/* Ensure exclusive access to a file by a pipeline*/
/* Usage: yada yada|exclusive >output_file
   or:    yada yada|exclusive name_of_file|yada name_of_file
   WARNING: Do not provide exclusive access to more than one file
	per pipeline, for risk of deadly embraces between processes.
*/
/*Started: RJ Botting, Sep 1999 SunOS ftp 4.1.3_U1 2 sun4m */
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>
#include <sys/file.h>
/*flock - apply or remove an advisory lock on an open file*/

main( argc, argv)
int argc; char *argv[];
{
   int ch; 
   int locker; /* file descriptor */
   int fl;/*reponse from flock(...)*/
   locker=1; /*stdout, default, allows exclusive >file */
   if(argc>1){
	/*fprintf(stderr, "DBUG1: %s\n",argv[1]);*/
	locker=open(argv[1], O_RDONLY);
	if( -1 == locker){
		fprintf(stderr, "File %s couldn't be openned.\n", argv[1]);
		perror("");
		exit(1);
	}
   }
   fl=flock( locker, LOCK_EX);

   /*if(argc>1)fprintf(stderr, "DBUG2: %s %d\n",argv[1], fl);*/

   /*copy stdin to stdout. From Kernighan & Ritchie Chapter 7 */
   while((ch=getc(stdin))!=EOF){
  	putc(ch, stdout); 
   }/*end while*/

   flock( locker, LOCK_UN);
   if(argc>1){
	/*fprintf(stderr, "DBUG3: %s\n",argv[1]);*/
	close(locker);
   }
}