sprintf crashing

Question

 int main()
 {
     char buf1[100], buf[100]="ddl";

     sprintf(buf1, "log_name = '%.*s'", buf);
  }

The above program is crashing. I am not able to understand why is this crashing. As far as I know before the character makes printf to skip the format code and assign buf to next format code. But here what is the significance of?

Answer 1

The format specifier "%.*s" requires the number of characters to be written to be specified:

sprintf(buf1, "log_name = '%.*s'", 3, buf); /* For example */
                                /* ^ */

In the posted code only buf is provided so there are missing arguments, which is undefined behaviour (in this case a crash).

Note, in this case, "%s" would serve just as well as buf is null terminated (no requirment for a length unless all characters in buf must not be copied).

Answer 2

You can see the effect of this as follows .

int main()
{   
printf("%.*s",13,"stackoverflow rocks");
}

which will give you only stackoverflow .