The Linux Programming Interface

286 Chapter 15

In Sections 14.8.1 and 15.5, we describe mount(2) options and per-file flags that pre-

vent updates to the last access time of a file. The open() O_NOATIME flag described in

Section 4.3.1 also serves a similar purpose. In some applications, this can be useful

for performance reasons, since it reduces the number of disk operations that are

required when a file is accessed.

Although most UNIX systems don’t record the creation time of a file, on

recent BSD systems, this time is recorded in a stat field named st_birthtime.

Table 15-2: Effect of various functions on file timestamps

Function

File or

directory

Parent

directory Notes

amc amc

chmod() • Same for fchmod()

chown() • Same for lchown() and fchown()

exec() •

link() •••Affects parent directory of second argument

mkdir() ••• ••

mkfifo() ••• ••

mknod() ••• ••

mmap() • •• st_mtime and st_ctime are changed only on updates

to MAP_SHARED mapping

msync() •• Changed only if file is modified

open(), creat() ••• ••When creating new file

open(), creat() •• When truncating existing file

pipe() •••

read() • Same for readv(), pread(), and preadv()

readdir() • readdir() may buffer directory entries; timestamps

updated only if directory is read

removexattr() • Same for fremovexattr() and lremovexattr()

rename() •• • Affects timestamps in both parent directories;

SUSv3 doesn’t specify file st_ctime change, but

notes that some implementations do this

rmdir() ••Same for remove(directory)

sendfile() • Timestamp changed for input file

setxattr() • Same for fsetxattr() and lsetxattr()

symlink() ••• ••Sets timestamps of link (not target file)

truncate() •• Same for ftruncate(); timestamps change only if

file size changes

unlink() •••Same for remove(file); file st_ctime changes if

previous link count was > 1

utime() ••• Same for utimes(), futimes(), futimens(), lutimes(),

and utimensat()

write() •• Same for writev(), pwrite(), and pwritev()