Unexpected output while adding characters from a string into other strings using sprintf

Question

I have a string which is a date and comes in the format yyyymmdd. I need to find out the day,month, year and store them in separate strings and use them further. I have written the following code

char *date="20151221";
char day[2];
char month[2];
char year[4];
sprintf(day, "%c%c", date[6], date[7]);
sprintf(month, "%c%c", date[4], date[5]);
sprintf(year, "%c%c%c%c", date[0], date[1],date[2],date[3]);
lr_output_message("day is %s",day);
lr_output_message("month is %s",month);
lr_output_message("year is %s",year);

But the output am getting is

day is 21122015

month is 122015

year is 2015

Maybe its a dumb question,but I am new to C. Can anybody please explain the reason for this?

When you use sprintf it will append a null to the end of the string. The problem is that you are overwriting neighboring variables with the nulls. This is undefined behavior. Check out the man page for sprintf. — David Hoelzer
– David Hoelzer, Commented Dec 23, 2015 at 3:24
Did you know that a 2-character string actually contains 3 characters? For example, "ab" is 'a', 'b' and '\0'. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Dec 23, 2015 at 3:24

Armali · Accepted Answer · 2018-01-17 13:01:15Z

1

As per the C11 standard, chapter §7.21.6.6, sprintf() function, (emphasis mine)

The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; [...]

which indicates, in case of

sprintf(day, "%c%c", date[6], date[7]);

day should have a minimum space allocated for 3 chars, including the terminating null to be written. Now, in your case, it does not have the room for the terminating null, and thereby, sprintf() try to write into past the allocated memory region, invoking undefined behavior.

You need to consider the space allocation for the terminating null also while defining the arrays.

Same goes for other arrays, too.

edited Jan 17, 2018 at 13:01

Armali

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answered Dec 23, 2015 at 4:31

Sourav Ghosh

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