3 files changed, 85 insertions, 0 deletions
diff --git a/src/lock.c b/src/lock.c
new file mode 100644
index 0000000..133d327
--- /dev/null
+++ b/src/lock.c
@@ -0,0 +1,71 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/file.h>
+#include "log.h"
+#include "xmalloc.h"
+
+static const char *default_cache_dir = ".cache/";
+static const char *lock_filename = "tofi.lock";
+
+[[nodiscard("memory leaked")]]
+static char *get_lock_path() {
+	char *lock_name = NULL;
+	const char *runtime_path = getenv("XDG_RUNTIME_DIR");
+	if (runtime_path == NULL) {
+		runtime_path = getenv("XDG_CACHE_HOME");
+	}
+	if (runtime_path == NULL) {
+		const char *home = getenv("HOME");
+		if (home == NULL) {
+			log_error("Couldn't retrieve HOME from environment.\n");
+			return NULL;
+		}
+		size_t len = strlen(home) + 1
+			+ strlen(default_cache_dir) + 1
+			+ strlen(lock_filename) + 1;
+		lock_name = xmalloc(len);
+		snprintf(
+			lock_name,
+			len,
+			"%s/%s/%s",
+			home,
+			default_cache_dir,
+			lock_filename);
+	} else {
+		size_t len = strlen(runtime_path) + 1
+			+ strlen(lock_filename) + 1;
+		lock_name = xmalloc(len);
+		snprintf(
+			lock_name,
+			len,
+			"%s/%s",
+			runtime_path,
+			lock_filename);
+	}
+	return lock_name;
+}
+
+bool lock_check(void)
+{
+	bool ret = false;
+	char *filename = get_lock_path();
+	errno = 0;
+	int fd = open(filename, O_RDONLY | O_CREAT, S_IRUSR | S_IWUSR);
+	if (fd == -1) {
+		log_error("Failed to open lock file %s: %s.\n", filename, strerror(errno));
+	} else if (flock(fd, LOCK_EX | LOCK_NB) == -1) {
+		if (errno == EWOULDBLOCK) {
+			/*
+			 * We can't lock the file because another tofi process
+			 * already has.
+			 */
+			ret = true;
+		}
+        }
+
+	free(filename);
+	return ret;
+}
diff --git a/src/lock.h b/src/lock.h
new file mode 100644
index 0000000..1ce402e
--- /dev/null
+++ b/src/lock.h
@@ -0,0 +1,8 @@
+#ifndef LOCK_H
+#define LOCK_H
+
+#include <stdbool.h>
+
+bool lock_check(void);
+
+#endif /* LOCK_H */
diff --git a/src/main.c b/src/main.c
index 2a9ce00..807f57c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -22,6 +22,7 @@
 #include "input.h"
 #include "log.h"
 #include "nelem.h"
+#include "lock.h"
 #include "shm.h"
 #include "string_vec.h"
 #include "string_vec.h"
@@ -905,6 +906,11 @@ int main(int argc, char *argv[])
 
 	parse_args(&tofi, argc, argv);
 
+	if (lock_check()) {
+		log_error("Another instance of tofi is already running.\n");
+		exit(EXIT_FAILURE);
+	}
+
 	/*
 	 * Initial Wayland & XKB setup.
 	 * The first thing to do is connect a listener to the global registry,