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Last update: 1997-05-20
9945-1-90 #16
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Topic: EISDIR
Relevant Sections: 5.3.1.4
Classification: No change.
Defect Report:
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Section 5.3.1.4 provides the errno EISDIR for the open() function
when the file being opened enabling writes and the file is a directory.
Section 2.4 (lines 521-524) allows implementations to generate error
numbers listed, under circumstances other than those described "if and
only if..."
We would like to know whether or not a conforming implementation
can return an EISDIR error for opening a directory for reading. It does
seem apparent that a portable application cannot use open, read,
and close on a directory because the standard does not define the format
of a directory file, and without that information, one cannot interpret
the results of the read.
If an implementation chooses to use file descriptors to implement
directory functions and a directory file descriptor number is passed
to close() or dup() or either parameter of dup2(), are these
functions permitted to fail? Our implementation of opendir(), readdir(),
rewinddir() and closedir() calls for these functions to be kernel
interfaces, so we could be fairly intelligent in preventing
a program from doing something that does not make sense(i.e. reading
a file whose contents cannot be interpretated). We'd like
to know what the standard permits.
WG15 response for ISO/IEC 9945-1:1990 (IEEE Std 1003.1-1990)
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An implementation must permit an application to open for reading a
filename referring to a directory. An implementation which returns an
error indication of EISDIR in this situation is nonconforming as
this error is limited to calling open() for write or read/write access.
The file descriptor returned by open() can be used normally with
other functions that take file descriptors as arguments.
An application which opens a directory with open() and reads it with
read() is not necessarily nonconforming, but the resulting contents of
the buffer are unspecified by POSIX.1. The standard does not specify
a relationship between file descriptors underlying the DIR datatype
from opendir() and a file descriptor obtained by calling open().
RATIONALE
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There are other ways a program might use a file descriptor referring
to a directory that do not involve read(), such as using fstat() to
periodically check whether the modification time of a directory has
changed and using this information to trigger rescanning the
directory with rewinddir() and readdir().
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