How to Create Single-Instance Applications in C or C : File Locks, Mutexes, and Beyond?

Creating Single Instance Applications in C or C

Introduction:

Ensuring that only one instance of an application runs simultaneously is crucial in various scenarios. This article explores methods for achieving single instance applications in C or C .

File Locks:

File locks can be applied to a specific file. If a lock is obtained, only the holding process can access the file. This can be used to prevent multiple instances of an application from running.

<code class="c">#include <sys/file.h>
int fd = open("lockfile", O_RDWR | O_CREAT);
int res = flock(fd, LOCK_EX | LOCK_NB);</code>

Mutexes:

Mutexes are used for synchronization. A mutex can be acquired by a single thread, preventing other threads from accessing shared resources until the mutex is released.

<code class="c">#include <pthread.h>
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);
pthread_mutex_lock(&mutex);
// Critical section
pthread_mutex_unlock(&mutex);</code>

Recommended Approach:

A common approach is to create a pidfile, a file containing the process ID of the running application. If the pidfile already exists, it suggests that another instance of the application is running. This method, however, has limitations due to stale pidfiles.

Advanced Methods:

More advanced methods include using a Unix domain socket or a unique identifier to ensure a single instance. A Unix domain socket can be bound to a specific name, and only the first instance of the application can bind successfully. A unique identifier can be generated and stored in a shared memory segment for verification.

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