Instead of creating /var/lib/pacman/db.lck before transactions and deleting it when pacman finishes, a better approach would be to make an exclusive lock on that file (db.lck would be present in the filesystem all the time). Lock is released when a process finishes (or crashes), so there would be no need to manually delete db.lck after pacman crashes or is killed.

#include <fcntl.h>
#include <sys/file.h>
#include <unistd.h>

int fd = open( "/var/lib/pacman/db.lck", O_RDONLY | O_NONBLOCK );
if( fd == -1 ) ... /// failed
int ret = flock( fd, LOCK_EX | LOCK_NB );
if( ret == -1 ) ... // failed

... DB transactions ...

close( fd );