Only open a unique instance of an application.

instance.c

include <stdlib.h>                                                                                                
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/file.h>
#include <sys/wait.h>

#define TRUE 1
#define FALSE 0
#define PID_FILENAME "/tmp/test_pid.lock"

typedef struct Instance {
    int unique;
    char pid[10];
    int pid_file;
} Instance;

int send_sig(int pid, int sig)
{
    char exec_str[128];
    sprintf(exec_str, "kill -%d %d >/dev/null 2>&1", sig, pid);
    int result = WEXITSTATUS(system(exec_str));
    if (result == 0)
        return 0;
    else
        return -1; 
}

static void signal_handler(int signal_id)
{
    if (signal_id == SIGTERM)
        exit(0);
}

int instance_lock(Instance *this) {
    this->unique = FALSE;
    this->pid_file = open(PID_FILENAME, O_CREAT | O_RDWR, 0666);

    if (this->pid_file != -1){

        if (flock(this->pid_file, LOCK_EX | LOCK_NB)) {
            if (EWOULDBLOCK == errno) {
                /* Already running, get the PID */
                read(this->pid_file, this->pid, 10);
            } else {
                /* Deal with an error */
            }   
        } else {
            /* Get lock */
            if (this->pid_file) {
                sprintf(this->pid, "%d", getpid());
                write(this->pid_file, this->pid, strlen(this->pid));
                this->unique = TRUE;
            }   
        }   
    }   
    return this->unique;
}

int instance_is_unique(Instance *this) {
    return this->unique;
}
void instance_signal(Instance *this, int signal_code)
{
    send_sig(atoi(this->pid), signal_code);
}

void instance_kill(Instance *this)
{
    instance_signal(this, SIGTERM);
}

void instance_free(Instance *this)
{
    if (this->pid_file)
        close(this->pid_file);
    unlink(PID_FILENAME);
}

int main(void)
{
    Instance instance;
    instance_lock(&instance);
    if (instance.unique != TRUE) {
        puts("Another instance is running");
        instance_kill(&instance);
    } else {
        puts("We are the only instance");

        while(1) {
            sleep(5);
            puts("Waiting");
        }
    }
}