I would like to see how much time it takes for connect syscall. I get the code for a simple TCP client. However, the program will wait for the server to respond after connect. How can I make it return right after syscall or using some other ways to time the syscall time?
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
int main(int argc, char *argv[]) {
int sockfd = 0, n = 0;
char recvBuff[1024];
struct sockaddr_in serv_addr;
if(argc != 2) {
printf("\n Usage: %s <ip of server> \n",argv[0]);
return 1;
}
memset(recvBuff, '0',sizeof(recvBuff));
if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
printf("\n Error : Could not create socket \n");
return 1;
}
memset(&serv_addr, '0', sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(5000);
if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0) {
printf("\n inet_pton error occured\n");
return 1;
}
if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
printf("\n Error : Connect Failed \n");
return 1;
}
printf("\nhello\n");
while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0) {
recvBuff[n] = 0;
if(fputs(recvBuff, stdout) == EOF) {
printf("\n Error : Fputs error\n");
}
}
if(n < 0)
{
printf("\n Read error \n");
}
return 0;
}
However, the program will wait for the server to respond after connect.
Yes, and it does so here:
while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0) {
recvBuff[n] = 0;
if(fputs(recvBuff, stdout) == EOF) {
printf("\n Error : Fputs error\n");
}
}
How can I make it return right after syscall
Err, remove the receive loop above?
I was writing a simple socket program for a server, where I got a hang in listen() call. Surprisingly, this piece of code hangs :
if((res = listen(sockfd, 5)) == -1)
{
perror("Error in listening over socket");
exit(1);
}
How come this is possible? Here's my full code for reference :
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#define MYPORT 7891
int main()
{
int sockfd, newfd, res;
struct sockaddr_in my_addr, their_addr;
socklen_t their_addr_size;
char msg[100] = {'\0'};
/* open a socket for the server */
if((sockfd = socket(AF_INET, SOCK_STREAM,0)) == -1)
{
perror("Error opening socket");
exit(1);
}
printf("Socket opened successfully\n");
/* specify the interface details, where the server should listen for incoming messages. It is set by bind */
my_addr.sin_family = AF_INET;
my_addr.sin_port = htons(MYPORT);
my_addr.sin_addr.s_addr = INADDR_ANY; /* listen on every interface, eth0, wlan, whatever f**kin place */
memset(&(my_addr.sin_zero),0,8);
if((res = bind(sockfd, (struct sockaddr *)&(my_addr), sizeof(struct sockaddr_in))) == -1)
{
perror("Error while bind()");
exit(1);
}
printf("Bind() is successfull\n");
/* listen on the socket, setting the waiting queue size to max 5 connections. Other connections will get ECONNREFUSED error */
if((res = listen(sockfd, 5)) == -1)
{
perror("Error in listening over socket");
exit(1);
}
// if(listen(sockfd,5)==0)
// printf("Listening\n");
// else
// printf("Error\n");
printf("Listening....");
/* accept incoming request */
their_addr_size = sizeof(struct sockaddr_in);
if((newfd = accept(sockfd, (struct sockaddr *)&their_addr, &their_addr_size)) == -1)
{
perror("Error accepting connection");
exit(1);
}
/* write data */
printf("Enter the data to be sent\n");
while(1)
{
scanf("%s",msg);
write(newfd, msg, strlen(msg));
}
/* though it never comes here due to infinite while loop */
close(newfd);
close(sockfd);
return 0;
}
I am not getting "Listening...".
It was due to the sdtout data buffered. Doing fflush(stdout), gave the proper print. And the process is now blocked at expected position accept().
What would be your suggestion in order to create a single instance application, so that only one process is allowed to run at a time? File lock, mutex or what?
A good way is:
#include <sys/file.h>
#include <errno.h>
int pid_file = open("/var/run/whatever.pid", O_CREAT | O_RDWR, 0666);
int rc = flock(pid_file, LOCK_EX | LOCK_NB);
if(rc) {
if(EWOULDBLOCK == errno)
; // another instance is running
}
else {
// this is the first instance
}
Note that locking allows you to ignore stale pid files (i.e. you don't have to delete them). When the application terminates for any reason the OS releases the file lock for you.
