path: root/man-pages-posix-2017/man3p/readdir.3p

'\" et
.TH READDIR "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
readdir,
readdir_r
\(em read a directory
.SH SYNOPSIS
.LP
.nf
#include <dirent.h>
.P
struct dirent *readdir(DIR *\fIdirp\fP);
int readdir_r(DIR *restrict \fIdirp\fP, struct dirent *restrict \fIentry\fP,
    struct dirent **restrict \fIresult\fP);
.fi
.SH DESCRIPTION
The type
.BR DIR ,
which is defined in the
.IR <dirent.h> 
header, represents a
.IR "directory stream" ,
which is an ordered sequence of all the directory entries in a
particular directory. Directory entries represent files; files may be
removed from a directory or added to a directory asynchronously to the
operation of
\fIreaddir\fR().
.P
The
\fIreaddir\fR()
function shall return a pointer to a structure representing the
directory entry at the current position in the directory stream
specified by the argument
.IR dirp ,
and position the directory stream at the next entry. It shall return a
null pointer upon reaching the end of the directory stream. The
structure
.BR dirent
defined in the
.IR <dirent.h> 
header describes a directory entry. The value of the structure's
.IR d_ino
member shall be set to the file serial number of the file named by the
.IR d_name
member. If the
.IR d_name
member names a symbolic link, the value of the
.IR d_ino
member shall be set to the file serial number of the symbolic link itself.
.P
The
\fIreaddir\fR()
function shall not return directory entries containing empty names. If
entries for dot or dot-dot exist, one entry shall be returned for dot
and one entry shall be returned for dot-dot; otherwise, they shall not
be returned.
.P
The application shall not modify the structure to which the return
value of
\fIreaddir\fR()
points, nor any storage areas pointed to by pointers within the
structure. The returned pointer, and pointers within the structure,
might be invalidated or the structure or the storage areas might be
overwritten by a subsequent call to
\fIreaddir\fR()
on the same directory stream. They shall not be affected by a call to
\fIreaddir\fR()
on a different directory stream. The returned pointer, and pointers
within the structure, might also be invalidated if the calling thread
is terminated.
.P
If a file is removed from or added to the directory after the most
recent call to
\fIopendir\fR()
or
\fIrewinddir\fR(),
whether a subsequent call to
\fIreaddir\fR()
returns an entry for that file is unspecified.
.P
The
\fIreaddir\fR()
function may buffer several directory entries per actual read
operation;
\fIreaddir\fR()
shall mark for update the last data access timestamp
of the directory each time the directory is actually read.
.P
After a call to
\fIfork\fR(),
either the parent or child (but not both) may continue processing the
directory stream using
\fIreaddir\fR(),
\fIrewinddir\fR(),
or
\fIseekdir\fR().
If both the parent and child processes use these functions, the result
is undefined.
.P
The
\fIreaddir\fR()
function need not be thread-safe.
.P
Applications wishing to check for error situations should set
.IR errno
to 0 before calling
\fIreaddir\fR().
If
.IR errno
is set to non-zero on return, an error occurred.
.P
The
\fIreaddir_r\fR()
function shall initialize the
.BR dirent
structure referenced by
.IR entry
to represent the directory entry at the current position in the
directory stream referred to by
.IR dirp ,
store a pointer to this structure at the location referenced by
.IR result ,
and position the directory stream at the next entry.
.P
The storage pointed to by
.IR entry
shall be large enough for a
.BR dirent
with an array of
.BR char
.IR d_name
members containing at least
{NAME_MAX}+1
elements.
.P
Upon successful return, the pointer returned at *\fIresult\fP shall have
the same value as the argument
.IR entry .
Upon reaching the end of the directory stream, this pointer shall
have the value NULL.
.P
The
\fIreaddir_r\fR()
function shall not return directory entries containing empty names.
.P
If a file is removed from or added to the directory after the most
recent call to
\fIopendir\fR()
or
\fIrewinddir\fR(),
whether a subsequent call to
\fIreaddir_r\fR()
returns an entry for that file is unspecified.
.P
The
\fIreaddir_r\fR()
function may buffer several directory entries per actual read
operation;
\fIreaddir_r\fR()
shall mark for update the last data access timestamp of the directory
each time the directory is actually read.
.SH "RETURN VALUE"
Upon successful completion,
\fIreaddir\fR()
shall return a pointer to an object of type
.BR "struct dirent" .
When an error is encountered, a null pointer shall be returned and
.IR errno
shall be set to indicate the error. When the end of the directory is
encountered, a null pointer shall be returned and
.IR errno
is not changed.
.P
If successful, the
\fIreaddir_r\fR()
function shall return zero; otherwise, an error number shall be
returned to indicate the error.
.SH ERRORS
These functions shall fail if:
.TP
.BR EOVERFLOW
One of the values in the structure to be returned cannot be represented
correctly.
.P
These functions may fail if:
.TP
.BR EBADF
The
.IR dirp
argument does not refer to an open directory stream.
.TP
.BR ENOENT
The current position of the directory stream is invalid.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
The following sample program searches the current directory for
each of the arguments supplied on the command line.
.sp
.RS 4
.nf

