fts_read(3) — Linux manual page

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FTS(3)                    Linux Programmer's Manual                   FTS(3)

NAME         top

       fts,  fts_open, fts_read, fts_children, fts_set, fts_close - traverse
       a file hierarchy

SYNOPSIS         top

       #include <sys/types.h>
       #include <sys/stat.h>
       #include <fts.h>

       FTS *fts_open(char * const *path_argv, int options,
                     int (*compar)(const FTSENT **, const FTSENT **));

       FTSENT *fts_read(FTS *ftsp);

       FTSENT *fts_children(FTS *ftsp, int instr);

       int fts_set(FTS *ftsp, FTSENT *f, int instr);

       int fts_close(FTS *ftsp);

DESCRIPTION         top

       The fts functions are provided for traversing file hierarchies.  A
       simple overview is that the fts_open() function returns a "handle"
       (of type FTS *) that refers to a file hierarchy "stream".  This
       handle is then supplied to the other fts functions.  The function
       fts_read() returns a pointer to a structure describing one of the
       files in the file hierarchy.  The function fts_children() returns a
       pointer to a linked list of structures, each of which describes one
       of the files contained in a directory in the hierarchy.

       In general, directories are visited two distinguishable times; in
       preorder (before any of their descendants are visited) and in
       postorder (after all of their descendants have been visited).  Files
       are visited once.  It is possible to walk the hierarchy "logically"
       (visiting the files that symbolic links point to) or physically
       (visiting the symbolic links themselves), order the walk of the
       hierarchy or prune and/or revisit portions of the hierarchy.

       Two structures (and associated types) are defined in the include file
       <fts.h>.  The first type is FTS, the structure that represents the
       file hierarchy itself.  The second type is FTSENT, the structure that
       represents a file in the file hierarchy.  Normally, an FTSENT
       structure is returned for every file in the file hierarchy.  In this
       manual page, "file" and "FTSENT structure" are generally
       interchangeable.

       The FTSENT structure contains fields describing a file.  The
       structure contains at least the following fields (there are
       additional fields that should be considered private to the
       implementation):

           typedef struct _ftsent {
               unsigned short  fts_info;     /* flags for FTSENT structure */
               char           *fts_accpath;  /* access path */
               char           *fts_path;     /* root path */
               short           fts_pathlen;  /* strlen(fts_path) +
                                                strlen(fts_name) */
               char           *fts_name;     /* filename */
               short           fts_namelen;  /* strlen(fts_name) */
               short           fts_level;    /* depth (-1 to N) */
               int             fts_errno;    /* file errno */
               long            fts_number;   /* local numeric value */
               void           *fts_pointer;  /* local address value */
               struct _ftsent *fts_parent;   /* parent directory */
               struct _ftsent *fts_link;     /* next file structure */
               struct _ftsent *fts_cycle;    /* cycle structure */
               struct stat    *fts_statp;    /* stat(2) information */
           } FTSENT;

       These fields are defined as follows:

       fts_info
              One of the following values describing the returned FTSENT
              structure and the file it represents.  With the exception of
              directories without errors (FTS_D), all of these entries are
              terminal, that is, they will not be revisited, nor will any of
              their descendants be visited.

              FTS_D  A directory being visited in preorder.

              FTS_DC A directory that causes a cycle in the tree.  (The
                     fts_cycle field of the FTSENT structure will be filled
                     in as well.)

              FTS_DEFAULT
                     Any FTSENT structure that represents a file type not
                     explicitly described by one of the other fts_info val‐
                     ues.

              FTS_DNR
                     A directory which cannot be read.  This is an error
                     return, and the fts_errno field will be set to indicate
                     what caused the error.

              FTS_DOT
                     A file named "."  or ".."  which was not specified as a
                     filename to fts_open() (see FTS_SEEDOT).

              FTS_DP A directory being visited in postorder.  The contents
                     of the FTSENT structure will be unchanged from when it
                     was returned in preorder, that is, with the fts_info
                     field set to FTS_D.

