effect expected behavior.
As you well know:
Edge Triggered: Once you set EPOLLET, events are triggered only if they change the state of the fd - meaning that only the first event is triggered and no new events will get triggered until that event is fully handled.
This design is explicitly meant to prevent epoll_wait from returning due to an event that is in the process of being handled (i.e., when new data arrives while the EPOLLIN was already raised but read hadn't been called or not all of the data was read).
The edge-triggered event rule is simple all same-type (i.e. EPOLLIN) events are merged until all available data was processed.
In the case of a listening socket, the EPOLLIN event won't be triggered again until all existing listen "backlog" sockets have been accepted using accept.
In the case of a byte stream, new events won't be triggered until all the the available bytes have been read from the stream (the buffer was emptied).
Level Triggered: On the other hand, level triggered events will behave closer to how legacy select (or poll) operates, allowing epoll to be used with older code.
The event-merger rule is more complex: events of the same type are only merged if no one is waiting for an event (no one is waiting for epoll_wait to return), or if multiple events happen before epoll_wait can return... otherwise any event causes epoll_wait to return.
In the case of a listening socket, the EPOLLIN event will be triggered every time a client connects... unless no one is waiting for epoll_wait to return, in which case the next call for epoll_wait will return immediately and all the EPOLLIN events that occurred during that time will have been merged into a single event.
In the case of a byte stream, new events will be triggered every time new data comes in... unless, of course, no one is waiting for epoll_wait to return, in which case the next call will return immediately for all the data that arrive util epoll_wait returned (even if it arrived in different chunks / events).
Exclusive return: The EPOLLEXCLUSIVE flag is used to prevent the "thundering heard" behavior, so only a single epoll_wait caller is woken up for each fd wake-up event.
As I pointed out before, for edge-triggered states, an fd wake-up event is a change in the fd state. So all EPOLLIN events will be raised until all data was read (the listening socket's backlog was emptied).
On the other hand, for level triggered events, each EPOLLIN will invoke a wake up event. If no one is waiting, these events will be merged.
Following the example in your question:
For level triggered events: every time a client connects, a single thread will return from epoll_wait... BUT, if two more clients were to connect while both threads were busy accepting the first two clients, these EPOLLIN events would merge into a single event and the next call to epoll_wait will return immediately with that merged event.
In the context of the example given in the question, thread B is expected to "wake up" due to epoll_wait returning.
In this case, both threads will "race" towards accept.
However, this doesn't defeat the EPOLLEXCLUSIVE directive or intent.
The EPOLLEXCLUSIVE directive is meant to prevent the "thundering heard" phenomenon. In this case, two threads are racing to accept two connections. Each thread can (presumably) call accept safely, with no errors. If three threads were used, the third would keep on sleeping.
If the EPOLLEXCLUSIVE weren't used, all the epoll_wait threads would have been woken up whenever a connection was available, meaning that as soon as the first connection arrived, both threads would have been racing to accept a single connection (resulting in a possible error for one of them).
For edge triggered events: only one thread is expected to receive the "wake up" call. That thread is expected to accept all waiting connections (empty the listen "backlog"). No more EPOLLIN events will be raised for that socket until the backlog is emptied.
The same applies to readable sockets and pipes. The thread that was woken up is expected to deal with all the readable data. This prevents to waiting threads from attempting to read the data concurrently and experiencing file lock race conditions.
I would recommend (and this is what I do) to set the listening socket to non-blocking mode and calling accept in a loop until an EAGAIN (or EWOULDBLOCK) error is raised, indicating that the backlog is empty. There is no way to avoid the risk of events being merged. The same is true for reading from a socket.
Testing this with code:
I wrote a simple test, with some sleep commands and blocking sockets. Client sockets are initiated only after both threads start waiting for epoll.
Client thread initiation is delayed, so client 1 and client 2 start a second apart.
Once a server thread is woken up, it will sleep for a second (allowing the second client to do it's thing) before calling accept. Maybe the servers should sleep a little more, but it seems close enough to manage the scheduler without resorting to conditional variables.
Here are the results of my test code (which might be a mess, I'm not the best person for test design)...
On Ubuntu 16.10, which supports EPOLLEXCLUSIVE, the test results show that the listening threads are woken up one after the other, in response to the clients. In the example in the question, thread B is woken up.
