touch

Hacking header file timestamps

May 7, 2015 C/C++ development and debugging. No comments , , ,

Suppose you change a header foo.h, but your build system has dependency checking that will force the world to be rebuilt. A handy way to avoid that is with judicious use of the touch command, such as:

touch -t197301010000 foo.h 

or (on Linux)

touch --reference another-file-that-is-early-enough-in-time foo.h

Both of these can be used to force the modification time for the file backwards.

This can be very useful way to save time.

It is also a great way to not build enough or break the build if misused!

I’d recommend this only for prototyping type work, to be followed up with a subsequent touch to restore the timestamps to normal, and then do a proper build.

Why does touch include a utimensat() syscall?

February 25, 2015 perl and general scripting hackery 1 comment , , , , , , , , ,

I’m seeing odd time sequencing of files when using clearcase version 8 dynamic views, which makes me wonder about an aspect of the (gnu) touch command. Running:

> cat u
rm -f touchedEarlier touchedLater

perl -e "open (F, '> touchedEarlier') || die"
touch touchedLater

ls --full-time touchedEarlier touchedLater

produces:

> ./u
-rw-r--r-- 1 peeterj pdxdb2 0 2015-02-25 11:42:05.833044000 -0500 touchedEarlier
-rw-r--r-- 1 peeterj pdxdb2 0 2015-02-25 11:42:05.000000000 -0500 touchedLater

Notice that the file that is touched by doing a perl “open” ends up with a later time, despite the fact that it was done logically earlier than the touch.

Running this command outside of a clearcase dynamic view shows zeros only in the subsecond times (also the behaviour of clearcase V7). Needless to say, this difference in file times from their creation sequence wreaks havoc on make.

I was curious how the two touch methods differed, and stracing them shows that the touch differs by including a utimesat() syscall. The perl touch is:

open(“touchedEarlier”, O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff29cd29f0) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=0, …}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
close(3) = 0
exit_group(0) = ?

whereas the touch command has:

open(“touchedLater”, O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
dup2(3, 0) = 0
close(3) = 0
dup2(0, 0) = 0
utimensat(0, NULL, NULL, 0) = 0
close(0) = 0
close(1) = 0
close(2) = 0
exit_group(0) = ?

It appears that the touch command explicitly zeros the subsecond portion of the files’ timestamp.

I also see that perl’s File::Touch module does the same thing, but uses a different mechanism. I see the following in a strace of such a Touch() call:

stat(“xxyyzz”, 0x656060)                = -1 ENOENT (No such file or directory)
open(“xxyyzz”, O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fff16bb3c60) = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR)                   = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=0, …}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
close(3)                                = 0
utimes(“xxyyzz”, {{1424884553, 0}, {1424884553, 0}}) = 0

I am very curious why touch and perl’s File::Touch() both explicitly zero the subsecond modification time for the file (using utimensat() or utimes() syscalls)?