*nix select and exceptfds/errorfds semantics

by

It is sometimes thought that exceptfds is needed to detect errors, but that is a misconception. Errors will be flagged in readfds. Although POSIX requires it (and even calls the parameter errorfds), it depends on the OS whether errors are also flagged in exceptfds. Really, this parameter is only needed if you care about exceptional conditions, but only rarely is there any need to detect those.

What qualifies as an exceptional condition depends on the kind of file descriptor, but by far the most common use is on a TCP socket where it indicates that out-of-band data is available to be read using recv() with the MSG_OOB flag. However, TCP out-of-band data has a number of quirks (e.g. only 1 byte can be outstanding) and as a result is rarely used.

In recent Linux kernels exceptfds can be used to detect when certain sysfs attributes change. The current value of the attribute can be read by reading the appropriate file under /sys, and a select() on the file descriptor will flag exceptfds when the attribute changes. However this currently only works for some of the attributes, and for mount changes (/proc/mounts).

Also some device drivers will flag certain device-specific conditions using exceptfds.

More Related Contents:

  1. Why does this program print “forked!” 4 times?
  2. What are the differences between poll and select?
  3. stat() error ‘No such file or directory’ when file name is returned by readdir()
  4. What is a bus error? Is it different from a segmentation fault?
  5. C Minishell Adding Pipelines
  6. In C how do you redirect stdin/stdout/stderr to files when making an execvp() or similar call?
  7. Color text in terminal applications in UNIX [duplicate]
  8. Please explain the exec() function and its family
  9. Non-blocking call for reading descriptor
  10. Why does ENOENT mean “No such file or directory”?
  11. Merge multiple .so shared libraries
  12. How can I copy a file on Unix using C?
  13. Getting absolute path of a file
  14. How do I get a thread ID from an arbitrary pthread_t?
  15. Why does open() create my file with the wrong permissions?
  16. Set environment variables in C
  17. Why are malloc() and printf() said as non-reentrant?
  18. Calculating elapsed time in a C program in milliseconds
  19. Waiting for all child processes before parent resumes execution UNIX
  20. Recursive mkdir() system call on Unix
  21. Understanding concurrent file writes from multiple processes
  22. Programmatically retrieving the absolute path of an OS X command-line app
  23. Seeking a simple description regarding ‘file descriptor’ after fork()
  24. Dropping root privileges
  25. To check the E2BIG error condition in exec
  26. What can cause a “Resource temporarily unavailable” on sock send() command
  27. Reading from stdin
  28. Using select() for non-blocking sockets
  29. Unix O_CREAT flag without mode specified
  30. Why can’t I ignore SIGSEGV signal?

Leave a Comment