It looks like if the other thread opened the same file you might get a false positive. struct stat { dev_t st_dev; /* ID of device containi

It looks like if the other thread opened the same file you might get a false positive. struct stat { dev_t st_dev; /* ID of device containing file */ ino_t st_ino; /* inode number */ mode_t st_mode; /* protection */ nlink_t st_nlink; /* number of hard links */ uid_t st_uid; /* user ID of owner */ gid_t st_gid; /* group ID of owner */ dev_t st_rdev; /* device ID (if special file) */ off_t st_size; /* total size, in bytes */ blksize_t st_blksize; /* blocksize for file system I/O */ blkcnt_t st_blocks; /* number of 512B blocks allocated */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last status change */ }; The only one of these likely to change is st_atime, but I don't think that's guaranteed to update. (I think different behaviours can be compiled into the linux kernel, including "only update atime if it equals ctime or mtime".)