Re: xctime()

[Bruno Haible]

Hi Robert,

Robert Millan wrote:
> I implemented xctime for GNU isofsmk.  xctime is an alternative to ctime()
> that:
> 
>   - Uses dynamic allocation
>   - Is thread-safe
>   - Returns localized strings

I agree that it's not a good idea to use ctime() or asctime(), because of
the problems mentioned in gnulib/doc/posix-functions/*ctime.texi.

> char *
> xctime (const time_t *time)
> {
>   struct tm *loctime;
>   char *str;
>   size_t len;
> 
>   loctime = localtime (time);
> 
>   len = strftime (NULL, INT32_MAX, "%c", loctime) + sizeof ("");
> 
>   str = xmalloc (len);
>   strftime (str, len, "%c", loctime);
> 
>   return str;
> }
>
> I find it much more comfortable than having to deal with strftime() every
> time.  If you're interested, I could make a Gnulib module out of it.

Hmm, if this is proposed for gnulib, I have 4 remarks:

  - Error handling: strftime can fail. Since you call xmalloc for the
    memory allocation, I think it would be reasonable to call error()
    in case strftime fails.

  - Flexibility: The three most often used formats are probably %c, %x, %X.
    Any function for date formatting should not have the format built-in.
    It should have a format indication as argument. But then you can also
    allow the user to pass an entire format string. And define shorthands
    as reminders like this:
      #define TIMEFORMAT_LOCALE_DATETIME "%c"
      #define TIMEFORMAT_LOCALE_DATE "%x"
      #define TIMEFORMAT_LOCALE_TIME "%X"

  - In the average case, you can get away with 1 strftime call instead of 2,
    if you preallocate a buffer on the stack:
      char buf[256];
      len = strftime (buf, sizeof (buf), "%c", loctime) + 1;
    In the case where 256 bytes are not enough, the function will need
    3 strftime calls instead of 1, but this case should be pretty rare
    (even in Chinese and GB18030 encoding, 64 characters should be enough
    for a date + time display).

  - sizeof ("") is confusing. You can write 1 instead.

Bruno