readdir(3)
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 us 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 4.09 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/.
2016-03-15 READDIR(3)
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
