Programming

hPrevInstance
    [in] Handle to the previous instance of the application. This parameter is always NULL.

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <stdio.h>
#include <stdlib.h>

// Have this called at exit
void cleanup(void)
{
        unlink("pidfile");
}

int get_lockfile(void)
{
        char buf[64];
        int len;
        // O_EXCL tells it to fail if the file already exists.
        int fd=open("pidfile", O_RDWR|O_CREAT|O_EXCL);
        if(fd < 0)
                return(-1);

        fprintf(stderr, "PID %d has the lockfile.\n", getpid());
        // Write the PID to the file
        len=sprintf(buf, "%d", getpid());
        write(fd, len, buf);
        close(fd);

        // Have cleanup() called automatically when main() returns
        atexit(cleanup);
        return(0);
}

int main(void)
{
        if(get_lockfile() < 0)
        {
                fprintf(stderr, "PID %d couldn't get lockfile\n", getpid());
                return(1);
        }

        sleep(10);
        fprintf(stderr, "PID %d releasing lockfile\n", getpid());
        return(0);
}