Test address: <null>:8000
Server thread 2 woke up with 1 events
Server thread 2 will sleep for a second, to let things happen.
client number 1 connected
Server thread 1 woke up with 1 events
Server thread 1 will sleep for a second, to let things happen.
client number 2 connected
Server thread 2 accepted a connection and saying hello.
client 1: Hello World - from server thread 2.
Server thread 1 accepted a connection and saying hello.
client 2: Hello World - from server thread 1.
To compare with Ubuntu 16.04 (without EPOLLEXCLUSIVE support), than both threads are woken up for the first connection. Since I use blocking sockets, the second thread hangs on accept until client # 2 connects.
main.c:178:2: warning: #warning EPOLLEXCLUSIVE undeclared, test is futile [-Wcpp]
#warning EPOLLEXCLUSIVE undeclared, test is futile
^
Test address: <null>:8000
Server thread 1 woke up with 1 events
Server thread 1 will sleep for a second, to let things happen.
Server thread 2 woke up with 1 events
Server thread 2 will sleep for a second, to let things happen.
client number 1 connected
Server thread 1 accepted a connection and saying hello.
client 1: Hello World - from server thread 1.
client number 2 connected
Server thread 2 accepted a connection and saying hello.
client 2: Hello World - from server thread 2.
For one more comparison, the results for level triggered kqueue show that both threads are awoken for the first connection. Since I use blocking sockets, the second thread hangs on accept until client # 2 connects.
Test address: <null>:8000
client number 1 connected
Server thread 2 woke up with 1 events
Server thread 1 woke up with 1 events
Server thread 2 will sleep for a second, to let things happen.
Server thread 1 will sleep for a second, to let things happen.
Server thread 2 accepted a connection and saying hello.
client 1: Hello World - from server thread 2.
client number 2 connected
Server thread 1 accepted a connection and saying hello.
client 2: Hello World - from server thread 1.
My test code was (sorry for the lack of comments and the messy code, I wasn't writing for future maintenance):
#ifndef _GNU_SOURCE
#define _GNU_SOURCE
#endif
#define ADD_EPOLL_OPTION 0 // define as EPOLLET or 0
#include <arpa/inet.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <netdb.h>
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <sys/types.h>
#include <time.h>
#include <unistd.h>
#if !defined(__linux__) && !defined(__CYGWIN__)
#include <sys/event.h>
#define reactor_epoll 0
#else
#define reactor_epoll 1
#include <sys/epoll.h>
#include <sys/timerfd.h>
#endif
int sock_listen(const char *address, const char *port);
void *listen_threard(void *arg);
void *client_thread(void *arg);
int server_fd;
char const *address = NULL;
char const *port = "8000";
int main(int argc, char const *argv[]) {
if (argc == 2) {
port = argv[1];
} else if (argc == 3) {
port = argv[2];
address = argv[1];
}
fprintf(stderr, "Test address: %s:%s\n", address ? address : "<null>", port);
server_fd = sock_listen(address, port);
/* code */
pthread_t threads[4];
for (size_t i = 0; i < 2; i++) {
if (pthread_create(threads + i, NULL, listen_threard, (void *)i))
perror("couldn't initiate server thread"), exit(-1);
}
for (size_t i = 2; i < 4; i++) {
sleep(1);
if (pthread_create(threads + i, NULL, client_thread, (void *)i))
perror("couldn't initiate client thread"), exit(-1);
}
// join only server threads.
for (size_t i = 0; i < 2; i++) {
pthread_join(threads[i], NULL);
}
close(server_fd);
sleep(1);
return 0;
}
/**
Sets a socket to non blocking state.
