.\" Copyright (C) 2008, Linux Foundation, written by Michael Kerrisk

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.\"

.TH UTIMENSAT 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

utimensat, futimens \- change file timestamps with nanosecond precision

.SH SYNOPSIS

.nf

.B #include <fcntl.h>           /* Definition of AT_* constants */

.B #include <sys/stat.h>

.PP

.BI "int utimensat(int " dirfd ", const char *" pathname ,

.BI "              const struct timespec " times "[2], int " flags );

.PP

.BI "int futimens(int " fd ", const struct timespec " times [2]);

.fi

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.ad l

.PD 0

.BR utimensat ():

.RS 4

.TP 4

Since glibc 2.10:

_POSIX_C_SOURCE\ >=\ 200809L

.TP

Before glibc 2.10:

_ATFILE_SOURCE

.RE

.PP

.BR futimens ():

.RS 4

.TP

Since glibc 2.10:

_POSIX_C_SOURCE\ >=\ 200809L

.TP

Before glibc 2.10:

_GNU_SOURCE

.RE

.PD

.ad

.SH DESCRIPTION

.BR utimensat ()

and

.BR futimens ()

update the timestamps of a file with nanosecond precision.

This contrasts with the historical

.BR utime (2)

and

.BR utimes (2),

which permit only second and microsecond precision, respectively,

when setting file timestamps.

.PP

With

.BR utimensat ()

the file is specified via the pathname given in

.IR pathname .

With

.BR futimens ()

the file whose timestamps are to be updated is specified via

an open file descriptor,

.IR fd .

.PP

For both calls, the new file timestamps are specified in the array

.IR times :

.IR times [0]

specifies the new "last access time" (\fIatime\fP);

.IR times [1]

specifies the new "last modification time" (\fImtime\fP).

Each of the elements of

.I times

specifies a time as the number of seconds and nanoseconds

since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

This information is conveyed in a structure of the following form:

.PP

.in +4n

.EX

struct timespec {

    time_t tv_sec;        /* seconds */

    long   tv_nsec;       /* nanoseconds */

};

.EE

.in

.PP

Updated file timestamps are set to the greatest value

supported by the filesystem that is not greater than the specified time.

.PP

If the

.I tv_nsec

field of one of the

.I timespec

structures has the special value

.BR UTIME_NOW ,

then the corresponding file timestamp is set to the current time.

If the

.I tv_nsec

field of one of the

.I timespec

structures has the special value

.BR UTIME_OMIT ,

then the corresponding file timestamp is left unchanged.

In both of these cases, the value of the corresponding

.I tv_sec

.\" 2.6.22 was broken: it is not ignored

field is ignored.

.PP

If

.I times

is NULL, then both timestamps are set to the current time.

.\"

.SS Permissions requirements

To set both file timestamps to the current time (i.e.,

.I times

is NULL, or both

.I tv_nsec

fields specify

.BR UTIME_NOW ),

either:

.IP 1. 3

the caller must have write access to the file;

.\" 2.6.22 was broken here -- for futimens() the check is

.\" based on whether or not the file descriptor is writable,

.\" not on whether the caller's effective UID has write

.\" permission for the file referred to by the descriptor.

.IP 2.

the caller's effective user ID must match the owner of the file; or

.IP 3.

the caller must have appropriate privileges.

.PP

To make any change other than setting both timestamps to the

current time (i.e.,

.I times

is not NULL, and neither

.I tv_nsec

field is

.B UTIME_NOW

.\" 2.6.22 was broken here:

.\" both must be something other than *either* UTIME_OMIT *or* UTIME_NOW.

and neither

.I tv_nsec

field is

.BR UTIME_OMIT ),

either condition 2 or 3 above must apply.

.PP

If both

.I tv_nsec

fields are specified as

.BR UTIME_OMIT ,

then no file ownership or permission checks are performed,

and the file timestamps are not modified,

but other error conditions may still be detected.

