I want to know what is access time . searched in web but get the same definition (read- gets changed). I know with touch we can change it. Could any one explain me more about it with an example how it is changed. Also is there any way file creating date/time be found in unix?
+3
A:
The stat(2) structure keeps track of all the file date/times:
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 */
};
st_atime is the access time, updated on read(2) calls (and probably also when open(2) opens a file for reading) -- it is NOT updated when files are read via mmap(2). (Which is why I assume open(2) will mark the access time.)
st_mtime is the data modification time, either via write(2) or truncate(2) or open(2) for writing. (Again, it is NOT updated when files are written via mmap(2).)
st_ctime is the metadata modification time: when any of the other data in the struct stat gets modified.
You can change the timestamps on files with utime(2):
struct utimbuf {
time_t actime; /* access time */
time_t modtime; /* modification time */
};
Note you can only change the access time and (data) modification time. You can set either of these to arbitrary times, but the ctime is going to be set to the current time -- because you changed the metadata for the file.
There is no create time in this structure, so it's not possible to find out when a file was created directly from the system.
If you really need to know the create time, you can narrow it down to a range by looking at your backups -- assuming the file you're interested in has been backed up, along with its metadata.
sarnold
2010-08-02 05:30:57
when i use vi editor or cat command does this function use read system calls. where can i find list of unix command that use read or utimes or mknod system calls
Arav
2010-08-04 01:23:09
@arav, `read(2)` is just short-hand for `read(2)`, `pread(2)`, `readv(2)`, `readdir(2)`, `readlink(2)`, `splice(2)`, `sendfile(2)`, and so forth. In other words, everything that reads file contents. So, if your program has read a file, then it may modify the access time of the read file. (Of course, some filesystems turn off access times for performance or reliability.) if you want to find programs that use specific system calls, you can do something like this: `for f in /bin/* /usr/bin/* /sbin/* /usr/sbin/* ; do readelf -s $f | grep -q readdir@GLIBC done 2> /dev/null`
sarnold
2010-08-04 02:13:29
does vi or cat uses the read system call. I tried but it was not modifying the access time
Arav
2010-08-05 04:14:58
A:
last access: Time when file data last accessed. Changed by the mknod(2), utimes(2) and read(2) system calls.
last modified: Time when file data last modified. Changed by the mknod(2), utimes(2) and write(2) system calls.
last changed: Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2) and write(2) system calls.
prodigitalson
2010-08-02 05:31:43
when i use vi editor or cat command does this function use read system calls. where can i find list of unix command that use read or utimes or mknod system calls
Arav
2010-08-03 23:59:37