futimens man page

Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

futimens, utimensat, utimes — set file access and modification times

Synopsis

#include <sys/stat.h>

int futimens(int fd, const struct timespec times[2]);
int utimensat(int fd, const char *path, const struct timespec times[2],
    int flag);

#include <sys/time.h>

int utimes(const char *path, const struct timeval times[2]);

Description

The futimens() and utimensat() functions shall set the access and modification times of a file to the values of the times argument. The futimens() function changes the times of the file associated with the file descriptor fd. The utimensat() function changes the times of the file pointed to by the path argument, relative to the directory associated with the file descriptor fd. Both functions allow time specifications accurate to the nanosecond.

For futimens() and utimensat(), the times argument is an array of two timespec structures. The first array member represents the date and time of last access, and the second member represents the date and time of last modification. The times in the timespec structure are measured in seconds and nanoseconds since the Epoch. The file's relevant timestamp shall be set to the greatest value supported by the file system that is not greater than the specified time.

If the tv_nsec field of a timespec structure has the special value UTIME_NOW, the file's relevant timestamp shall be set to the greatest value supported by the file system that is not greater than the current time. If the tv_nsec field has the special value UTIME_OMIT, the file's relevant timestamp shall not be changed. In either case, the tv_sec field shall be ignored.

If the times argument is a null pointer, both the access and modification timestamps shall be set to the greatest value supported by the file system that is not greater than the current time. If utimensat() is passed a relative path in the path argument, the file to be used shall be relative to the directory associated with the file descriptor fd instead of the current working directory. If the file descriptor was opened without O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the file descriptor was opened with O_SEARCH, the function shall not perform the check.

If utimensat() is passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used.

Only a process with the effective user ID equal to the user ID of the file, or with write access to the file, or with appropriate privileges may use futimens() or utimensat() with a null pointer as the times argument or with both tv_nsec fields set to the special value UTIME_NOW. Only a process with the effective user ID equal to the user ID of the file or with appropriate privileges may use futimens() or utimensat() with a non-null times argument that does not have both tv_nsec fields set to UTIME_NOW and does not have both tv_nsec fields set to UTIME_OMIT. If both tv_nsec fields are set to UTIME_OMIT, no ownership or permissions check shall be performed for the file, but other error conditions may still be detected (including [EACCES] errors related to the path prefix).

Values for the flag argument of utimensat() are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

AT_SYMLINK_NOFOLLOW
If path names a symbolic link, then the access and modification times of the symbolic link are changed.

Upon completion, futimens() and utimensat() shall mark the last file status change timestamp for update.

The utimes() function shall be equivalent to the utimensat() function with the special value AT_FDCWD as the fd argument and the flag argument set to zero, except that the times argument is a timeval structure rather than a timespec structure, and accuracy is only to the microsecond, not nanosecond, and rounding towards the nearest second may occur.

Return Value

Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1 and set errno to indicate the error. If -1 is returned, the file times shall not be affected.

Errors

These functions shall fail if:

EACCES
The times argument is a null pointer, or both tv_nsec values are UTIME_NOW, and the effective user ID of the process does not match the owner of the file and write access is denied.
EINVAL
Either of the times argument structures specified a tv_nsec value that was neither UTIME_NOW nor UTIME_OMIT, and was a value less than zero or greater than or equal to 1000 million.
EINVAL
A new file timestamp would be a value whose tv_sec component is not a value supported by the file system.
EPERM
The times argument is not a null pointer, does not have both tv_nsec fields set to UTIME_NOW, does not have both tv_nsec fields set to UTIME_OMIT, the calling process' effective user ID does not match the owner of the file, and the calling process does not have appropriate privileges.
EROFS
The file system containing the file is read-only.

The futimens() function shall fail if:

EBADF
The fd argument is not a valid file descriptor.

The utimensat() function shall fail if:

EACCES
fd was not opened with O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.
EBADF
The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file descriptor associated with a non-directory file.

The utimensat() and utimes() functions shall fail if:

EACCES
Search permission is denied by a component of the path prefix.
ELOOP
A loop exists in symbolic links encountered during resolution of the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than {NAME_MAX}.
ENOENT
A component of path does not name an existing file or path is an empty string.
ENOTDIR
A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.

The utimensat() and utimes() functions may fail if:

ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

The utimensat() function may fail if:

EINVAL
The value of the flag argument is not valid.

The following sections are informative.

Rationale

The purpose of the utimensat() function is to set the access and modification time of files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to utimes(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the utimensat() function it can be guaranteed that the changed file is located relative to the desired directory.

The standard developers considered including a special case for the permissions required by utimensat() when one tv_nsec field is UTIME_NOW and the other is UTIME_OMIT. One possibility would be to include this case in with the cases where times is a null pointer or both fields are UTIME_NOW, where the call is allowed if the process has write permission for the file. However, associating write permission with an update to just the last data access timestamp (which is normally updated by read()) did not seem appropriate. The other possibility would be to specify that this one case is allowed if the process has read permission, but this was felt to be too great a departure from the utime() and utimes() functions on which utimensat() is based. If an application needs to set the last data access timestamp to the current time for a file on which it has read permission but is not the owner, it can do so by opening the file, reading one or more bytes (or reading a directory entry, if the file is a directory), and then closing it.

See Also

read(), utime()

The Base Definitions volume of POSIX.1‐2008, <fcntl.h>, <sys_stat.h>, <sys_time.h>

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2013 IEEE/The Open Group POSIX Programmer's Manual