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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       fdopendir, opendir -- open directory associated with file descriptor

SYNOPSIS
       #include <dirent.h>

       DIR *fdopendir(int fd);
       DIR *opendir(const char *dirname);

DESCRIPTION
       The fdopendir() function shall be equivalent to the opendir() function
       except that the directory is specified by a file descriptor rather than
       by a name. The file offset associated with the file descriptor at the
       time of the call determines which entries are returned.

       Upon successful return from fdopendir(), the file descriptor is under
       the control of the system, and if any attempt is made to close the file
       descriptor, or to modify the state of the associated description, other
       than by means of closedir(), readdir(), readdir___r(), rewinddir(), or
       seekdir(), the behavior is undefined. Upon calling closedir() the file
       descriptor shall be closed.

       It is unspecified whether the FD_CLOEXEC flag will be set on the file
       descriptor by a successful call to fdopendir().

       The opendir() function shall open a directory stream corresponding to
       the directory named by the dirname argument. The directory stream is
       positioned at the first entry. If the type DIR is implemented using a
       file descriptor, applications shall only be able to open up to a total
       of {OPEN_MAX} files and directories.

       If the type DIR is implemented using a file descriptor, the descriptor
       shall be obtained as if the O_DIRECTORY flag was passed to open().

RETURN VALUE
       Upon successful completion, these functions shall return a pointer to
       an object of type DIR.  Otherwise, these functions shall return a null
       pointer and set errno to indicate the error.

ERRORS
       The fdopendir() function shall fail if:

       EBADF  The fd argument is not a valid file descriptor open for reading.

       ENOTDIR
              The descriptor fd is not associated with a directory.

       The opendir() function shall fail if:

       EACCES Search permission is denied for the component of the path prefix
              of dirname or read permission is denied for dirname.

       ELOOP  A loop exists in symbolic links encountered during resolution of
              the dirname argument.

       ENAMETOOLONG
              The length of a component of a pathname is longer than
              {NAME_MAX}.

       ENOENT A component of dirname does not name an existing directory or
              dirname is an empty string.

       ENOTDIR
              A component of dirname names an existing file that is neither a
              directory nor a symbolic link to a directory.

       The opendir() function may fail if:

       ELOOP  More than {SYMLOOP_MAX} symbolic links were encountered during
              resolution of the dirname argument.

       EMFILE All file descriptors available to the process are currently
              open.

       ENAMETOOLONG
              The length of a pathname exceeds {PATH_MAX}, or pathname
              resolution of a symbolic link produced an intermediate result
              with a length that exceeds {PATH_MAX}.

       ENFILE Too many files are currently open in the system.

       The following sections are informative.

EXAMPLES
   Open aa Directory Stream
       The following program fragment demonstrates how the opendir() function
       is used.


           #include <dirent.h>
           ...
               DIR *dir;
               struct dirent *dp;
           ...
               if ((dir = opendir (".")) == NULL) {
                   perror ("Cannot open .");
                   exit (1);
               }

               while ((dp = readdir (dir)) != NULL) {
           ...

   Find And Open aa File
       The following program searches through a given directory looking for
       files whose name does not begin with a dot and whose size is larger
       than 1 MiB.


           #include <stdio.h>
           #include <dirent.h>
           #include <fcntl.h>
           #include <sys/stat.h>
           #include <stdint.h>
           #include <stdlib.h>
           #include <unistd.h>

           int
           main(int argc, char *argv[])
           {
               struct stat statbuf;
               DIR *d;
               struct dirent *dp;
               int dfd, ffd;

               if ((d = fdopendir((dfd = open("./tmp", O_RDONLY)))) == NULL) {
                   fprintf(stderr, "Cannot open ./tmp directory\n");
                   exit(1);
               }
               while ((dp = readdir(d)) != NULL) {
                   if (dp->d_name[0] == '.')
                       continue;
                   /* there is a possible race condition here as the file
                    * could be renamed between the readdir and the open */
                   if ((ffd = openat(dfd, dp->d_name, O_RDONLY)) == -1) {
                       perror(dp->d_name);
                       continue;
                   }
                   if (fstat(ffd, &statbuf) == 0 && statbuf.st_size > (1024*1024)) {
                       /* found it ... */
                       printf("%s: %jdK\n", dp->d_name,
                           (intmax_t)(statbuf.st_size / 1024));
                   }
                   close(ffd);
               }
               closedir(d); // note this implicitly closes dfd
               return 0;
           }

APPLICATION USAGE
       The opendir() function should be used in conjunction with readdir(),
       closedir(), and rewinddir() to examine the contents of the directory
       (see the EXAMPLES section in readdir()).  This method is recommended
       for portability.

RATIONALE
       The purpose of the fdopendir() function is to enable opening files in
       directories other than the current working directory without exposure
       to race conditions. Any part of the path of a file could be changed in
       parallel to a call to opendir(), resulting in unspecified behavior.

       Based on historical implementations, the rules about file descriptors
       apply to directory streams as well. However, this volume of
       POSIX.1-2017 does not mandate that the directory stream be implemented
       using file descriptors. The description of closedir() clarifies that if
       a file descriptor is used for the directory stream, it is mandatory
       that closedir() deallocate the file descriptor. When a file descriptor
       is used to implement the directory stream, it behaves as if the
       FD_CLOEXEC had been set for the file descriptor.

       The directory entries for dot and dot-dot are optional. This volume of
       POSIX.1-2017 does not provide a way to test _a priori for their
       existence because an application that is portable must be written to
       look for (and usually ignore) those entries. Writing code that presumes
       that they are the first two entries does not always work, as many
       implementations permit them to be other than the first two entries,
       with a ``normal'' entry preceding them. There is negligible value in
       providing a way to determine what the implementation does because the
       code to deal with dot and dot-dot must be written in any case and
       because such a flag would add to the list of those flags (which has
       proven in itself to be objectionable) and might be abused.

       Since the structure and buffer allocation, if any, for directory
       operations are defined by the implementation, this volume of
       POSIX.1-2017 imposes no portability requirements for erroneous program
       constructs, erroneous data, or the use of unspecified values such as
       the use or referencing of a dirp value or a dirent structure value
       after a directory stream has been closed or after a fork() or one of
       the exec function calls.

FUTURE DIRECTIONS
       None.

SEE ALSO
       closedir(), dirfd(), fstatat(), open(), readdir(), rewinddir(),
       symlink()

       The Base Definitions volume of POSIX.1-2017, <dirent.h>, <sys_types.h>

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 .

       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                  2017                         FDOPENDIR(3)