Pid files are not terribly useful because they can be stale (the file exists but the process does not). Hence, the application executable itself can be locked instead of creating and locking a pid file.
A more advanced method is to create and bind a unix domain socket using a predefined socket name. Bind succeeds for the first instance of your application. Again, the OS unbinds the socket when the application terminates for any reason. When bind() fails another instance of the application can connect() and use this socket to pass its command line arguments to the first instance.
Here is a solution in C++. It uses the socket recommendation of Maxim. I like this solution better than the file based locking solution, because the file based one fails if the process crashes and does not delete the lock file. Another user will not be able to delete the file and lock it. The sockets are automatically deleted when the process exits.
Usage:
int main()
{
SingletonProcess singleton(5555); // pick a port number to use that is specific to this app
if (!singleton())
{
cerr << "process running already. See " << singleton.GetLockFileName() << endl;
return 1;
}
... rest of the app
}
Code:
#include <netinet/in.h>
class SingletonProcess
{
public:
SingletonProcess(uint16_t port0)
: socket_fd(-1)
, rc(1)
, port(port0)
{
}
~SingletonProcess()
{
if (socket_fd != -1)
{
close(socket_fd);
}
}
bool operator()()
{
if (socket_fd == -1 || rc)
{
socket_fd = -1;
rc = 1;
if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
{
throw std::runtime_error(std::string("Could not create socket: ") + strerror(errno));
}
else
{
struct sockaddr_in name;
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
rc = bind (socket_fd, (struct sockaddr *) &name, sizeof (name));
}
}
return (socket_fd != -1 && rc == 0);
}
std::string GetLockFileName()
{
return "port " + std::to_string(port);
}
private:
int socket_fd = -1;
int rc;
uint16_t port;
};
For windows, a named kernel object (e.g. CreateEvent, CreateMutex). For unix, a pid-file - create a file and write your process ID to it.
You can create an "anonymous namespace" AF_UNIX socket. This is completely Linux-specific, but has the advantage that no filesystem actually has to exist.
Read the man page for unix(7) for more info.
Avoid file-based locking
It is always good to avoid a file based locking mechanism to implement the singleton instance of an application. The user can always rename the lock file to a different name and run the application again as follows:
mv lockfile.pid lockfile1.pid
Where lockfile.pid is the lock file based on which is checked for existence before running the application.
So, it is always preferable to use a locking scheme on object directly visible to only the kernel. So, anything which has to do with a file system is not reliable.
So the best option would be to bind to a inet socket. Note that unix domain sockets reside in the filesystem and are not reliable.
Alternatively, you can also do it using DBUS.
It's seems to not be mentioned - it is possible to create a mutex in shared memory but it needs to be marked as shared by attributes (not tested):
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_t *mutex = shmat(SHARED_MEMORY_ID, NULL, 0);
pthread_mutex_init(mutex, &attr);
There is also shared memory semaphores (but I failed to find out how to lock one):
int sem_id = semget(SHARED_MEMORY_KEY, 1, 0);
No one has mentioned it, but sem_open() creates a real named semaphore under modern POSIX-compliant OSes. If you give a semaphore an initial value of 1, it becomes a mutex (as long as it is strictly released only if a lock was successfully obtained).
With several sem_open()-based objects, you can create all of the common equivalent Windows named objects - named mutexes, named semaphores, and named events. Named events with "manual" set to true is a bit more difficult to emulate (it requires four semaphore objects to properly emulate CreateEvent(), SetEvent(), and ResetEvent()). Anyway, I digress.
Alternatively, there is named shared memory. You can initialize a pthread mutex with the "shared process" attribute in named shared memory and then all processes can safely access that mutex object after opening a handle to the shared memory with shm_open()/mmap(). sem_open() is easier if it is available for your platform (if it isn't, it should be for sanity's sake).
Regardless of the method you use, to test for a single instance of your application, use the trylock() variant of the wait function (e.g. sem_trywait()). If the process is the only one running, it will successfully lock the mutex. If it isn't, it will fail immediately.
