Creating C formatted strings (not printing them)

Question

I have a function that accepts a string, that is:

void log_out(char *);

In calling it, I need to create a formatted string on the fly like:

Answer 1

http://www.gnu.org/software/hello/manual/libc/Variable-Arguments-Output.html gives the following example to print to stderr. You can modify it to use your log function instead:

 #include 
 #include 

 void
 eprintf (const char *template, ...)
 {
   va_list ap;
   extern char *program_invocation_short_name;

   fprintf (stderr, "%s: ", program_invocation_short_name);
   va_start (ap, template);
   vfprintf (stderr, template, ap);
   va_end (ap);
 }

Instead of vfprintf you will need to use vsprintf where you need to provide an adequate buffer to print into.