#include <dirent.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
.P
static void lookup(const char *arg)
{
    DIR *dirp;
    struct dirent *dp;
.P
    if ((dirp = opendir(".")) == NULL) {
        perror("couldn\(aqt open \(aq.\(aq");
        return;
    }
.P
    do {
        errno = 0;
        if ((dp = readdir(dirp)) != NULL) {
            if (strcmp(dp->d_name, arg) != 0)
                continue;
.P
            (void) printf("found %s\en", arg);
            (void) closedir(dirp);
                return;
.P
        }
    } while (dp != NULL);
.P
    if (errno != 0)
        perror("error reading directory");
    else
        (void) printf("failed to find %s\en", arg);
    (void) closedir(dirp);
    return;
}
.P
int main(int argc, char *argv[])
{
    int i;
    for (i = 1; i < argc; i++)
        lookup(argv[i]);
    return (0);
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
\fIreaddir\fR()
function should be used in conjunction with
\fIopendir\fR(),
\fIclosedir\fR(),
and
\fIrewinddir\fR()
to examine the contents of the directory.
.P
The
\fIreaddir_r\fR()
function is thread-safe and shall return values in a user-supplied
buffer instead of possibly using a static data area that may be
overwritten by each call.
.SH RATIONALE
The returned value of
\fIreaddir\fR()
merely \fIrepresents\fP a directory entry. No equivalence should be
inferred.
.P
Historical implementations of
\fIreaddir\fR()
obtain multiple directory entries on a single read operation, which
permits subsequent
\fIreaddir\fR()
operations to operate from the buffered information. Any wording that
required each successful
\fIreaddir\fR()
operation to mark the directory last data access timestamp
for update would disallow such historical performance-oriented
implementations.
.P
When returning a directory entry for the root of a mounted file system,
some historical implementations of
\fIreaddir\fR()
returned the file serial number of the underlying mount point, rather
than of the root of the mounted file system. This behavior is considered
to be a bug, since the underlying file serial number has no significance
to applications.
.P
Since
\fIreaddir\fR()
returns NULL
when it detects an error and when the end of the directory is
encountered, an application that needs to tell the difference must set
.IR errno
to zero before the call and check it if NULL is returned.
Since the function must not change
.IR errno
in the second case and must set it to a non-zero value in the first
case, a zero
.IR errno
after a call returning NULL indicates end-of-directory; otherwise, an
error.
.P
Routines to deal with this problem more directly were proposed:
.sp
.RS 4
.nf

int derror (\fIdirp\fP)
DIR *\fIdirp\fP;
.P
void clearderr (\fIdirp\fP)
DIR *\fIdirp\fP;
.fi
.P
.RE
.P
The first would indicate whether an error had occurred, and the second
would clear the error indication. The simpler method involving
.IR errno
was adopted instead by requiring that
\fIreaddir\fR()
not change
.IR errno
when end-of-directory is encountered.
.P
An error or signal indicating that a directory has changed while open
was considered but rejected.
.P
The thread-safe version of the directory reading function returns
values in a user-supplied buffer instead of possibly using a static
data area that may be overwritten by each call. Either the
{NAME_MAX}
compile-time constant or the corresponding
\fIpathconf\fR()
option can be used to determine the maximum sizes of returned
pathnames.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIclosedir\fR\^(\|)",
.IR "\fIdirfd\fR\^(\|)",
.IR "\fIexec\fR\^",
.IR "\fIfdopendir\fR\^(\|)",
.IR "\fIfstatat\fR\^(\|)",
.IR "\fIrewinddir\fR\^(\|)",
.IR "\fIsymlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<dirent.h>\fP",
.IR "\fB<sys_types.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .