How to create a single instance application in C or C++

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;
}