Don't forget to unlock and close the mutex on application exit.
It will depend on which problem you want to avoid by forcing your application to have only one instance and the scope on which you consider instances.
For a daemon — the usual way is to have a /var/run/app.pid file.
For user application, I've had more problems with applications which prevented me to run them twice than with being able to run twice an application which shouldn't have been run so. So the answer on "why and on which scope" is very important and will probably bring answer specific on the why and the intended scope.
Here is a solution based on sem_open
/*
*compile with :
*gcc single.c -o single -pthread
*/
/*
* run multiple instance on 'single', and check the behavior
*/
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <unistd.h>
#include <errno.h>
#define SEM_NAME "/mysem_911"
int main()
{
sem_t *sem;
int rc;
sem = sem_open(SEM_NAME, O_CREAT, S_IRWXU, 1);
if(sem==SEM_FAILED){
printf("sem_open: failed errno:%d\n", errno);
}
rc=sem_trywait(sem);
if(rc == 0){
printf("Obtained lock !!!\n");
sleep(10);
//sem_post(sem);
sem_unlink(SEM_NAME);
}else{
printf("Lock not obtained\n");
}
}
One of the comments on a different answer says "I found sem_open() rather lacking". I am not sure about the specifics of what's lacking
Based on the hints in maxim's answer here is my POSIX solution of a dual-role daemon (i.e. a single application that can act as daemon and as a client communicating with that daemon). This scheme has the advantage of providing an elegant solution of the problem when the instance started first should be the daemon and all following executions should just load off the work at that daemon. It is a complete example but lacks a lot of stuff a real daemon should do (e.g. using syslog for logging and fork to put itself into background correctly, dropping privileges etc.), but it is already quite long and is fully working as is. I have only tested this on Linux so far but IIRC it should be all POSIX-compatible.
In the example the clients can send integers passed to them as first command line argument and parsed by atoi via the socket to the daemon which prints it to stdout. With this kind of sockets it is also possible to transfer arrays, structs and even file descriptors (see man 7 unix).
#include <stdio.h>
#include <stddef.h>
#include <stdbool.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SOCKET_NAME "/tmp/exampled"
static int socket_fd = -1;
static bool isdaemon = false;
static bool run = true;
/* returns
* -1 on errors
* 0 on successful server bindings
* 1 on successful client connects
*/
int singleton_connect(const char *name) {
int len, tmpd;
struct sockaddr_un addr = {0};
if ((tmpd = socket(AF_UNIX, SOCK_DGRAM, 0)) < 0) {
printf("Could not create socket: '%s'.\n", strerror(errno));
return -1;
}
/* fill in socket address structure */
addr.sun_family = AF_UNIX;
strcpy(addr.sun_path, name);
len = offsetof(struct sockaddr_un, sun_path) + strlen(name);
int ret;
unsigned int retries = 1;
do {
/* bind the name to the descriptor */
ret = bind(tmpd, (struct sockaddr *)&addr, len);
/* if this succeeds there was no daemon before */
if (ret == 0) {
socket_fd = tmpd;
isdaemon = true;
return 0;
} else {
if (errno == EADDRINUSE) {
ret = connect(tmpd, (struct sockaddr *) &addr, sizeof(struct sockaddr_un));
if (ret != 0) {
if (errno == ECONNREFUSED) {
printf("Could not connect to socket - assuming daemon died.\n");
unlink(name);
continue;
}
printf("Could not connect to socket: '%s'.\n", strerror(errno));
continue;
}
printf("Daemon is already running.\n");
socket_fd = tmpd;
return 1;
}
printf("Could not bind to socket: '%s'.\n", strerror(errno));
continue;
}
} while (retries-- > 0);
