crypt.3: srcfix: rewrap source lines

[thirdparty/man-pages.git] / man3 / readdir.3

.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\" and Copyright (C) 2008, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 16:09:49 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 11 June 1995 by Andries Brouwer (aeb@cwi.nl)
.\" Modified 22 July 1996 by Andries Brouwer (aeb@cwi.nl)
.\" 2007-07-30 Ulrich Drepper <drepper@redhat.com>, mtk:
.\"     Rework discussion of nonstandard structure fields.
.\"
.TH READDIR 3  2017-09-15 "" "Linux Programmer's Manual"
.SH NAME
readdir \- read a directory
.SH SYNOPSIS
.nf
.B #include <dirent.h>
.PP
.BI "struct dirent *readdir(DIR *" dirp );
.fi
.SH DESCRIPTION
The
.BR readdir ()
function returns a pointer to a \fIdirent\fP structure
representing the next directory entry in the directory stream pointed
to by \fIdirp\fP.
It returns NULL on reaching the end of the directory stream or if
an error occurred.
.PP
In the glibc implementation, the
.I dirent
structure is defined as follows:
.PP
.in +4n
.EX
struct dirent {
    ino_t          d_ino;       /* Inode number */
    off_t          d_off;       /* Not an offset; see below */
    unsigned short d_reclen;    /* Length of this record */
    unsigned char  d_type;      /* Type of file; not supported
                                   by all filesystem types */
    char           d_name[256]; /* Null-terminated filename */
};
.EE
.in
.PP
The only fields in the
.I dirent
structure that are mandated by POSIX.1 are
.IR d_name
and
.IR d_ino .
The other fields are unstandardized, and not present on all systems;
see NOTES below for some further details.
.PP
The fields of the
.I dirent
structure are as follows:
.TP
.I d_ino
This is the inode number of the file.
.TP
.I d_off
The value returned in
.I d_off
is the same as would be returned by calling
.BR telldir (3)
at the current position in the directory stream.
Be aware that despite its type and name, the
.I d_off
field is seldom any kind of directory offset on modern filesystems.
.\" https://lwn.net/Articles/544298/
Applications should treat this field as an opaque value,
making no assumptions about its contents; see also
.BR telldir (3).
.TP
.I d_reclen
This is the size (in bytes) of the returned record.
This may not match the size of the structure definition shown above;
see NOTES.
.TP
.I d_type
This field contains a value indicating the file type,
making it possible to avoid the expense of calling
.BR lstat (2)
if further actions depend on the type of the file.
.IP
When a suitable feature test macro is defined
.RB ( _DEFAULT_SOURCE
on glibc versions since 2.19, or
.BR _BSD_SOURCE
on glibc versions 2.19 and earlier),
glibc defines the following macro constants for the value returned in
.IR d_type :
.RS
.TP 12
.B DT_BLK
This is a block device.
.TP
.B DT_CHR
This is a character device.
.TP
.B DT_DIR
This is a directory.
.TP
.B DT_FIFO
This is a named pipe (FIFO).
.TP
.B DT_LNK
This is a symbolic link.
.TP
.B DT_REG
This is a regular file.
.TP
.B DT_SOCK
This is a UNIX domain socket.
.TP
.B DT_UNKNOWN
The file type could not be determined.
.RE
.IP
Currently,
.\" kernel 2.6.27
.\" The same sentence is in getdents.2
only some filesystems (among them: Btrfs, ext2, ext3, and ext4)
have full support for returning the file type in
.IR d_type .
All applications must properly handle a return of
.BR DT_UNKNOWN .
.TP
.I d_name
This field contains the null terminated filename.
.IR "See NOTES" .
.PP
The data returned by
.BR readdir ()
may be overwritten by subsequent calls to
.BR readdir ()
for the same directory stream.
.SH RETURN VALUE
On success,
.BR readdir ()
returns a pointer to a
.I dirent
structure.
(This structure may be statically allocated; do not attempt to
.BR free (3)
it.)
.PP
If the end of the directory stream is reached, NULL is returned and
.I errno
is not changed.
If an error occurs, NULL is returned and
.I errno
is set appropriately.
To distinguish end of stream and from an error, set
.I errno
to zero before calling
.BR readdir ()
and then check the value of
.I errno
if NULL is returned.
.SH ERRORS
.TP
.B EBADF
Invalid directory stream descriptor \fIdirp\fP.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR readdir ()
T}	Thread safety	MT-Unsafe race:dirstream
.TE
.sp 1
.PP
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.
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 the deprecated
.BR readdir_r (3)
function.
It is expected that a future version of POSIX.1
.\" FIXME .
.\" http://www.austingroupbugs.net/view.php?id=696
will require that
.BR readdir ()
be thread-safe when concurrently employed on different directory streams.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.SH NOTES
A directory stream is opened using
.BR opendir (3).
.PP
The order in which filenames are read by successive calls to
.BR readdir ()
depends on the filesystem implementation;
it is unlikely that the names will be sorted in any fashion.
.PP
Only the fields
.I d_name
and (as an XSI extension)
.I d_ino
are specified in POSIX.1.
.\" POSIX.1-2001, POSIX.1-2008
Other than Linux, the
.I d_type
field is available mainly only on BSD systems.
The remaining fields are available on many, but not all systems.
Under glibc,
programs can check for the availability of the fields not defined
in POSIX.1 by testing whether the macros
.BR _DIRENT_HAVE_D_NAMLEN ,
.BR _DIRENT_HAVE_D_RECLEN ,
.BR _DIRENT_HAVE_D_OFF ,
or
.B _DIRENT_HAVE_D_TYPE
are defined.
.\"
.SS The d_name field
The
.I dirent
structure definition shown above is taken from the glibc headers,
and shows the
.I d_name
field with a fixed size.
.PP
.IR Warning :
applications should avoid any dependence on the size of the
.I d_name
field.
POSIX defines it as
.IR "char\ d_name[]",
a character array of unspecified size, with at most
.B NAME_MAX
characters preceding the terminating null byte (\(aq\\0\(aq).
.PP
POSIX.1 explicitly notes that this field should not be used as an lvalue.
The standard also notes that the use of
.I sizeof(d_name)
is incorrect; use
.IR strlen(d_name)
instead.
(On some systems, this field is defined as
.IR char\ d_name[1] !)
By implication, the use
.IR "sizeof(struct dirent)"
to capture the size of the record including the size of
.IR d_name
is also incorrect.
.PP
Note that while the call
.PP
    fpathconf(fd, _PC_NAME_MAX)
.PP
returns the value 255 for most filesystems,
on some filesystems (e.g., CIFS, Windows SMB servers),
the null-terminated filename that is (correctly) returned in
.I d_name
can actually exceed this size.
In such cases, the
.I d_reclen
field will contain a value that exceeds the size of the glibc
.I dirent
structure shown above.
.SH SEE ALSO
.BR getdents (2),
.BR read (2),
.BR closedir (3),
.BR dirfd (3),
.BR ftw (3),
.BR offsetof (3),
.BR opendir (3),
.BR readdir_r (3),
.BR rewinddir (3),
.BR scandir (3),
.BR seekdir (3),
.BR telldir (3)