# fdopendir

## 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



           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 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>
    
               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(3p)).  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.
    


* * *