printf("Could neither connect to an existing daemon nor become one.\n");
close(tmpd);
return -1;
}
static void cleanup(void) {
if (socket_fd >= 0) {
if (isdaemon) {
if (unlink(SOCKET_NAME) < 0)
printf("Could not remove FIFO.\n");
} else
close(socket_fd);
}
}
static void handler(int sig) {
run = false;
}
int main(int argc, char **argv) {
switch (singleton_connect(SOCKET_NAME)) {
case 0: { /* Daemon */
struct sigaction sa;
sa.sa_handler = &handler;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGINT, &sa, NULL) != 0 || sigaction(SIGQUIT, &sa, NULL) != 0 || sigaction(SIGTERM, &sa, NULL) != 0) {
printf("Could not set up signal handlers!\n");
cleanup();
return EXIT_FAILURE;
}
struct msghdr msg = {0};
struct iovec iovec;
int client_arg;
iovec.iov_base = &client_arg;
iovec.iov_len = sizeof(client_arg);
msg.msg_iov = &iovec;
msg.msg_iovlen = 1;
while (run) {
int ret = recvmsg(socket_fd, &msg, MSG_DONTWAIT);
if (ret != sizeof(client_arg)) {
if (errno != EAGAIN && errno != EWOULDBLOCK) {
printf("Error while accessing socket: %s\n", strerror(errno));
exit(1);
}
printf("No further client_args in socket.\n");
} else {
printf("received client_arg=%d\n", client_arg);
}
/* do daemon stuff */
sleep(1);
}
printf("Dropped out of daemon loop. Shutting down.\n");
cleanup();
return EXIT_FAILURE;
}
case 1: { /* Client */
if (argc < 2) {
printf("Usage: %s <int>\n", argv[0]);
return EXIT_FAILURE;
}
struct iovec iovec;
struct msghdr msg = {0};
int client_arg = atoi(argv[1]);
iovec.iov_base = &client_arg;
iovec.iov_len = sizeof(client_arg);
msg.msg_iov = &iovec;
msg.msg_iovlen = 1;
int ret = sendmsg(socket_fd, &msg, 0);
if (ret != sizeof(client_arg)) {
if (ret < 0)
printf("Could not send device address to daemon: '%s'!\n", strerror(errno));
else
printf("Could not send device address to daemon completely!\n");
cleanup();
return EXIT_FAILURE;
}
printf("Sent client_arg (%d) to daemon.\n", client_arg);
break;
}
default:
cleanup();
return EXIT_FAILURE;
}
cleanup();
return EXIT_SUCCESS;
}
All credits go to Mark Lakata. I merely did some very minor touch up only.
main.cpp
#include "singleton.hpp"
#include <iostream>
using namespace std;
int main()
{
SingletonProcess singleton(5555); // pick a port number to use that is specific to this app
if (!singleton())
{
cerr << "process running already. See " << singleton.GetLockFileName() << endl;
return 1;
}
// ... rest of the app
}
singleton.hpp
#include <netinet/in.h>
#include <unistd.h>
#include <cerrno>
#include <string>
#include <cstring>
#include <stdexcept>
using namespace std;
class SingletonProcess
{
public:
SingletonProcess(uint16_t port0)
: socket_fd(-1)
, rc(1)
, port(port0)
{
}
~SingletonProcess()
{
if (socket_fd != -1)
{
close(socket_fd);
}
}
bool operator()()
{
if (socket_fd == -1 || rc)
{
socket_fd = -1;
rc = 1;
if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
{
throw std::runtime_error(std::string("Could not create socket: ") + strerror(errno));
}
else
{
struct sockaddr_in name;
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
rc = bind (socket_fd, (struct sockaddr *) &name, sizeof (name));
}
}
return (socket_fd != -1 && rc == 0);
}
std::string GetLockFileName()
{
return "port " + std::to_string(port);
}
private:
int socket_fd = -1;
int rc;
uint16_t port;
};
#include <windows.h>
int main(int argc, char *argv[])
{
// ensure only one running instance
HANDLE hMutexH`enter code here`andle = CreateMutex(NULL, TRUE, L"my.mutex.name");
if (GetLastError() == ERROR_ALREADY_EXISTS)
{
return 0;
}
// rest of the program
ReleaseMutex(hMutexHandle);
CloseHandle(hMutexHandle);
return 0;
}
FROM: HERE
On Windows you could also create a shared data segment and use an interlocked function to test for the first occurence, e.g.
