Question

Learning C and I’m trying to get a visual comparison of the variable types and sizes that are returned by stat() for the atime/mtime attributes and for the nsec precision values.

I’m running stat() on a file and want to get the mtime and mtime nsec values from the returned stat structure and then store these values in separate variables (which I then want to pass to utimes()… long story!).
According to http://www.kernel.org/doc/man-pages/online/pages/man2/stat.2.html#NOTES I can get the value from st_mtim.tv_nsec or st_mtimensec depending on various OS/build conditions. In my actual program I’ll check for both and use whichever is set, or just fallback to the normal second precision of st_mtime

What variable type and size do I need to declare in order to store a normal timestamp as returned by st_mtime?

What variable type and size do I need to declare to store an nsec value from st_mtim.tv_nsec or st_mtimensec?
Are these a decimal, including the number of whole seconds of the time? Or do they just return the nsec portion of the time?

Do I need to declare different variable sizes for the nsecs depending on my system’s architecture?

And finally, what conversion specifiers do I need for outputting these variables using printf()?

Cheers, B

Answer 1

1. st_mtime should be a time_t.
2. According to POSIX <time.h>, the type of tv_nsec is just long.
3. The fields like st_mtim.tv_nsec will return the number of nanoseconds.
4. For the long, you need l; for time_t, it is not clearly defined, AFAIK.