[MAN] readdir

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READDIR

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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NAME

readdir - read directory entry  

SYNOPSIS

int readdir(unsigned int fd, struct old_linux_dirent *dirp,
            unsigned int count);

Note: There is no glibc wrapper for this system call; see NOTES.  

DESCRIPTION

This is not the function you are interested in. Look at readdir(3) for the POSIX conforming C library interface. This page documents the bare kernel system call interface, which is superseded by getdents(2).

readdir() reads one old_linux_dirent structure from the directory referred to by the file descriptor fd into the buffer pointed to by dirp. The argument count is ignored; at most one old_linux_dirent structure is read.

The old_linux_dirent structure is declared as follows:

struct old_linux_dirent {
    long  d_ino;              /* inode number */
    off_t d_off;              /* offset to this old_linux_dirent */
    unsigned short d_reclen;  /* length of this d_name */
    char  d_name[NAME_MAX+1]; /* filename (null-terminated) */ }

d_ino is an inode number. d_off is the distance from the start of the directory to this old_linux_dirent. d_reclen is the size of d_name, not counting the terminating null byte ('\0'). d_name is a null-terminated filename.  

RETURN VALUE

On success, 1 is returned. On end of directory, 0 is returned. On error, -1 is returned, and errno is set appropriately.  

ERRORS

EBADF

Invalid file descriptor fd.

EFAULT

Argument points outside the calling process's address space.

EINVAL

Result buffer is too small.

ENOENT

No such directory.

ENOTDIR

File descriptor does not refer to a directory.

 

CONFORMING TO

This system call is Linux-specific.  

NOTES

Glibc does not provide a wrapper for this system call; call it using syscall(2). You will need to define the old_linux_dirent structure yourself. However, probably you should use readdir(3) instead.

This system call does not exist on x86-64.  

SEE ALSO

getdents(2), readdir(3)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

CONFORMING TO

NOTES

SEE ALSO

COLOPHON

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READDIR

Section: GNU Awk Extension Modules (3am)
Updated: Jan 15 2013
Index Return to Main Contents

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NAME

readdir - directory input parser for gawk  

SYNOPSIS

@load "readdir"  

DESCRIPTION

The readdir extension adds an input parser for directories.

When this extension is in use, instead of skipping directories named on the command line (or with getline), they are read, with each entry returned as a record.

The record consists of three fields. The first two are the inode number and the filename, separated by a forward slash character. On systems where the directory entry contains the file type, the record has a third field which is a single letter indicating the type of the file: f for file, d for directory, b for a block device, c for a character device, p for a FIFO, l for a symbolic link, s for a socket, and u (unknown) for anything else.

On systems without the file type information, the third field is always u.  

NOTES

On GNU/Linux systems, there are filesystems that don't support the d_type entry (see readdir(3)), and so the file type is always u. You can use the filefuncs extension to call stat() in order to get correct type information.  

EXAMPLE

@load "readdir"
...
BEGIN { FS = "/" }
{ print "file name is", $2 }

 

SEE ALSO

GAWK: Effective AWK Programming, filefuncs(3am), fnmatch(3am), fork(3am), inplace(3am), ordchr(3am), readfile(3am), revoutput(3am), rwarray(3am), time(3am).

opendir(3), readdir(3), stat(2).  

AUTHOR

Arnold Robbins, arnold@skeeve.com.  

COPYING PERMISSIONS

Copyright © 2012, 2013, Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual page under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual page into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation.

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Index

NAME

SYNOPSIS

DESCRIPTION

NOTES

EXAMPLE

SEE ALSO

AUTHOR

COPYING PERMISSIONS

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READDIR

Section: Linux Programmer's Manual (3)
Updated: 2017-09-15
Index Return to Main Contents

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NAME

readdir - read a directory  

SYNOPSIS

#include <dirent.h>

struct dirent *readdir(DIR *dirp);

 

DESCRIPTION

The readdir() function returns a pointer to a dirent structure representing the next directory entry in the directory stream pointed to by dirp. It returns NULL on reaching the end of the directory stream or if an error occurred.