#include <Windows.h>
#include <stdio.h>
#include <conio.h>
#pragma data_seg("Shared")
volatile LONG lock = 0;
#pragma data_seg()
#pragma comment(linker, "/SECTION:Shared,RWS")
void main()
{
if (InterlockedExchange(&lock, 1) == 0)
printf("first\n");
else
printf("other\n");
getch();
}
I have just written one, and tested.
#define PID_FILE "/tmp/pidfile"
static void create_pidfile(void) {
int fd = open(PID_FILE, O_RDWR | O_CREAT | O_EXCL, 0);
close(fd);
}
int main(void) {
int fd = open(PID_FILE, O_RDONLY);
if (fd > 0) {
close(fd);
return 0;
}
// make sure only one instance is running
create_pidfile();
}
Just run this code on a seperate thread:
void lock() {
while(1) {
ofstream closer("myapplock.locker", ios::trunc);
closer << "locked";
closer.close();
}
}
Run this as your main code:
int main() {
ifstream reader("myapplock.locker");
string s;
reader >> s;
if (s != "locked") {
//your code
}
return 0;
}
I am writing a program on a Linux system using Lazarus IDE. The program is supposed to connect to the Internet or Intranet. So, I want to display to the user list of all the available network connections that they can use to connect to the Internet or Intranet like wifi, if there are two active network cards on the system, then this program should display their available connections.
At the moment, I don't know where to start or what tool(s) to use.
Any hints, clues or advice will be greatly appreciated.
You can use ifconfig to list all available network interfaces and their status.
Edit: For doing it programmatically you have to use function ioctl with SIOCGIFCONF.
#include <sys/types.h>
#include <sys/socket.h>
#include <net/if.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <string.h>
#include <arpa/inet.h>
int main()
{
int sockfd, len, lastlen;
char *ptr, *buf;
struct ifconf ifc;
struct ifreq *ifr;
char ifname[IFNAMSIZ + 1];
char str[INET6_ADDRSTRLEN];
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
lastlen = 0;
len = 100 * sizeof(struct ifreq); /* initial buffer size guess */
for ( ; ; )
{
buf = malloc(len);
ifc.ifc_len = len;
ifc.ifc_buf = buf;
if (ioctl(sockfd, SIOCGIFCONF, &ifc) < 0)
{
if (errno != EINVAL || lastlen != 0)
exit(-1);
}
else
{
if (ifc.ifc_len == lastlen)
break; /* success, len has not changed */
lastlen = ifc.ifc_len;
}
len += 10 * sizeof(struct ifreq); /* increment */
free(buf);
}
printf("LEN: %d\n", ifc.ifc_len);
for (ptr = buf; ptr < buf + ifc.ifc_len; )
{
ifr = (struct ifreq *) ptr;
ptr += sizeof(struct ifreq); /* for next one in buffer */
memcpy(ifname, ifr->ifr_name, IFNAMSIZ);
printf("Interface name: %s\n", ifname);
const char *res;
switch (ifr->ifr_addr.sa_family)
{
case AF_INET6:
res = inet_ntop(ifr->ifr_addr.sa_family, &(((struct sockaddr_in6 *)&ifr->ifr_addr)->sin6_addr), str, INET6_ADDRSTRLEN);
break;
case AF_INET:
res = inet_ntop(ifr->ifr_addr.sa_family, &(((struct sockaddr_in *)&ifr->ifr_addr)->sin_addr), str, INET_ADDRSTRLEN);
break;
default:
printf("OTHER\n");
str[0] = 0;
res = 0;
}
if (res != 0)
{
printf("IP Address: %s\n", str);
}
else
{
printf("ERROR\n");
}
}
return 0;
}
ioctl SIOCGIFCONF will return, if success, a struct ifconf which has a pointer to an array of struct ifreq.
These structs are defined in net/if.h
Using this code, from ifc.ifc_req you can get all interfaces, please look at the declaration of struct ifreq in order to determine the length and type of each array element. I think from here you can continue alone, if not please let me know.
