Error reporting

• flc_attach() returns 1 if OK, 0 on error
• flc_open() returns NULL on error
• flc_read() may return 0 in case of concurrent write access
• Reason for failure recorded in standard C errno variable:
  • EBUSY ("Device or resource busy"): write lock unavailable
  • EINVAL ("Invalid argument"): size, type, or magic number mismatch
  • Others, see libflcomms(3), shmget(2) and shmat(2) manual pages for full list
• Can use perror("...") to report libflcomms-related errors