*/
inline int sock_set_non_block(int fd) // Thanks to Bjorn Reese
{
/* If they have O_NONBLOCK, use the Posix way to do it */
#if defined(O_NONBLOCK)
/* Fixme: O_NONBLOCK is defined but broken on SunOS 4.1.x and AIX 3.2.5. */
int flags;
if (-1 == (flags = fcntl(fd, F_GETFL, 0)))
flags = 0;
// printf("flags initial value was %d\n", flags);
return fcntl(fd, F_SETFL, flags | O_NONBLOCK);
#else
/* Otherwise, use the old way of doing it */
static int flags = 1;
return ioctl(fd, FIOBIO, &flags);
#endif
}
/* open a listenning socket */
int sock_listen(const char *address, const char *port) {
int srvfd;
// setup the address
struct addrinfo hints;
struct addrinfo *servinfo; // will point to the results
memset(&hints, 0, sizeof hints); // make sure the struct is empty
hints.ai_family = AF_UNSPEC; // don't care IPv4 or IPv6
hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
if (getaddrinfo(address, port, &hints, &servinfo)) {
perror("addr err");
return -1;
}
// get the file descriptor
srvfd =
socket(servinfo->ai_family, servinfo->ai_socktype, servinfo->ai_protocol);
if (srvfd <= 0) {
perror("socket err");
freeaddrinfo(servinfo);
return -1;
}
// // keep the server socket blocking for the test.
// // make sure the socket is non-blocking
// if (sock_set_non_block(srvfd) < 0) {
// perror("couldn't set socket as non blocking! ");
// freeaddrinfo(servinfo);
// close(srvfd);
// return -1;
// }
// avoid the "address taken"
{
int optval = 1;
setsockopt(srvfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval));
}
// bind the address to the socket
{
int bound = 0;
for (struct addrinfo *p = servinfo; p != NULL; p = p->ai_next) {
if (!bind(srvfd, p->ai_addr, p->ai_addrlen))
bound = 1;
}
if (!bound) {
// perror("bind err");
freeaddrinfo(servinfo);
close(srvfd);
return -1;
}
}
freeaddrinfo(servinfo);
// listen in
if (listen(srvfd, SOMAXCONN) < 0) {
perror("couldn't start listening");
close(srvfd);
return -1;
}
return srvfd;
}
/* will start listenning, sleep for 5 seconds, then accept all the backlog and
* finish */
void *listen_threard(void *arg) {
int epoll_fd;
ssize_t event_count;
#if reactor_epoll
#ifndef EPOLLEXCLUSIVE
#warning EPOLLEXCLUSIVE undeclared, test is futile
#define EPOLLEXCLUSIVE 0
#endif
// create the epoll wait fd
epoll_fd = epoll_create1(0);
if (epoll_fd < 0)
perror("couldn't create epoll fd"), exit(1);
// add the server fd to the epoll watchlist
{
struct epoll_event chevent = {0};
chevent.data.ptr = (void *)((uintptr_t)server_fd);
chevent.events =
EPOLLOUT | EPOLLIN | EPOLLERR | EPOLLEXCLUSIVE | ADD_EPOLL_OPTION;
epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_fd, &chevent);
}
// wait with epoll
struct epoll_event events[10];
event_count = epoll_wait(epoll_fd, events, 10, 5000);
#else
// testing on BSD, use kqueue
epoll_fd = kqueue();
if (epoll_fd < 0)
perror("couldn't create kqueue fd"), exit(1);
// add the server fd to the kqueue watchlist
{
struct kevent chevent[2];
EV_SET(chevent, server_fd, EVFILT_READ, EV_ADD | EV_ENABLE, 0, 0,
(void *)((uintptr_t)server_fd));
EV_SET(chevent + 1, server_fd, EVFILT_WRITE, EV_ADD | EV_ENABLE, 0, 0,
(void *)((uintptr_t)server_fd));
kevent(epoll_fd, chevent, 2, NULL, 0, NULL);
}
// wait with kqueue
static struct timespec reactor_timeout = {.tv_sec = 5, .tv_nsec = 0};
struct kevent events[10];
event_count = kevent(epoll_fd, NULL, 0, events, 10, &reactor_timeout);
#endif
close(epoll_fd);
if (event_count <= 0) {
fprintf(stderr, "Server thread %lu wakeup no events / error\n",
(size_t)arg + 1);
perror("errno ");
return NULL;
}
fprintf(stderr, "Server thread %lu woke up with %lu events\n",
(size_t)arg + 1, event_count);
fprintf(stderr,
"Server thread %lu will sleep for a second, to let things happen.\n",
(size_t)arg + 1);
sleep(1);
int connfd;
struct sockaddr_storage client_addr;
socklen_t client_addrlen = sizeof client_addr;
/* accept up all connections. we're non-blocking, -1 == no more connections */
if ((connfd = accept(server_fd, (struct sockaddr *)&client_addr,
&client_addrlen)) >= 0) {
fprintf(stderr,
"Server thread %lu accepted a connection and saying hello.\n",
(size_t)arg + 1);
if (write(connfd, arg ? "Hello World - from server thread 2."
