C strftime has ending hex characters

Question

I am trying to add a Date header to my HTTP response. So far, I have been doing something like that:

char timebuf[37];
time_t now = time(0);
struct tm tm = *gmtime(&now);
strftime(timebuf, sizeof timebuf,"Date: %a, %d %b %Y %H:%M:%S %Z\r\n", &tm);)

However, when inspecting timebuf, I am, sometimes (like on the 1st request of the server), receiving a \x7f ending character.

Why is that inconsistent? I have double checked the size of timebuf and it should be exactly 37.

Answer 1

The format string you've shown us will result in a string of length 37. Excluding the NULL terminator.

Your array should have 38 elements!

Currently your timebuf is left unterminated, so you have undefined behaviour anywhere that expects a null-terminated string. That you're seeing \x7f sometimes and other characters sometimes (presumably even \0 sometimes) is just pure chance.

---

From cppreference.com's C documentation on strftime:

Converts the date and time information from a given calendar time time to a null-terminated multibyte character string str according to format string format. Up to count bytes are written.

And here are some sums:

Substr     Length
---------+--------
"Date: "     6
"%a"         3
", "         2
"%d"         2
" "          1
"%b"         3
" "          1
"%Y"         4
" "          1
"%H"         2
":"          1
"%M"         2
":"          1
"%S"         2
" "          1
"%Z"         4
"\r\n"       2

NULL term.   1
-----------------
         =   38