how does Python get the time()?

Skaperen · 07-06-2020, 10:06 PM

i put together this short script in Python:

Code:

import time
for _ in range(8):
    print(int(time.time()*3906250))

then i ran it on Xubuntu 18.04 on Python 3.6 under strace and got:

Code:

...
stat("tt.py", {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
openat(AT_FDCWD, "tt.py", O_RDONLY)     = 3
ioctl(3, FIOCLEX)                       = 0
fstat(3, {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
fstat(3, {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
lseek(3, 0, SEEK_SET)                   = 0
read(3, "import time\nfor _ in range(8):\n "..., 45) = 45
read(3, "time.time()*3906250))\n", 4096) = 22
close(3)                                = 0
stat("tt.py", {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
readlink("tt.py", 0x7fff6a7bced0, 4096) = -1 EINVAL (Invalid argument)
getcwd("/home/forums", 4096)            = 13
lstat("/home/forums/tt.py", {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
openat(AT_FDCWD, "tt.py", O_RDONLY)     = 3
fcntl(3, F_GETFD)                       = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fstat(3, {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
ioctl(3, TCGETS, 0x7fff6a7cde50)        = -1 ENOTTY (Inappropriate ioctl for device)
lseek(3, 0, SEEK_CUR)                   = 0
fstat(3, {st_mode=S_IFREG|0444, st_size=67, ...}) = 0
read(3, "import time\nfor _ in range(8):\n "..., 4096) = 67
lseek(3, 0, SEEK_SET)                   = 0
brk(0x29b5000)                          = 0x29b5000
read(3, "import time\nfor _ in range(8):\n "..., 4096) = 67
read(3, "", 4096)                       = 0
brk(0x29ac000)                          = 0x29ac000
brk(0x29aa000)                          = 0x29aa000
close(3)                                = 0
openat(AT_FDCWD, "/usr/share/zoneinfo/America/New_York", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=3545, ...}) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=3545, ...}) = 0
read(3, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\5\0\0\0\5\0\0\0\0"..., 4096) = 3545
lseek(3, -2261, SEEK_CUR)               = 1284
read(3, "TZif2\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\5\0\0\0\5\0\0\0\0"..., 4096) = 2261
close(3)                                = 0
write(1, "6226917864829328\n", 176226917864829328
)      = 17
write(1, "6226917864829508\n", 176226917864829508
)      = 17
write(1, "6226917864829629\n", 176226917864829629
)      = 17
write(1, "6226917864829744\n", 176226917864829744
)      = 17
write(1, "6226917864829858\n", 176226917864829858
)      = 17
write(1, "6226917864829970\n", 176226917864829970
)      = 17
write(1, "6226917864830081\n", 176226917864830081
)      = 17
write(1, "6226917864830191\n", 176226917864830191
)      = 17
brk(0x29a8000)                          = 0x29a8000
rt_sigaction(SIGINT, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f81e9921f20}, {sa_handler=0x62ffc0, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f81e9921f20}, 8) = 0
sigaltstack(NULL, {ss_sp=0x28db540, ss_flags=0, ss_size=8192}) = 0
sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}, NULL) = 0
exit_group(0)                           = ?
+++ exited with 0 +++
lt2a/forums /home/forums 27> cat tt.py
import time
for _ in range(8):
    print(int(time.time()*3906250))
lt2a/forums /home/forums 28>

it looks like no syscall is needed to get the time. what C library call can do that? or is it just a special memory access location?

tinfoil3d · 07-06-2020, 10:33 PM

Confirmed on debian too. Peculiar. If you add time.sleep(1) before that it'll use select(0,0,0,0,[1,0]) instead of nanosleep in linux. But where does it pull the time in clock_gettime/gettimeofday format i wonder?

tinfoil3d · 07-06-2020, 10:39 PM

If you run it with ltrace(warning: LOTS of data would be printed) you can actually see clock_gettime() there BUT it's not coming up in strace, which doesn't seem right.

syg00 · 07-06-2020, 11:58 PM

Code:

man vdso

tinfoil3d · 07-07-2020, 12:34 AM

Quote:

Originally Posted by syg00

Code:

man vdso

Thanks man, learned something new today. I thought it's gotta be somehow mapped onto memory otherwise it can't be explained but I've never heard of vdso before.

dugan · 07-07-2020, 04:05 PM

It's, clock_gettime, gettimeofday, etc.

https://github.com/python/cpython/bl.../pytime.c#L695

onebuck · 07-07-2020, 04:24 PM

Moved: This thread is more suitable in <Programming> and has been moved accordingly to help your thread/question get the exposure it deserves.

Skaperen · 07-10-2020, 04:47 PM

i read man vdso but i didn't get much out of it besides it saying strace won't show it. i'm still curious how this code manages to get the time value. i assume it does not do that expensive INT (or its equivalent on other architectures). is it doing a virtual memory trap and code in the kernels handles a vdso memory trap specially to copy the time value into some designated place like a place in the vdso or some register?

tinfoil3d · 07-10-2020, 07:35 PM

Yeah, that's what it basically says. You'll have to study vdso code to figure out more but it's only natural it's some kind of map that's probably doesn't have any overhead until queried to be read out from userlevel.

Skaperen · 07-11-2020, 08:42 AM

then that method should be used for all syscalls.