: "Hello World - from server thread 1.",
35) < 35)
perror("server write failed");
close(connfd);
} else {
fprintf(stderr, "Server thread %lu failed to accept a connection",
(size_t)arg + 1);
perror(": ");
}
return NULL;
}
void *client_thread(void *arg) {
int fd;
// setup the address
struct addrinfo hints;
struct addrinfo *addrinfo; // will point to the results
memset(&hints, 0, sizeof hints); // make sure the struct is empty
hints.ai_family = AF_UNSPEC; // don't care IPv4 or IPv6
hints.ai_socktype = SOCK_STREAM; // TCP stream sockets
hints.ai_flags = AI_PASSIVE; // fill in my IP for me
if (getaddrinfo(address, port, &hints, &addrinfo)) {
perror("client couldn't initiate address");
return NULL;
}
// get the file descriptor
fd =
socket(addrinfo->ai_family, addrinfo->ai_socktype, addrinfo->ai_protocol);
if (fd <= 0) {
perror("client couldn't create socket");
freeaddrinfo(addrinfo);
return NULL;
}
// // // Leave the socket blocking for the test.
// // make sure the socket is non-blocking
// if (sock_set_non_block(fd) < 0) {
// freeaddrinfo(addrinfo);
// close(fd);
// return -1;
// }
if (connect(fd, addrinfo->ai_addr, addrinfo->ai_addrlen) < 0 &&
errno != EINPROGRESS) {
fprintf(stderr, "client number %lu FAILED\n", (size_t)arg - 1);
perror("client connect failure");
close(fd);
freeaddrinfo(addrinfo);
return NULL;
}
freeaddrinfo(addrinfo);
fprintf(stderr, "client number %lu connected\n", (size_t)arg - 1);
char buffer[128];
if (read(fd, buffer, 35) < 35) {
perror("client: read error");
close(fd);
} else {
buffer[35] = 0;
fprintf(stderr, "client %lu: %s\n", (size_t)arg - 1, buffer);
close(fd);
}
return NULL;
}
P.S.
As a final recommendation, I would consider having no more than a single thread and a single epoll fd per process. This way the "thundering heard" is a non-issue and EPOLLEXCLUSIVE (which is still very new and isn't widely supported) can be disregarded... the only "thundering heard" this still exposes is for the limited amount of shared sockets, where the race condition might be good for load balancing.
Original Answer
I'm not sure I understand the confusion, so I'll go over EPOLLET and EPOLLEXCLUSIVE to show their combined expected behavior.
As you well know:
Once you set EPOLLET (edge triggered), events are triggered on fd state changes rather than fd events.
This design is explicitly meant to prevent epoll_wait from returning due to an event that is in the process of being handled (i.e., when new data arrives while the EPOLLIN was already raised but read hadn't been called or not all of the data was read).
In the case of a listening socket, the EPOLLIN event won't be triggered again until all existing listen "backlog" sockets have been accepted using accept.
The EPOLLEXCLUSIVE flag is used to prevent the "thundering heard" behavior, so only a single epoll_wait caller is woken up for each fd wake-up event.
As I pointed out before, for edge-triggered states, an fd wake-up event is a change in the fd state. So all EPOLLIN events will be raised until all data was read (the listening socket's backlog was emptied).
When merging these behaviors, and following the example in your question, only one thread is expected to receive the "wake up" call. That thread is expected to accept all waiting connections (empty the listen "backlog") or no more EPOLLIN events will be raised for that socket.
The same applies to readable sockets and pipes. The thread that was woken up is expected to deal with all the readable data. This prevents to waiting threads from attempting to read the data concurrently and experiencing file lock race conditions.
I would recommend that you consider avoiding the edge triggered events if you mean to call accept only once for each epoll_wait wake-up event. Regardless of using EPOLLEXCLUSIVE, you run the risk of not emptying the existing "backlog", so that no new wake-up events will be raised.