              FTS_ERR
                     This is an error return, and the fts_errno field will
                     be set to indicate what caused the error.

              FTS_F  A regular file.

              FTS_NS A file for which no stat(2) information was available.
                     The contents of the fts_statp field are undefined.
                     This is an error return, and the fts_errno field will
                     be set to indicate what caused the error.

              FTS_NSOK
                     A file for which no stat(2) information was requested.
                     The contents of the fts_statp field are undefined.

              FTS_SL A symbolic link.

              FTS_SLNONE
                     A symbolic link with a nonexistent target.  The con‐
                     tents of the fts_statp field reference the file charac‐
                     teristic information for the symbolic link itself.

       fts_accpath
              A path for accessing the file from the current directory.

       fts_path
              The path for the file relative to the root of the traversal.
              This path contains the path specified to fts_open() as a pre‐
              fix.

       fts_pathlen
              The sum of the lengths of the strings referenced by fts_path
              and fts_name.

       fts_name
              The name of the file.

       fts_namelen
              The length of the string referenced by fts_name.

       fts_level
              The depth of the traversal, numbered from -1 to N, where this
              file was found.  The FTSENT structure representing the parent
              of the starting point (or root) of the traversal is numbered
              -1, and the FTSENT structure for the root itself is numbered
              0.

       fts_errno
              If fts_children() or fts_read() returns an FTSENT structure
              whose fts_info field is set to FTS_DNR, FTS_ERR, or FTS_NS,
              the fts_errno field contains the error number (i.e., the errno
              value) specifying the cause of the error.  Otherwise, the con‐
              tents of the fts_errno field are undefined.

       fts_number
              This field is provided for the use of the application program
              and is not modified by the fts functions.  It is initialized
              to 0.

       fts_pointer
              This field is provided for the use of the application program
              and is not modified by the fts functions.  It is initialized
              to NULL.

       fts_parent
              A pointer to the FTSENT structure referencing the file in the
              hierarchy immediately above the current file, that is, the
              directory of which this file is a member.  A parent structure
              for the initial entry point is provided as well, however, only
              the fts_level, fts_number, and fts_pointer fields are guaran‐
              teed to be initialized.

       fts_link
              Upon return from the fts_children() function, the fts_link
              field points to the next structure in the NULL-terminated
              linked list of directory members.  Otherwise, the contents of
              the fts_link field are undefined.

       fts_cycle
              If a directory causes a cycle in the hierarchy (see FTS_DC),
              either because of a hard link between two directories, or a
              symbolic link pointing to a directory, the fts_cycle field of
              the structure will point to the FTSENT structure in the hier‐
              archy that references the same file as the current FTSENT
              structure.  Otherwise, the contents of the fts_cycle field are
              undefined.

       fts_statp
              A pointer to stat(2) information for the file.

       A single buffer is used for all of the paths of all of the files in
       the file hierarchy.  Therefore, the fts_path and fts_accpath fields
       are guaranteed to be null-terminated only for the file most recently
       returned by fts_read().  To use these fields to reference any files
       represented by other FTSENT structures will require that the path
       buffer be modified using the information contained in that FTSENT
       structure's fts_pathlen field.  Any such modifications should be
       undone before further calls to fts_read() are attempted.  The
       fts_name field is always null-terminated.

   fts_open()
       The fts_open() function takes a pointer to an array of character
       pointers naming one or more paths which make up a logical file hier‐
       archy to be traversed.  The array must be terminated by a null
       pointer.

       There are a number of options, at least one of which (either FTS_LOG‐
       ICAL or FTS_PHYSICAL) must be specified.  The options are selected by
       ORing the following values:

       FTS_COMFOLLOW
              This option causes any symbolic link specified as a root path
              to be followed immediately whether or not FTS_LOGICAL is also
              specified.

       FTS_LOGICAL
              This option causes the fts routines to return FTSENT struc‐
              tures for the targets of symbolic links instead of the sym‐
              bolic links themselves.  If this option is set, the only sym‐
              bolic links for which FTSENT structures are returned to the
              application are those referencing nonexistent files.  Either
              FTS_LOGICAL or FTS_PHYSICAL must be provided to the fts_open()
              function.

