i finally found the actual answer to the TMPDIR content deletion behavior after observing my un-accessed 3-day old files in TMPDIR getting deleted at 3:35am. it's using a different mechanism from the /tmp/ directory. and if you don't want your per-user files to get deleted and still want it in /var/folders/ (and don't want to use the home directory ~/Library/Caches/), you might want to consider using the "cache" (DARWIN_USER_CACHE_DIR) directory instead.
"3:35am" and "$TMPDIR" were mentioned in the page below.
http://www.mobileread.mobi/forums/showthread.php?t=273999
"on Mac OS X, there is a dirhelper daemon that looks for files not accessed within 3 days inside the official temp directory and will delete them even if the app using them is still running."
the dirhelper daemon runs at 3:35am and deletes files that didn't not get accessed for 3 days.
/System/Library/LaunchDaemons/com.apple.bsd.dirhelper.plist
% getconf DARWIN_USER_TEMP_DIR
/var/folders/xx/xxxx_xxxx/T/
% getconf DARWIN_USER_CACHE_DIR
/var/folders/xx/xxxx_xxxx/C/
% man confstr
...
_CS_DARWIN_USER_TEMP_DIR
Provides the path to a user's temporary items directory. The directory will be created it if does
not already exist. This directory is created with access permissions of 0700 and restricted by the
umask(2) of the calling process and is a good location for temporary files.
By default, files in this location may be cleaned (removed) by the system if they are not accessed
in 3 days.
_CS_DARWIN_USER_CACHE_DIR
Provides the path to the user's cache directory. The directory will be created if it does not
already exist. This directory is created with access permissions of 0700 and restricted by the
umask(2) of the calling process and is a good location for user cache data as it will not be
automatically cleaned by the system.
Files in this location will be removed during safe boot.
...
% cat confstr.c
// cc confstr.c -o confstr -mmacosx-version-min=10.11 -arch x86_64 -arch arm64
#include <unistd.h>
#include <stdio.h>
#include <sys/syslimits.h>
int main() {
char buf[PATH_MAX];
size_t len = PATH_MAX;
size_t s = confstr(_CS_DARWIN_USER_TEMP_DIR, buf, len);
printf("_CS_DARWIN_USER_TEMP_DIR(len:%d): %s\n", (int)s, buf);
s = confstr(_CS_DARWIN_USER_CACHE_DIR, buf, len);
printf("_CS_DARWIN_USER_CACHE_DIR(len:%d): %s\n", (int)s, buf);
return 0;
}
% ./confstr
_CS_DARWIN_USER_TEMP_DIR(len:50): /var/folders/xx/xxxx_xxxx/T/
_CS_DARWIN_USER_CACHE_DIR(len:50): /var/folders/xx/xxxx_xxxx/C/
