Answering questions that may arise related to the meaning of portions of an IEEE standard concerning specific applications.

IEEE Standards Interpretation for IEEE Std 1003.1™-1990 IEEE Standard for Information Technology--Portable Operating System Interfaces (POSIX®)

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Interpretation Request #14
format of directory entries, use of read/write Relevant Sections: 5.1.1 Classification: No change required

In 5.1.1 Format of Directory Entries, page 83, line 10:
"The internal format of directories is unspecified."

In Errors [Open a File], page 90, lines 256-257:
"[EISDIR] The named file is a directory, and the oflag argument specifies write or read/write access."

Traditional implementations permitted the use of the read() function on directory files, so no error condition is defined in POSIX.1 for this case. May a conforming application open a directory file and read it using the open() and read() functions?

Interpretation Response
Yes, a conforming application can invoke read() on a directory, however, the result of any such read() is unspecified, and may be an error return, including [EISDIR].

Rationale for Interpretation
Nothing in the standard prevents an application from using the open() and read() functions on a directory, but an application which uses knowledge of a particular implementation's format for directories is using a non-portable extension. Implementations are free to return anything they want from a read() to a directory. For example, read() could always return zero, or -1 with some appropriate errno.