formated time string for file naming

#include <stdio.h>
#include <time.h>

int main()
{
  time_t rawtime;
  struct tm *timeinfo;
  char buffer[];

  time(&rawtime);
  timeinfo = localtime(&rawtime);

  strftime(buffer, , "result-%Y_%m_%d_%H_%M_%S.jpg", timeinfo);
  puts(buffer);

  return ;
}