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When opening a FIFO with O_RDONLY or O_WRONLY set: If O_NONBLOCK is set:
  An open() for reading only will return without delay. An open() for writing only will return an error if no process currently has the file open for reading.
If O_NONBLOCK is clear:
  An open() for reading only will block the calling thread until a thread opens the file for writing. An open() for writing only will block the calling thread until a thread opens the file for reading.

When opening a block special or character special file that supports non-blocking opens:
If O_NONBLOCK is set:
  The open() function will return without blocking for the device to be ready or available. Subsequent behaviour of the device is device-specific.
If O_NONBLOCK is clear:
  The open() function will block the calling thread until the device is ready or available before returning.

Otherwise, the behaviour of O_NONBLOCK is unspecified.

Once the file is open, programmers can use the POSIX {{lstat()}}/{{fstat()

}} functions to obtain information about a named file, and the {{S_ISREG()}} macro to determine if the file is a regular file. (see \[[FIO05-A. Identify files using multiple file attributes]\])

Since the behavior of O_NONBLOCK on subsequent calls to read() or write() is unspecified, it is advisable to disable the flag once we are sure the file in question is not a special device.

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\[[Garfinkel 96|AA. C References#Garfinkel 96]\] Section 5.6, "Device Files"
\[[Howard 02|AA. C References#Howard 02]\] Chapter 11, "Canonical Representation Issues"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section

Related Rules and Recommendations

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FIO31-C. Do not simultaneously open the same file multiple times      09. Input Output (FIO)       FIO33-C. Detect and handle input output errors resulting in undefined behavior