       FTS_NOCHDIR
              As a performance optimization, the fts functions change direc‐
              tories as they walk the file hierarchy.  This has the side-
              effect that an application cannot rely on being in any partic‐
              ular directory during the traversal.  The FTS_NOCHDIR option
              turns off this optimization, and the fts functions will not
              change the current directory.  Note that applications should
              not themselves change their current directory and try to
              access files unless FTS_NOCHDIR is specified and absolute
              pathnames were provided as arguments to fts_open().

       FTS_NOSTAT
              By default, returned FTSENT structures reference file charac‐
              teristic information (the statp field) for each file visited.
              This option relaxes that requirement as a performance opti‐
              mization, allowing the fts functions to set the fts_info field
              to FTS_NSOK and leave the contents of the statp field unde‐
              fined.

       FTS_PHYSICAL
              This option causes the fts routines to return FTSENT struc‐
              tures for symbolic links themselves instead of the target
              files they point to.  If this option is set, FTSENT structures
              for all symbolic links in the hierarchy are returned to the
              application.  Either FTS_LOGICAL or FTS_PHYSICAL must be pro‐
              vided to the fts_open() function.

       FTS_SEEDOT
              By default, unless they are specified as path arguments to
              fts_open(), any files named "."  or ".."  encountered in the
              file hierarchy are ignored.  This option causes the fts rou‐
              tines to return FTSENT structures for them.

       FTS_XDEV
              This option prevents fts from descending into directories that
              have a different device number than the file from which the
              descent began.

       The argument compar() specifies a user-defined function which may be
       used to order the traversal of the hierarchy.  It takes two pointers
       to pointers to FTSENT structures as arguments and should return a
       negative value, zero, or a positive value to indicate if the file
       referenced by its first argument comes before, in any order with
       respect to, or after, the file referenced by its second argument.
       The fts_accpath, fts_path, and fts_pathlen fields of the FTSENT
       structures may never be used in this comparison.  If the fts_info
       field is set to FTS_NS or FTS_NSOK, the fts_statp field may not
       either.  If the compar() argument is NULL, the directory traversal
       order is in the order listed in path_argv for the root paths, and in
       the order listed in the directory for everything else.

   fts_read()
       The fts_read() function returns a pointer to an FTSENT structure
       describing a file in the hierarchy.  Directories (that are readable
       and do not cause cycles) are visited at least twice, once in preorder
       and once in postorder.  All other files are visited at least once.
       (Hard links between directories that do not cause cycles or symbolic
       links to symbolic links may cause files to be visited more than once,
       or directories more than twice.)

       If all the members of the hierarchy have been returned, fts_read()
       returns NULL and sets the external variable errno to 0.  If an error
       unrelated to a file in the hierarchy occurs, fts_read() returns NULL
       and sets errno appropriately.  If an error related to a returned file
       occurs, a pointer to an FTSENT structure is returned, and errno may
       or may not have been set (see fts_info).

       The FTSENT structures returned by fts_read() may be overwritten after
       a call to fts_close() on the same file hierarchy stream, or, after a
       call to fts_read() on the same file hierarchy stream unless they rep‐
       resent a file of type directory, in which case they will not be over‐
       written until after a call to fts_read() after the FTSENT structure
       has been returned by the function fts_read() in postorder.

   fts_children()
       The fts_children() function returns a pointer to an FTSENT structure
       describing the first entry in a NULL-terminated linked list of the
       files in the directory represented by the FTSENT structure most
       recently returned by fts_read().  The list is linked through the
       fts_link field of the FTSENT structure, and is ordered by the user-
       specified comparison function, if any.  Repeated calls to fts_chil‐
       dren() will re-create this linked list.