Alternatively, I would recommend (and this is what I do) to set the listening socket to non-blocking mode and calling accept in a loop until and an EAGAIN (or EWOULDBLOCK) error is raised, indicating that the backlog is empty.
EDIT 1: Level Triggered Events
It seems, as Nathaniel pointed out in the comment, that I totally misunderstood the question... I guess I'm used to EPOLLET being the misunderstood element.
So, what happens with normal, level-triggered, events (NOT EPOLLET)?
Well... the expected behavior is the exact mirror image (opposite) of edge triggered events.
For listenning sockets, the epoll_wait is expected return whenever a new connected is available, whether accept was called after a previous event or not.
Events are only "merged" if no-one is waiting with epoll_wait... in which case the next call for epoll_wait will return immediately.
In the context of the example given in the question, thread B is expected to "wake up" due to epoll_wait returning.
In this case, both threads will "race" towards accept.
However, this doesn't defeat the EPOLLEXCLUSIVE directive or intent.
The EPOLLEXCLUSIVE directive is meant to prevent the "thundering heard" phenomenon. In this case, two threads are racing to accept two connections. Each thread can (presumably) call accept safely, with no errors. If three threads were used, the third would keep on sleeping.
If the EPOLLEXCLUSIVE weren't used, all the epoll_wait threads would have been woken up whenever a connection was available, meaning that as soon as the first connection arrived, both threads would have been racing to accept a single connection (resulting in a possible error for one of them).
This is only a partial answer, but Jason Baron (the author of the EPOLLEXCLUSIVE patch) just responded to an email I sent him to confirm that when using EPOLLEXCLUSIVE in level-triggered mode he does think it's possible that two connections will arrive but only one thread will be woken (thread B keeps sleeping). So when using EPOLLEXCLUSIVE you have to use the same kinds of defensive programming as you use for edge-trigged epoll, regardless of whether you set EPOLLET.
My understanding of epoll and edge triggered behaviour was that you are notified when a state change occurs for a given file descriptor. Ie, when data becomes available on the fd, you get notified but you only get notified again after you drain all the data (you get EAGAIN) and data becomes present again.
Based on this understanding, I'd implemented a server which does the following:
Listens for client connections
For each connections, add fd into epoll
When data available on fd, spawn a separate thread to read until EAGAIN
Repeat until data reads are done
Semi pseudo-code:
n = epoll_wait(efd, events, MAXEVENTS, -1);
for (i = 0; i < n; ++i) {
check for errors;
if (server_sock == events[i].data.fd) {
conn = accept( ... );
make_nonblocking(conn);
event.data.fd = conn;
event.events = EPOLLIN | EPOLLRDHUP;
epoll_ctl(efd, EPOLL_CTL_ADD, conn, &event);
} else {
spawn_reader(events[i].data.fd);
}
With the above code, my expectation was that until the reader thread hits EAGAIN, epoll will not spawn another thread for this fd. This doesn't seem to be the case and I get many threads being spawned.
Am I using epoll incorrectly? I shouldn't need to remove the fd via EPOLL_CTL_DEL each time I spawn the reader thread, correct?
Yes, its working as it should. epoll allows asynchronous IO programming in a single thread of control (event driven). Using a thread to process reads on an fd does not remove it from the list of monitored fds.
I'm writing a client-server (TCP) program in C on a Unix system. The client sends some information and the server answers. There's only one connection per child process. New connections use pre-running processes from a pool, and the pool size is dynamic, so if the number of free processes (processes not servicing a client) drops too low, it should create new processes, and likewise if it gets too high extra processes should be terminated.
This is my server code. Every connection make a new child process using fork(). Each connection runs in a new process. How can I make a dynamic pool like I explained above?
int main(int argc, char * argv[])
{
int cfd;
int listener = socket(AF_INET, SOCK_STREAM, 0); //create listener socket
if(listener < 0){
perror("socket error");
return 1;
}
struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_port = htons(PORT);
addr.sin_addr.s_addr = htonl(INADDR_ANY);
int binding = bind(listener, (struct sockaddr *)&addr, sizeof(addr));
if(binding < 0){
perror("binding error");
return 1;
}
listen(listener, 1); //listen for new clients
signal(SIGCHLD,handler);
int pid;
for(;;) // infinity loop on server
{
cfd = accept(listener, NULL, NULL); //client socket descriptor
pid = fork(); //make child proc
if(pid == 0) //in child proc...
