Issue 0045: Can one freopen an already closed file?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: David J. Hendricksen, WG14
Date: 1992-12-10
Reference document: X3J11/92-036
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_045.html

Under subclause 7.9.5.4 The freopen function, the C Standard states on page 130, lines 24-29:

The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function.

The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file successfully is ignored. The error and end-of-file indicators for the stream are cleared.

It is not clear whether the following situations have defined behavior:

1. Calling freopen where stream points to uninitialized storage. For example:

           { FILE a, *b;
           b = freopen("c.d", "r", &a);
           }
   

   (It may not be possible to detect that the information contained within a is not valid when the close for freopen is attempted.)

2. Calling freopen where stream is associated with a previously closed file. (The storage pointed to by stream may have been deallocated.)

Comment from WG14 on 1997-09-23:

Response

The behavior is undefined in both cases; case (2) is clear from subclause 7.9.3 Files, page 126, lines 24-27, “A file may be disassociated from a controlling stream by closing the file... The value of a pointer to a FILE object is indeterminate after the associated file is closed (including the standard text streams).” Also subclause 7.9.3 Files, page 126, lines 2-3 and lines 37-39, “A stream is associated with an external file... by opening a file, ... At program startup, three text streams are predefined and need not be opened explicitly...” Also subclause 7.9.5.3 The fopen function, and, similarly, subclause 7.9.5.4 The freopen function: “The ... function opens the file ... and associates a stream with it...” Thus when subclause 7.9.5.4 says “The freopen function ... associates the stream pointed to by stream with it,” the intention is certainly that stream already points to a valid stream. Extending this to case (1), we observe that a (or &a) might not refer to a stream, since none has been “associated” by any means specified in the C Standard.