fdopendir man page on Archlinux

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

       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.

       fdopendir, opendir — open directory associated with file descriptor

       #include <dirent.h>

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

       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().

       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.

       The fdopendir() function shall fail if:

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

	      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.

	      The  length  of  a  component  of	 a  pathname  is  longer  than

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

	      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

	      The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
	      tion 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.

   Open a 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 a 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>

	   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");
	       while ((dp = readdir(d)) != NULL) {
		   if (dp->d_name[0] == '.')
		   /* 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) {
		   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));
	       closedir(d); // note this implicitly closes dfd
	       return 0;

       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.

       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.1‐2008 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.1‐2008 does not provide a way to test a priori  for  their	 exis‐
       tence  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 implemen‐
       tations 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 opera‐
       tions are defined by the implementation, this  volume  of  POSIX.1‐2008
       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 direc‐
       tory stream has been closed or after a fork() or one of the exec	 func‐
       tion calls.


       closedir(),  dirfd(),  fstatat(),  open(), readdir(), rewinddir(), sym‐

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

       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 Electri‐
       cal 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.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			 FDOPENDIR(3P)

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