In the glibc implementation, the dirent structure is defined as follows:

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 */ };

The only fields in the dirent structure that are mandated by POSIX.1 are d_name and d_ino. The other fields are unstandardized, and not present on all systems; see NOTES below for some further details.

The fields of the dirent structure are as follows:

d_ino

This is the inode number of the file.

d_off

The value returned in d_off is the same as would be returned by calling telldir(3) at the current position in the directory stream. Be aware that despite its type and name, the d_off field is seldom any kind of directory offset on modern filesystems. Applications should treat this field as an opaque value, making no assumptions about its contents; see also telldir(3).

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.

d_type

This field contains a value indicating the file type, making it possible to avoid the expense of calling lstat(2) if further actions depend on the type of the file.

When a suitable feature test macro is defined (_DEFAULT_SOURCE on glibc versions since 2.19, or _BSD_SOURCE on glibc versions 2.19 and earlier), glibc defines the following macro constants for the value returned in d_type:

DT_BLK

This is a block device.

DT_CHR

This is a character device.

DT_DIR

This is a directory.

DT_FIFO

This is a named pipe (FIFO).

DT_LNK

This is a symbolic link.

DT_REG

This is a regular file.

DT_SOCK

This is a UNIX domain socket.

DT_UNKNOWN

The file type could not be determined.

Currently, only some filesystems (among them: Btrfs, ext2, ext3, and ext4) have full support for returning the file type in d_type. All applications must properly handle a return of DT_UNKNOWN.

d_name

This field contains the null terminated filename. See NOTES.

The data returned by readdir() may be overwritten by subsequent calls to readdir() for the same directory stream.  

RETURN VALUE

On success, readdir() returns a pointer to a dirent structure. (This structure may be statically allocated; do not attempt to free(3) it.)

If the end of the directory stream is reached, NULL is returned and errno is not changed. If an error occurs, NULL is returned and errno is set appropriately. To distinguish end of stream and from an error, set errno to zero before calling readdir() and then check the value of errno if NULL is returned.  

ERRORS

EBADF

Invalid directory stream descriptor dirp.

 

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
readdir()	Thread safety	MT-Unsafe race:dirstream

In the current POSIX.1 specification (POSIX.1-2008), readdir() is not required to be thread-safe. However, in modern implementations (including the glibc implementation), concurrent calls to readdir() that specify different directory streams are thread-safe. In cases where multiple threads must read from the same directory stream, using readdir() with external synchronization is still preferable to the use of the deprecated readdir_r(3) function. It is expected that a future version of POSIX.1 will require that readdir() be thread-safe when concurrently employed on different directory streams.  

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  

NOTES

A directory stream is opened using opendir(3).

The order in which filenames are read by successive calls to readdir() depends on the filesystem implementation; it is unlikely that the names will be sorted in any fashion.

Only the fields d_name and (as an XSI extension) d_ino are specified in POSIX.1. Other than Linux, the 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 _DIRENT_HAVE_D_NAMLEN, _DIRENT_HAVE_D_RECLEN, _DIRENT_HAVE_D_OFF, or _DIRENT_HAVE_D_TYPE are defined.  

The d_name field

The dirent structure definition shown above is taken from the glibc headers, and shows the d_name field with a fixed size.

Warning: applications should avoid any dependence on the size of the d_name field. POSIX defines it as char d_name[], a character array of unspecified size, with at most NAME_MAX characters preceding the terminating null byte ('\0').

POSIX.1 explicitly notes that this field should not be used as an lvalue. The standard also notes that the use of sizeof(d_name) is incorrect; use strlen(d_name) instead. (On some systems, this field is defined as char d_name[1]!) By implication, the use sizeof(struct dirent) to capture the size of the record including the size of d_name is also incorrect.

Note that while the call

    fpathconf(fd, _PC_NAME_MAX)

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 d_name can actually exceed this size. In such cases, the d_reclen field will contain a value that exceeds the size of the glibc dirent structure shown above.  

SEE ALSO

getdents(2), read(2), closedir(3), dirfd(3), ftw(3), offsetof(3), opendir(3), readdir_r(3), rewinddir(3), scandir(3), seekdir(3), telldir(3)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

NOTES

The d_name field

SEE ALSO

COLOPHON

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