Taken from: https://sources.debian.org/patches/c2esp/27-8/0002-Add-extern-stanzas-to-variable-definitions-in-header.patch/
Author: Didier Raboud <odyx@debian.org>

--- a/src/c2espcommon.c
+++ b/src/c2espcommon.c
@@ -42,11 +42,11 @@
  * Globals...
  */
 char		CallerName[50];  	/* String that identifies the calling program */
-int		DoBack;			/* Enables the back channel comms */ 
+extern int	DoBack;			/* Enables the back channel comms */
 char 		BackBuf[32000]; //for the back channel replies from the printer
 int 		BackBufLen=sizeof(BackBuf)-1;
 FILE 		*LogFile = NULL; //file descriptor for log file
-time_t		StartTime;
+extern time_t	StartTime;
 int		BlackPercent, ColourPercent;
 
 time_t KeepAwake(time_t Start, int Interval, FILE *PrintFile)
--- a/src/c2espcommon.h
+++ b/src/c2espcommon.h
@@ -35,8 +35,8 @@
 /*
  * Globals...
 */
-char 		BackBuf[32000]; //for the back channel replies from the printer
-int		ColourPercent, BlackPercent; //to store the detected marker levels
+extern char BackBuf[32000]; //for the back channel replies from the printer
+extern int ColourPercent, BlackPercent; //to store the detected marker levels
 
 
 time_t KeepAwake(time_t Start, int Interval, FILE *PrintFile);