readdir

READDIR(3POSIX)             POSIX Programmer's Manual            READDIR(3POSIX)



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.


NAME
       readdir, readdir_r — read a directory

SYNOPSIS
       #include <dirent.h>

       struct dirent *readdir(DIR *dirp);
       int readdir_r(DIR *restrict dirp, struct dirent *restrict entry,
           struct dirent **restrict result);

DESCRIPTION
       The type DIR, which is defined in the <dirent.h> header, represents a
       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 readdir().

       The readdir() function shall return a pointer to a structure representing
       the directory entry at the current position in the directory stream
       specified by the argument 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 dirent defined in the <dirent.h> header
       describes a directory entry. The value of the structure's d_ino member
       shall be set to the file serial number of the file named by the d_name
       member. If the d_name member names a symbolic link, the value of the
       d_ino member shall be set to the file serial number of the symbolic link
       itself.

       The readdir() 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.

       The application shall not modify the structure to which the return value
       of readdir() 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 readdir() on the same directory
       stream. They shall not be affected by a call to readdir() on a different
       directory stream.

       If a file is removed from or added to the directory after the most recent
       call to opendir() or rewinddir(), whether a subsequent call to readdir()
       returns an entry for that file is unspecified.

       The readdir() function may buffer several directory entries per actual
       read operation; readdir() shall mark for update the last data access
       timestamp of the directory each time the directory is actually read.

       After a call to fork(), either the parent or child (but not both) may
       continue processing the directory stream using readdir(), rewinddir(), or
       seekdir().  If both the parent and child processes use these functions,
       the result is undefined.

       The readdir() function need not be thread-safe.

       Applications wishing to check for error situations should set errno to 0
       before calling readdir().  If errno is set to non-zero on return, an
       error occurred.

       The readdir_r() function shall initialize the dirent structure referenced
       by entry to represent the directory entry at the current position in the
       directory stream referred to by dirp, store a pointer to this structure
       at the location referenced by result, and position the directory stream
       at the next entry.

       The storage pointed to by entry shall be large enough for a dirent with
       an array of char d_name members containing at least {NAME_MAX}+1
       elements.

       Upon successful return, the pointer returned at *result shall have the
       same value as the argument entry.  Upon reaching the end of the directory
       stream, this pointer shall have the value NULL.

       The readdir_r() function shall not return directory entries containing
       empty names.

       If a file is removed from or added to the directory after the most recent
       call to opendir() or rewinddir(), whether a subsequent call to
       readdir_r() returns an entry for that file is unspecified.

       The readdir_r() function may buffer several directory entries per actual
       read operation; readdir_r() shall mark for update the last data access
       timestamp of the directory each time the directory is actually read.

RETURN VALUE
       Upon successful completion, readdir() shall return a pointer to an object
       of type struct dirent.  When an error is encountered, a null pointer
       shall be returned and errno shall be set to indicate the error. When the
       end of the directory is encountered, a null pointer shall be returned and
       errno is not changed.

       If successful, the readdir_r() function shall return zero; otherwise, an
       error number shall be returned to indicate the error.

ERRORS
       These functions shall fail if:

       EOVERFLOW
              One of the values in the structure to be returned cannot be
              represented correctly.

       These functions may fail if:

       EBADF  The dirp argument does not refer to an open directory stream.

       ENOENT The current position of the directory stream is invalid.

       The following sections are informative.

EXAMPLES
       The following sample program searches the current directory for each of
       the arguments supplied on the command line.

           #include <dirent.h>
           #include <errno.h>
           #include <stdio.h>
           #include <string.h>

           static void lookup(const char *arg)
           {
               DIR *dirp;
               struct dirent *dp;

               if ((dirp = opendir(".")) == NULL) {
                   perror("couldn't open '.'");
                   return;
               }

               do {
                   errno = 0;
                   if ((dp = readdir(dirp)) != NULL) {
                       if (strcmp(dp->d_name, arg) != 0)
                           continue;

                       (void) printf("found %s\n", arg);
                       (void) closedir(dirp);
                           return;

                   }
               } while (dp != NULL);

               if (errno != 0)
                   perror("error reading directory");
               else
                   (void) printf("failed to find %s\n", arg);
               (void) closedir(dirp);
               return;
           }

           int main(int argc, char *argv[])
           {
               int i;
               for (i = 1; i < argc; i++)
                   lookup(argv[i]);
               return (0);
           }

APPLICATION USAGE
       The readdir() function should be used in conjunction with opendir(),
       closedir(), and rewinddir() to examine the contents of the directory.

       The readdir_r() 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.

RATIONALE
       The returned value of readdir() merely represents a directory entry. No
       equivalence should be inferred.

       Historical implementations of readdir() obtain multiple directory entries
       on a single read operation, which permits subsequent readdir() operations
       to operate from the buffered information. Any wording that required each
       successful readdir() operation to mark the directory last data access
       timestamp for update would disallow such historical performance-oriented
       implementations.

       When returning a directory entry for the root of a mounted file system,
       some historical implementations of readdir() 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.

       Since readdir() 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 errno to zero before the call and check it if NULL is
       returned.  Since the function must not change errno in the second case
       and must set it to a non-zero value in the first case, a zero errno after
       a call returning NULL indicates end-of-directory; otherwise, an error.

       Routines to deal with this problem more directly were proposed:

           int derror (dirp)
           DIR *dirp;

           void clearderr (dirp)
           DIR *dirp;

       The first would indicate whether an error had occurred, and the second
       would clear the error indication. The simpler method involving errno was
       adopted instead by requiring that readdir() not change errno when end-of-
       directory is encountered.

       An error or signal indicating that a directory has changed while open was
       considered but rejected.

       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 pathconf() option can be used to determine
       the maximum sizes of returned pathnames.

FUTURE DIRECTIONS
       None.

SEE ALSO
       closedir(), dirfd(), exec, fdopendir(), fstatat(), rewinddir(), symlink()

       The Base Definitions volume of POSIX.1‐2008, <dirent.h>, <sys_types.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical
       and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008
       with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .

       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 .



IEEE/The Open Group                   2013                       READDIR(3POSIX)