       As a special case, if fts_read() has not yet been called for a hier‐
       archy, fts_children() will return a pointer to the files in the logi‐
       cal directory specified to fts_open(), that is, the arguments speci‐
       fied to fts_open().  Otherwise, if the FTSENT structure most recently
       returned by fts_read() is not a directory being visited in preorder,
       or the directory does not contain any files, fts_children() returns
       NULL and sets errno to zero.  If an error occurs, fts_children()
       returns NULL and sets errno appropriately.

       The FTSENT structures returned by fts_children() may be overwritten
       after a call to fts_children(), fts_close(), or fts_read() on the
       same file hierarchy stream.

       The instr argument is either zero or the following value:

       FTS_NAMEONLY
              Only the names of the files are needed.  The contents of all
              the fields in the returned linked list of structures are unde‐
              fined with the exception of the fts_name and fts_namelen
              fields.

   fts_set()
       The function fts_set() allows the user application to determine fur‐
       ther processing for the file f of the stream ftsp.  The fts_set()
       function returns 0 on success, and -1 if an error occurs.

       The instr argument is either 0 (meaning "do nothing") or one of the
       following values:

       FTS_AGAIN
              Revisit the file; any file type may be revisited.  The next
              call to fts_read() will return the referenced file.  The
              fts_stat and fts_info fields of the structure will be reini‐
              tialized at that time, but no other fields will have been
              changed.  This option is meaningful only for the most recently
              returned file from fts_read().  Normal use is for postorder
              directory visits, where it causes the directory to be revis‐
              ited (in both preorder and postorder) as well as all of its
              descendants.

       FTS_FOLLOW
              The referenced file must be a symbolic link.  If the refer‐
              enced file is the one most recently returned by fts_read(),
              the next call to fts_read() returns the file with the fts_info
              and fts_statp fields reinitialized to reflect the target of
              the symbolic link instead of the symbolic link itself.  If the
              file is one of those most recently returned by fts_children(),
              the fts_info and fts_statp fields of the structure, when
              returned by fts_read(), will reflect the target of the sym‐
              bolic link instead of the symbolic link itself.  In either
              case, if the target of the symbolic link does not exist, the
              fields of the returned structure will be unchanged and the
              fts_info field will be set to FTS_SLNONE.

              If the target of the link is a directory, the preorder return,
              followed by the return of all of its descendants, followed by
              a postorder return, is done.

       FTS_SKIP
              No descendants of this file are visited.  The file may be one
              of those most recently returned by either fts_children() or
              fts_read().

   fts_close()
       The fts_close() function closes the file hierarchy stream referred to
       by ftsp and restores the current directory to the directory from
       which fts_open() was called to open ftsp.  The fts_close() function
       returns 0 on success, and -1 if an error occurs.

ERRORS         top

       The function fts_open() may fail and set errno for any of the errors
       specified for open(2) and malloc(3).

       The function fts_close() may fail and set errno for any of the errors
       specified for chdir(2) and close(2).

       The functions fts_read() and fts_children() may fail and set errno
       for any of the errors specified for chdir(2), malloc(3), opendir(3),
       readdir(3), and stat(2).

       In addition, fts_children(), fts_open(), and fts_set() may fail and
       set errno as follows:

       EINVAL options or instr was invalid.

VERSIONS         top

       These functions are available in Linux since glibc2.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────────────────────────┬───────────────┬───────────┐
       │Interface                          Attribute     Value     │
       ├───────────────────────────────────┼───────────────┼───────────┤
       │fts_open(), fts_set(), fts_close() │ Thread safety │ MT-Safe   │
       ├───────────────────────────────────┼───────────────┼───────────┤
       │fts_read(), fts_children()         │ Thread safety │ MT-Unsafe │
       └───────────────────────────────────┴───────────────┴───────────┘

CONFORMING TO         top

       4.4BSD.

BUGS         top

       In versions of glibc before 2.23, all of the APIs described in this
       man page are not safe when compiling a program using the LFS APIs
       (e.g., when compiling with -D_FILE_OFFSET_BITS=64).

SEE ALSO         top

       find(1), chdir(2), stat(2), ftw(3), qsort(3)

COLOPHON         top

       This page is part of release 5.07 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/.

Linux                            2020-04-11                           FTS(3)