{
close(listener); //close listener socket descriptor
... //some server actions that I do.(receive or send)
close(cfd); // close client fd
return 0;
}
close(cfd);
}
If you have several processes blocked in accept on the same listen socket, then a new connection that comes in will get delivered to one of them. (Depending, several may wake up, but only one will actually get the connection). So you need to fork several children after listen, but before accept. After handling a request, the child goes back to accept instead of exit. That handles (1) and (2).
(3) is harder. You need some form of IPC. Typically, you'd have a parent process that just manages having the right number of children. Your child processes need to use IPC to tell the parent how busy they are. The parent can then either fork more children (which go into the accept loop above) or send signals to children to tell them to finish up and exit. It should also handle waiting on children, handle unexpected deaths, etc.
The IPC you want to use is probably shared memory. Your two options are SysV (shmget) and POSIX (shm_open`) shared memory. You probably want the latter if available. You'll have to deal with synchronizing access (both POSIX and SysV provide semaphores to help with this, again prefer POSIX) or using atomic access only.
(You probably don't actually want a process to exit the instant there are more than X free children, that'll lead to repeatedly reaping and spawning them, which is expensive. Instead you probably want some measure of how utilized they were over the last second... So your data is more complicated than a bitmap of in use/free.)
There are a lot of daemons that work like this, so you can fairly easily find code examples. Of course, if you go look at Apache, you'll probably find it more complicated, to get good performance and be portable everywhere.
In Unix Network Programming there is an example of a Pre-forked server which uses message passing on a Unix Domain Pipe to instruct child processes to handle an incoming connection:
for ( ; ; ) {
rset = masterset;
if (navail <= 0)
FD_CLR(listenfd, &rset); /* turn off if no available children */
nsel = Select(maxfd + 1, &rset, NULL, NULL, NULL);
/* 4check for new connections */
if (FD_ISSET(listenfd, &rset)) {
clilen = addrlen;
connfd = Accept(listenfd, cliaddr, &clilen);
for (i = 0; i < nchildren; i++)
if (cptr[i].child_status == 0)
break; /* available */
if (i == nchildren)
err_quit("no available children");
cptr[i].child_status = 1; /* mark child as busy */
cptr[i].child_count++;
navail--;
n = Write_fd(cptr[i].child_pipefd, "", 1, connfd);
Close(connfd);
if (--nsel == 0)
continue; /* all done with select() results */
}
As you can see, the parent writes the file descriptor number for the socket to the pipe, and then calls close on the file descriptor. When the preforked children finish with the socket they also call close on the descriptor. The thing which is throwing me for a loop is that because these children are preforked I would assume that only file descriptors which existed at the time the children were forked would be shared. However, if that was true, then this example would fail spectacularly, yet it works.
Can someone shed some light on how it is that file descriptors created by the parent after the fork end up being shared with the children process?
Take a look at the Write_fd implementation. It uses something like
union {
struct cmsghdr cm;
char control[CMSG_SPACE(sizeof(int))];
} control_un;
struct cmsghdr *cmptr;
msg.msg_control = control_un.control;
msg.msg_controllen = sizeof(control_un.control);
cmptr = CMSG_FIRSTHDR(&msg);
cmptr->cmsg_len = CMSG_LEN(sizeof(int));
cmptr->cmsg_level = SOL_SOCKET;
cmptr->cmsg_type = SCM_RIGHTS;
*((int *) CMSG_DATA(cmptr)) = sendfd;
That is, sending a control message with type SCM_RIGHTS is a way unixes can share a file descriptor with an unreleated process.
You can send (most) arbitrary file descriptors to a potentially unrelated process using the FD passing mechanism in Unix sockets.
This is typically a little-used mechanism and rather tricky to get right - both processes need to cooperate.
Most prefork servers do NOT do this, rather, they have the child process call accept() on a shared listen socket, and create its own connected socket this way. Other processes cannot see this connected socket, and there is only one copy of it, so when the child closes it, it's gone.
One disadvantage is that the process cannot tell what the client is going to request BEFORE calling accept, so you cannot handle different types of requests in different children etc. Once one child has accept()ed it, another child cannot.