.\"

.\"

.SS utimensat() specifics

If

.I pathname

is relative, then by default it is interpreted relative to the

directory referred to by the open file descriptor,

.IR dirfd

(rather than relative to the current working directory of

the calling process, as is done by

.BR utimes (2)

for a relative pathname).

See

.BR openat (2)

for an explanation of why this can be useful.

.PP

If

.I pathname

is relative and

.I dirfd

is the special value

.BR AT_FDCWD ,

then

.I pathname

is interpreted relative to the current working

directory of the calling process (like

.BR utimes (2)).

.PP

If

.I pathname

is absolute, then

.I dirfd

is ignored.

.PP

The

.I flags

field is a bit mask that may be 0, or include the following constant,

defined in

.IR <fcntl.h> :

.TP

.B AT_SYMLINK_NOFOLLOW

If

.I pathname

specifies a symbolic link, then update the timestamps of the link,

rather than the file to which it refers.

.SH RETURN VALUE

On success,

.BR utimensat ()

and

.BR futimens ()

return 0.

On error, \-1 is returned and

.I errno

is set to indicate the error.

.SH ERRORS

.TP

.B EACCES

.I times

is NULL,

or both

.I tv_nsec

values are

.BR UTIME_NOW ,

and either:

.RS

.IP * 3

the effective user ID of the caller does not match

the owner of the file,

the caller does not have write access to the file,

and the caller is not privileged

(Linux: does not have either the

.B CAP_FOWNER

or the

.B CAP_DAC_OVERRIDE

capability); or,

.\" But Linux 2.6.22 was broken here.

.\" Traditionally, utime()/utimes() gives the error EACCES for the case

.\" where the timestamp pointer argument is NULL (i.e., set both timestamps

.\" to the current time), and the file is owned by a user other than the

.\" effective UID of the caller, and the file is not writable by the

.\" effective UID of the program.  utimensat() also gives this error in the

.\" same case.  However, in the same circumstances, when utimensat() is

.\" given a 'times' array in which both tv_nsec fields are UTIME_NOW, which

.\" provides equivalent functionality to specifying 'times' as NULL, the

.\" call succeeds.  It should fail with the error EACCES in this case.

.\"

.\" POSIX.1-2008 has the following:

.\" .TP

.\" .B EACCES

.\" .RB ( utimensat ())

.\" .I fd

.\" was not opened with

.\" .B O_SEARCH

.\" and the permissions of the directory to which

.\" .I fd

.\" refers do not allow searches.

.IP *

the file is marked immutable (see

.BR chattr (1)).

.\" EXT2_IMMUTABLE_FL and similar flags for other filesystems.

.RE

.TP

.B EBADF

.RB ( futimens ())

.I fd

is not a valid file descriptor.

.TP

.B EBADF

.RB ( utimensat ())

.I pathname

is a relative pathname, but

.I dirfd

is neither

.BR AT_FDCWD

nor a valid file descriptor.

.TP

.B EFAULT

.I times

pointed to an invalid address; or,

.I dirfd

was

.BR AT_FDCWD ,

and

.I pathname

is NULL or an invalid address.

.TP

.B EINVAL

Invalid value in

.IR flags .

.TP

.B EINVAL

Invalid value in one of the

.I tv_nsec

fields (value outside range 0 to 999,999,999, and not

.B UTIME_NOW

or

.BR UTIME_OMIT );

or an invalid value in one of the

.I tv_sec

fields.

.TP

.B EINVAL

.\" SUSv4 does not specify this error.

.I pathname

is NULL,

.I dirfd

is not

.BR AT_FDCWD ,

and

.I flags

contains

.BR AT_SYMLINK_NOFOLLOW .

.TP

.B ELOOP

.RB ( utimensat ())

Too many symbolic links were encountered in resolving

.IR pathname .

.TP

.B ENAMETOOLONG

.RB ( utimensat ())

.I pathname

is too long.

.TP

.B ENOENT

.RB ( utimensat ())

A component of

.I pathname

does not refer to an existing directory or file,

or

.I pathname

is an empty string.

