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readdir.3: Explain why readdir_r() is deprecated and readdir() is preferred 

Based on Florian Weimer's text that was added to the glibc manual;
relicensed for use in this man page by permission of Florian.
See http://www.austingroupbugs.net/view.php?id=696
and https://sourceware.org/bugzilla/show_bug.cgi?id=14699

Cowritten-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2016-03-01 00:54:25 +01:00
parent cff459dea2
commit 0c52f6d623
1 changed files with 61 additions and 2 deletions
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63
man3/readdir.3
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@ -1,5 +1,6 @@
.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2008, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2016 Florian Weimer <fweimer@redhat.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -124,7 +125,7 @@ field, and other nonstandard fields may precede that field within the
.I dirent
structure, portable applications that use
.BR readdir_r ()
should allocate the buffer whose address is passed in
could allocate the buffer whose address is passed in
.IR entry
as follows:
.in +4n
@ -142,6 +143,61 @@ entryp = malloc(len);
.I d_name
is the last field in a
.IR "struct dirent" .)
However, the above approach has problems,
and it is recommended that applications use
.BR readdir ()
instead of
.BR readdir_r ().
Furthermore, since version 2.24, glibc deprecates
.BR readdir_r ().
The reasons are as follows:
.IP * 3
On systems where
.BR NAME_MAX
is undefined, calling
.BR readdir_r ()
may be unsafe because the interface does not allow the caller to specify
the length of the buffer used for the returned directory entry.
.IP *
On some systems,
.BR readdir_r ()
can't read directory entries with very long names.
When the glibc implementation encounters such a name,
.BR readdir_r ()
fails with the error
.B ENAMETOOLONG
.IR "after the final directory entry has been read" .
On some other systems,
.BR readdir_r ()
may return a success status, but the returned
.IR d_name
field may not be null terminated or may be truncated.
.IP *
In the current POSIX.1 specification (POSIX.1-2008),
.BR readdir ()
is not required to be thread-safe.
However, in modern implementations (including the glibc implementation),
concurrent calls to
.BR readdir ()
that specify different directory streams are thread-safe.
Therefore, the use of
.BR readdir_r ()
is generally unnecessary in multithreaded programs.
In cases where multiple threads must read from the same directory stream,
using
.BR readdir ()
with external synchronization is still preferable to the use of
.BR readdir_r (),
for the reasons given in the points above.
.IP *
It is expected that a future version of POSIX.1
.\" http://www.austingroupbugs.net/view.php?id=696
will make
.BR readdir_r ()
obsolete, and require that
.BR readdir ()
be thread-safe when concurrently employed on different directory streams.
.SH RETURN VALUE
On success,
.BR readdir ()
@ -170,6 +226,9 @@ returns 0, and returns NULL in
.TP
.B EBADF
Invalid directory stream descriptor \fIdirp\fP.
.TP
.B ENAMETOOLOMG
A directory entry whose name was too long to be read was encountered.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).