IEEE Standards AssociationIEEE Standards Interpretations for IEEE Std 1003.1™-2001 IEEE Standard for Information Technology - Portable Operating System Interface (POSIX®)
Copyright © 2006 by the Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue New York, New York 10016-5997 USA All Rights Reserved.
Interpretations are issued to explain and clarify the intent of a standard and do not constitute an alteration to the original standard. In addition, interpretations are not intended to supply consulting information. Permission is hereby granted to download and print one copy of this document. Individuals seeking permission to reproduce and/or distribute this document in its entirety or portions of this document must contact the IEEE Standards Department for the appropriate license. Use of the information contained in this document is at your own risk.
IEEE Standards Department Copyrights and Permissions 445 Hoes Lane, Piscataway, New Jersey 08855-1331, USA
Interpretation Request #20
Topic: c99 Relevant Sections: Output Files
In earlier versions of this standard, it was generally assumed, but not explicitly specified that object files and executable files were regular files. With the changes that were made to XBD subclause 1.7.1.4 (File Read, Write, and Creation) in this revision and wording in c99 that doesn't quite match the templates in XBD 1.7.1.4, some people have interpreted the standard to require that c99 be able to write object files and executable files to files of type fattach()-ed STREAM, block special, character special, and FIFO special as well as to regular files. I do not believe that this was the intent when the c89 and c99 utilities were originally drafted. I do not object to implementations supporting all of these file types, but I see no reason for the standard to require that compilers work correctly with anything but regular files (and symlinks pointing to regular files).
Add a new sentence to the end of the paragraph in the OUTPUT FILES section on P214, L8400: If an existing file that does not resolve to a regular file matches the name of an object file being written or matches the name of an executable file being created by c99, it is unspecified whether c99 shall attempt to write the object file or create the executable file, or shall issue a diagnostic and exit with a non-zero exit status.
Add a new paragraph to the rationale after P218, L8602:
This standard specifies that the c99 utility must be
able to use regular files for *.o files and for a.out
files. Implementations are free to overwrite existing
files of other types when attempting to create object
files and executable files, but are not required to do
so. If something other than a regular file is specified and using it fails for
any reason, c99 is
required to issue a diagnostic message and exit with a
non-zero exit status. But for some file types, the
problem may not be noticed for a long time. For
example, if a FIFO named a.out exists in the current
directory, c99 may attempt to open a.out and will hang
in the open() call until another process opens the
FIFO for reading. Then c99 may write most of the a.out
to the FIFO and fail when it tries to seek back close
to the start of the file to insert a timestamp (FIFOs
are not seekable files). The c99 utility is also
allowed to issues a diagnostic immediately if it
encounters an a.out or *.o file that is not a regular
file. For portable use, applications should ensure
that any a.out, -o option-argument, or *.o files
corresponding to any *.c files do not conflict with
names already in use that are not regular files or
symbolic links that point to regular files.
Interpretation Response
The standards states the requirements for the c99 utility, and conforming
implementations must conform to this. However, concerns have been raised
about this which are being referred to the sponsor.
Rationale for Interpretation
None.
Notes to the Editor (not part of this interpretation)
A future revision should consider the following additions to c99:
Add a new sentence to the end of the paragraph in the OUTPUT FILES section
on P214, L8400:
If an existing file that does not resolve to a regular file matches the name
of an object file being written or matches the name of an executable file being
created by c99, it is unspecified whether c99 shall attempt to write the object
file or create the executable file, or shall issue a diagnostic and exit with
a non-zero exit status.
Add a new paragraph to the rationale after P218, L8602: This standard specifies that the c99 utility must be able to use regular files for *.o files and for a.out files. Implementations are free to overwrite existing files of other types when attempting to create object files and executable files, but are not required to do so. If something other than a regular file is specified and using it fails for any reason, c99 is required to issue a diagnostic message and exit with a non-zero exit status. But for some file types, the problem may not be noticed for a long time. For example, if a FIFO named a.out exists in the current directory, c99 may attempt to open a.out and will hang in the open() call until another process opens the FIFO for reading. Then c99 may write most of the a.out to the FIFO and fail when it tries to seek back close to the start of the file to insert a timestamp (FIFOs are not seekable files). The c99 utility is also allowed to issues a diagnostic immediately if it encounters an a.out or *.o file that is not a regular file. For portable use, applications should ensure that any a.out, -o option-argument, or *.o files corresponding to any *.c files do not conflict with names already in use that are not regular files or symbolic links that point to regular files.