The following code does work on my Linux system. It outputs all the available connection point through which you can connect to the Internet or intranet. I modified the code to print out its name and ip address.
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <arpa/inet.h>
// you may need to include other headers
int main()
{
struct ifaddrs* interfaces = NULL;
struct ifaddrs* temp_addr = NULL;
int success;
char *name;
char *address;
// retrieve the current interfaces - returns 0 on success
success = getifaddrs(&interfaces);
if (success == 0)
{
// Loop through linked list of interfaces
temp_addr = interfaces;
while (temp_addr != NULL)
{
if (temp_addr->ifa_addr->sa_family == AF_INET) // internetwork only
{
name = temp_addr->ifa_name;
address = inet_ntoa(((struct sockaddr_in *)temp_addr->ifa_addr)->sin_addr);
printf("%s %s\n",name,address);
}
temp_addr = temp_addr->ifa_next;
}
}
// Free memory
freeifaddrs(interfaces);
}
I have had a rather strange observation about behavior of setsockopt on Linux for SO_REUSEADDR. In one line: if I apply the sockopt to an fd returned by accept on a "listening socket" the socketoption is reflected on the port held by the listening socket.
Ok some code.
Server : Opens a socket, applies SO_REUSEADDR to be true. Accepts a connection and then applies SO_REUSEADDR to be false on the fd on the fd returned by accept.
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <string.h>
int main(void)
{
int s, len;
int sin_size;
int reuse = 1;
int ret;
struct sockaddr_in my_addr;
memset(&my_addr, 0, sizeof(my_addr));
my_addr.sin_family = AF_INET;
my_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
my_addr.sin_port = htons(33235);
if( (s = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("Socket Error\n");
return -1;
}
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(int));
if( bind(s, (struct sockaddr*)&my_addr, sizeof(struct sockaddr)) < 0)
{
printf("Bind Error\n");
return -1;
}
listen(s, 6);
reuse = 0;
memset(&my_addr, 0, sizeof(my_addr));
while(1) {
ret = accept(s, (struct sockaddr*)&my_addr, &len);
if (ret<0) {
printf("Accept failed\n");
} else {
printf("Accepted a client setting reuse add to 0\n");
setsockopt(ret, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(int));
}
}
printf("Server exiting\n");
return 0;
}
Client : Client connects to the server, and doesn't do anything after that ensuring that the server socket stays in TIME_WAIT state.
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <string.h>
#include <errno.h>
int main(void)
{
int s, len;
int sin_size;
struct sockaddr_in my_addr;
memset(&my_addr, 0, sizeof(my_addr));
my_addr.sin_family = AF_INET;
my_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
my_addr.sin_port = htons(33235);
if( (s = socket(AF_INET, SOCK_STREAM, 0)) < 0)
{
printf("Socket Error\n");
return -1;
}
if (!connect(s,(struct sockaddr*)&my_addr, sizeof(struct sockaddr)))
{
printf("Client Connected successfully\n");
}
else
{
printf("%s\n",strerror(errno));
}
while(1) sleep(1);
return 0;
}
Steps that I do reproduce the issue.
Run server.
Connect client.
Kill and restart server. The server fails with Bind Failure
I tested this on mac os. And the bind didn't fail. I have digged up all Posix specifications and none of them say that this code is undefined.
Question:
Can someone with more experience on this share their understanding of the issue?
One way to think about it is that SO_REUSEADDR determines if you can have another socket bound to that same address. It's a property of any socket (listen or connection), but very commonly inherited from listen via accept. In linux it's mapped to the struct sock "sk_reuse" flag.
If you clear this flag on a FD you "accepted" then from that point on the IP/Port pair is considered busy-and-non-reusable. The SO_REUSEADDR flag on the listen socket does not change, but the flag on the accepted socket affects bind logic. You could probably check this with getsockopt.
If you want to know more you can try to read the inet_csk_get_port function: http://lxr.free-electrons.com/source/net/ipv4/inet_connection_sock.c#L100. This is where the actual "binding" takes place.