getauxval.3: wfix

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

.\" Copyright (C) 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
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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.
.\" 2008-09-11, mtk, Document readdir_r().
.\"
.TH READDIR 3  2015-08-08 "" "Linux Programmer's Manual"
.SH NAME
readdir, readdir_r \- read a directory
.SH SYNOPSIS
.nf
.B #include <dirent.h>
.sp
.BI "struct dirent *readdir(DIR *" dirp );
.sp
.BI "int readdir_r(DIR *" dirp ", struct dirent *" entry \
", struct dirent **" result );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.ad l
.in
.sp
.BR readdir_r ():
.RS 4
_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
_SVID_SOURCE || _POSIX_SOURCE
.RE
.ad b
.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
On Linux, the
.I dirent
structure is defined as follows:
.PP
.in +4n
.nf
struct dirent {
    ino_t          d_ino;       /* inode number */
    off_t          d_off;       /* not an offset; see NOTES */
    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]; /* filename */
};
.fi
.in
.PP
The only fields in the
.I dirent
structure that are mandated by POSIX.1 are:
.IR d_name [],
of unspecified size, with at most
.B NAME_MAX
characters preceding the terminating null byte (\(aq\\0\(aq);
and (as an XSI extension)
.IR d_ino .
The other fields are unstandardized, and not present on all systems;
see NOTES below for some further details.
.PP
The data returned by
.BR readdir ()
may be overwritten by subsequent calls to
.BR readdir ()
for the same directory stream.

The
.BR readdir_r ()
function is a reentrant version of
.BR readdir ().
It reads the next directory entry from the directory stream
.IR dirp ,
and returns it in the caller-allocated buffer pointed to by
.IR entry .
(See NOTES for information on allocating this buffer.)
A pointer to the returned item is placed in
.IR *result ;
if the end of the directory stream was encountered,
then NULL is instead returned in
.IR *result .
.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.)
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.

The
.BR readdir_r ()
function returns 0 on success.
On error, it returns a positive error number (listed under ERRORS).
If the end of the directory stream is reached,
.BR readdir_r ()
returns 0, and returns NULL in
.IR *result .
.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
T{
.BR readdir_r ()
T}	Thread safety	MT-Safe
.TE
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
.SH NOTES
Only the fields
.I d_name
and
.I d_ino
are specified in POSIX.1.
.\" POSIX.1-2001, POSIX.1-2008
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.

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).

Other than Linux, the
.I d_type
field is available mainly only on BSD systems.
This field makes it possible to avoid the expense of calling
.BR lstat (2)
if further actions depend on the type of the file.
If the
.B _BSD_SOURCE
feature test macro is defined,
then glibc defines the following macro constants
for the value returned in
.IR d_type :
.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 is unknown.
.\" The glibc manual says that on some systems this is the only
.\" value returned
.PP
If the file type could not be determined, the value
.B DT_UNKNOWN
is returned in
.IR d_type .

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 .

Since POSIX.1 does not specify the size of the
.I d_name
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
.IR entry
as follows:
.in +4n
.nf

name_max = pathconf(dirpath, _PC_NAME_MAX);
if (name_max == \-1)         /* Limit not defined, or error */
    name_max = 255;         /* Take a guess */
len = offsetof(struct dirent, d_name) + name_max + 1;
entryp = malloc(len);

.fi
.in
(POSIX.1 requires that
.I d_name
is the last field in a
.IR "struct dirent" .)
.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 rewinddir (3),
.BR scandir (3),
.BR seekdir (3),
.BR telldir (3)