How to implement futimes in terms of utimes?

Question

Given that in Linux utimes(2) is a system call and futimes(3) is a library function, I would think that futimes is implemented in terms of utimes. However, utimes takes a pathname, whereas futimes takes a file descriptor.

Since, it is "not possible" to determine a pathname from the file descriptor or i-node number I wonder how this can be done? Does the "real" system call always work on i-node numbers?

Answer 1

First, you likely wrongly mentioned Posix because the latter doesn't differ system calls and library functions. The putting of futimes() to library calls is Linux specific. In glibc (file sysdeps/unix/sysv/linux/futimes.c), there is the comment:

/* Change the access time of the file associated with FD to TVP[0] and
   the modification time of FILE to TVP[1].

   Starting with 2.6.22 the Linux kernel has the utimensat syscall which
   can be used to implement futimes.  Earlier kernels have no futimes()
   syscall so we use the /proc filesystem.  */

So, this is done using utimensat() with the specified descriptor as the reference one as for all *at() calls. Previously, this worked using utimes() for the path /proc/${pid}/fd/${fd} (too cumbersome and only if /proc is mounted). This is a reply to your second question: despite it isn't generally possible to detect a file name from its descriptor, the file still could be accessed separately. (BTW, the initial path used to open the file is sometimes stored; see /proc/$pid/{cwd,exe} for a Linux process.)

To compare with, FreeBSD provides explicit futimes() and futimesat() syscalls (but I wonder why the latter isn't named "utimesat").