.TP

.B ENOTDIR

.RB ( utimensat ())

.I pathname

is a relative pathname, but

.I dirfd

is neither

.B AT_FDCWD

nor a file descriptor referring to a directory;

or, one of the prefix components of

.I pathname

is not a directory.

.TP

.B EPERM

The caller attempted to change one or both timestamps to a value

other than the current time,

or to change one of the timestamps to the current time while

leaving the other timestamp unchanged,

(i.e.,

.I times

is not NULL, neither

.I tv_nsec

field is

.BR UTIME_NOW ,

and neither

.I tv_nsec

field is

.BR UTIME_OMIT )

and either:

.RS

.IP * 3

the caller's effective user ID does not match the owner of file,

and the caller is not privileged

(Linux: does not have the

.BR CAP_FOWNER

capability); or,

.IP *

.\" Linux 2.6.22 was broken here:

.\" it was not consistent with the old utimes() implementation,

.\" since the case when both tv_nsec fields are UTIME_NOW, was not

.\" treated like the (times == NULL) case.

the file is marked append-only or immutable (see

.BR chattr (1)).

.\" EXT2_IMMUTABLE_FL EXT_APPPEND_FL and similar flags for

.\" other filesystems.

.\"

.\" Why the inconsistency (which is described under NOTES) between

.\" EACCES and EPERM, where only EPERM tests for append-only.

.\" (This was also so for the older utimes() implementation.)

.RE

.TP

.B EROFS

The file is on a read-only filesystem.

.TP

.B ESRCH

.RB ( utimensat ())

Search permission is denied for one of the prefix components of

.IR pathname .

.SH VERSIONS

.BR utimensat ()

was added to Linux in kernel 2.6.22;

glibc support was added with version 2.6.

.PP

Support for

.BR futimens ()

first appeared in glibc 2.6.

.SH ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw23 lb lb

l l l.

Interface	Attribute	Value

T{

.BR utimensat (),

.BR futimens ()

T}	Thread safety	MT-Safe

.TE

.sp 1

.SH CONFORMING TO

.BR futimens ()

and

.BR utimensat ()

are specified in POSIX.1-2008.

.SH NOTES

.BR utimensat ()

obsoletes

.BR futimesat (2).

.PP

On Linux, timestamps cannot be changed for a file marked immutable,

and the only change permitted for files marked append-only is to

set the timestamps to the current time.

(This is consistent with the historical behavior of

.BR utime (2)

and

.BR utimes (2)

on Linux.)

.PP

If both

.I tv_nsec

fields are specified as

.BR UTIME_OMIT ,

then the Linux implementation of

.BR utimensat ()

succeeds even if the file referred to by

.IR dirfd

and

.I pathname

does not exist.

.SS C library/kernel ABI differences

On Linux,

.BR futimens ()

is a library function implemented on top of the

.BR utimensat ()

system call.

To support this, the Linux

.BR utimensat ()

system call implements a nonstandard feature: if

.I pathname

is NULL, then the call modifies the timestamps of

the file referred to by the file descriptor

.I dirfd

(which may refer to any type of file).

Using this feature, the call

.I "futimens(fd,\ times)"

is implemented as:

.PP

.in +4n

.EX

utimensat(fd, NULL, times, 0);

.EE

.in

.PP

Note, however, that the glibc wrapper for

.BR utimensat ()

disallows passing NULL as the value for

.IR pathname :

the wrapper function returns the error

.IR EINVAL

in this case.

.SH BUGS

Several bugs afflict

.BR utimensat ()

and

.BR futimens ()

on kernels before 2.6.26.

These bugs are either nonconformances with the POSIX.1 draft specification

or inconsistencies with historical Linux behavior.

.IP * 3

POSIX.1 specifies that if one of the

.I tv_nsec

fields has the value

.B UTIME_NOW

or

.BR UTIME_OMIT ,

then the value of the corresponding

.I tv_sec

field should be ignored.

