How to access the QUEST common block in a C program ?

You should use cfortran.h.

 #include        "cfortran.h"

 typedef struct {
         int     iquest[100];
 } quest_def;

 #define QUEST COMMON_BLOCK(QUEST,quest)

 COMMON_BLOCK_DEF(quest_def,QUEST);

Then in the C program the iquest variable can be used as follows:

           .
           .
           .
 QUEST.iquest[9] = 0;
           .
           .
           .

 if (QUEST.iquest[0] != 1 ) {
           .
           .
           .

Note that IQUEST(1) in fortran correspond to QUEST.iquest[0] in C.