Question

I would like to use the syntax that printf uses, using the %d, %s and adding values after to assign a value to a char[]. Is this possible?

e.g. Given an output of:

printf("now: %d-%d-%d %d:%d:%d\n", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);

I’d like to assign that to char[] output;

How can this be done?

I tried:

sprintf(output, "now: %d-%d-%d %d:%d:%d\n", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);

but that didn’t seem to work. Is sprintf used differently… or is that not what I should be using?

Thanks!

EDIT: It turned out I simply needed to increase the initialized length of output…

Answer 1

That should work fine provided you have enough space in the output buffer (at least 26 bytes) to take the largest string you’ll create:

char output[26];
sprintf(output, "now: %d-%d-%d %d:%d:%d\n",
    tm.tm_year + 1900,
    tm.tm_mon + 1,
    tm.tm_mday,
    tm.tm_hour,
    tm.tm_min,
    tm.tm_sec);

You need:

• 5 for "now: ".
• 4 for the year (for at least the next 79 centuries).
• 2 each for the other five numerics.
• 5 for the separators ("-- ::").
• 1 for the newline.
• 1 for the terminating null character.

You may also want to consider changing the format string to "now: %04d-%02d-%02d %02d:%02d:%02d" so that the datetimes will line up if you’re outputting them, to avoid something like:

2009-12-31 23:59:58 blah blah blah
2010-1-1 0:0:7 yada yada yada

when

2009-12-31 23:59:58 blah blah blah
2010-01-01 00:00:07 yada yada yada

looks so much better. You’ll notice I’ve left the "\n" off the end – since this is the sort of code you see a lot in logging, you’d want the datetime to be in a format you could easily append to, something like:

fprintf (logFile, "%s Cannot write to '%s', error = %d\n",
    output, fileSpec, errno);

If I’ve misunderstood what you’re using this for, feel free to ignore the last few paragraphs.