Instead, the value of the

.I tv_sec

field is required to be 0 (or the error

.B EINVAL

results).

.IP *

Various bugs mean that for the purposes of permission checking,

the case where both

.I tv_nsec

fields are set to

.BR UTIME_NOW

isn't always treated the same as specifying

.I times

as NULL,

and the case where one

.I tv_nsec

value is

.B UTIME_NOW

and the other is

.B UTIME_OMIT

isn't treated the same as specifying

.I times

as a pointer to an array of structures containing arbitrary time values.

As a result, in some cases:

a) file timestamps can be updated by a process that shouldn't have

permission to perform updates;

b) file timestamps can't be updated by a process that should have

permission to perform updates; and

c) the wrong

.I errno

value is returned in case of an error.

.\" Below, the long description of the errors from the previous bullet

.\" point (abridged because it's too much detail for a man page).

.\" .IP *

.\" If one of the

.\" .I tv_nsec

.\" fields is

.\" .BR UTIME_OMIT

.\" and the other is

.\" .BR UTIME_NOW ,

.\" then the error

.\" .B EPERM

.\" should occur if the process's effective user ID does not match

.\" the file owner and the process is not privileged.

.\" Instead, the call successfully changes one of the timestamps.

.\" .IP *

.\" If file is not writable by the effective user ID of the process and

.\" the process's effective user ID does not match the file owner and

.\" the process is not privileged,

.\" and

.\" .I times

.\" is NULL, then the error

.\" .B EACCES

.\" results.

.\" This error should also occur if

.\" .I times

.\" points to an array of structures in which both

.\" .I tv_nsec

.\" fields are

.\" .BR UTIME_NOW .

.\" Instead the call succeeds.

.\" .IP *

.\" If a file is marked as append-only (see

.\" .BR chattr (1)),

.\" then Linux traditionally

.\" (i.e.,

.\" .BR utime (2),

.\" .BR utimes (2)),

.\" permits a NULL

.\" .I times

.\" argument to be used in order to update both timestamps to the current time.

.\" For consistency,

.\" .BR utimensat ()

.\" and

.\" .BR futimens ()

.\" should also produce the same result when given a

.\" .I times

.\" argument that points to an array of structures in which both

.\" .I tv_nsec

.\" fields are

.\" .BR UTIME_NOW .

.\" Instead, the call fails with the error

.\" .BR EPERM .

.\" .IP *

.\" If a file is marked as immutable (see

.\" .BR chattr (1)),

.\" then Linux traditionally

.\" (i.e.,

.\" .BR utime (2),

.\" .BR utimes (2)),

.\" gives an

.\" .B EACCES

.\" error if

.\" .I times

.\" is NULL.

.\" For consistency,

.\" .BR utimensat ()

.\" and

.\" .BR futimens ()

.\" should also produce the same result when given a

.\" .I times

.\" that points to an array of structures in which both

.\" .I tv_nsec

.\" fields are

.\" .BR UTIME_NOW .

.\" Instead, the call fails with the error

.\" .BR EPERM .

.IP *

POSIX.1 says that a process that has \fIwrite access to the file\fP

can make a call with

.I times

as NULL, or with

.I times

pointing to an array of structures in which both

.I tv_nsec

fields are

.BR UTIME_NOW ,

in order to update both timestamps to the current time.

However,

.BR futimens ()

instead checks whether the

.IR "access mode of the file descriptor allows writing" .

.\" This means that a process with a file descriptor that allows

.\" writing could change the timestamps of a file for which it

.\" does not have write permission;

.\" conversely, a process with a read-only file descriptor won't

.\" be able to update the timestamps of a file,

.\" even if it has write permission on the file.

.SH SEE ALSO

.BR chattr (1),

.BR touch (1),

.BR futimesat (2),

.BR openat (2),

.BR stat (2),

.BR utimes (2),

.BR futimes (3),

.BR inode (7),

.BR path_resolution (7),

.BR symlink (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.