/* Define the record */
     #define SIZE_A 10   
            /* Max size of string including the NUL terminator */

     struct s_record
          {
          char a[SIZE_A];
          int xx;
          ...
          };

     struct s_record record;
     int record_number;

     /* Write a record */
     record_number = 12;
     fseek(file,(long) RECORD_SIZE * record_number, 0);
     fwrite(&record,RECORD_SIZE,1,file);
     
     /* Read a record */
     record_number = 12;
     fseek(file,(long) RECORD_SIZE * record_number, 0);
     fread(&record,RECORD_SIZE,1,file);