lockme.c

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

#include <sys/file.h>

static const char *filename = "/tmp/lockme";

int
main(int argc, char *argv[]) {

	int fd;

	fd = open(filename, O_RDWR | O_CREAT, S_IRWXU);

	if (fd < 0) {
		fprintf(stderr, "failed to open %s\n", filename);
		return EXIT_FAILURE;
	}

	flock(fd, LOCK_EX);

	printf("obtained lock on %s, press any key to unlock\n", filename);
	getchar();

	flock(fd, LOCK_UN);
	
	close(fd);
}