man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

nftw(3C)

Name

ftw, nftw - walk a file tree

Synopsis

#include <ftw.h>

int ftw(const char *path, int (*fn) (const char *,
     const struct stat *, int), int depth);
int nftw(const char *path, int (*fn) (const char *,
     const struct stat *, int, struct FTW *), int depth,
     int flags);

Description

The ftw() function recursively descends the directory hierarchy rooted in path. For each object in the hierarchy, ftw() calls the user-defined function fn, passing it a pointer to a null-terminated character string containing the name of the object, a pointer to a stat structure (see stat(2)) containing information about the object, and an integer. Possible values of the integer, defined in the <ftw.h> header, are:

FTW_F

The object is a file.

FTW_D

The object is a directory.

FTW_DNR

The object is a directory that cannot be read. Descendants of the directory are not processed.

FTW_NS

The stat() function failed on the object because of lack of appropriate permission or the object is a symbolic link that points to a non-existent file. The stat buffer passed to fn is undefined.

The ftw() function visits a directory before visiting any of its descendants.

The tree traversal continues until the tree is exhausted, an invocation of fn returns a non-zero value, or some error is detected within ftw() (such as an I/O error). If the tree is exhausted, ftw() returns 0. If fn returns a non-zero value, ftw() stops its tree traversal and returns whatever value was returned by fn.

The nftw() function is similar to ftw() except that it takes the additional argument flags, which is a bitwise-inclusive OR of zero or more of the following flags:

FTW_CHDIR

If set, nftw() changes the current working directory to each directory as it reports files in that directory. If clear, nftw() does not change the current working directory.

FTW_DEPTH

If set, nftw() reports all files in a directory before reporting the directory itself. If clear, nftw() reports any directory before reporting the files in that directory.

FTW_MOUNT

If set, nftw() reports only files in the same file system as path. If clear, nftw() reports all files encountered during the walk.

FTW_PHYS

If set, nftw() performs a physical walk and does not follow symbolic links.

If FTW_PHYS is clear and FTW_DEPTH is set, nftw() follows links instead of reporting them, but does not report any directory that would be a descendant of itself. If FTW_PHYS is clear and FTW_DEPTH is clear, nftw() follows links instead of reporting them, but does not report the contents of any directory that would be a descendant of itself.

At each file it encounters, nftw() calls the user-supplied function fn with four arguments:

  • The first argument is the pathname of the object.

  • The second argument is a pointer to the stat buffer containing information on the object.

  • The third argument is an integer giving additional information. Its value is one of the following:

    FTW_F

    The object is a file.

    FTW_D

    The object is a directory.

    FTW_DP

    The object is a directory and subdirectories have been visited. (This condition only occurs if the FTW_DEPTH flag is included in flags.)

    FTW_SL

    The object is a symbolic link. (This condition only occurs if the FTW_PHYS flag is included in flags.)

    FTW_SLN

    The object is a symbolic link that points to a non-existent file. (This condition only occurs if the FTW_PHYS flag is not included in flags.)

    FTW_DNR

    The object is a directory that cannot be read. The user-defined function fn will not be called for any of its descendants.

    FTW_NS

    The stat() function failed on the object because of lack of appropriate permission. The stat buffer passed to fn is undefined. Failure of stat() for any other reason is considered an error and nftw() returns −1.

  • The fourth argument is a pointer to an FTW structure that contains the following members:

    int   base;
    int   level;

    The base member is the offset of the object's filename in the pathname passed as the first argument to fn(). The value of level indicates the depth relative to the root of the walk, where the root level is 0.

    The results are unspecified if the application-supplied fn() function does not preserve the current working directory.

Both ftw() and nftw() use one file descriptor for each level in the tree. The depth argument limits the number of file descriptors used. If depth is zero or negative, the effect is the same as if it were 1. It must not be greater than the number of file descriptors currently available for use. The ftw() function runs faster if depth is at least as large as the number of levels in the tree. Both ftw() and nftw() are able to descend to arbitrary depths in a file hierarchy and do not fail due to path length limitations unless either the length of the path name pointed to by the path argument exceeds {PATH_MAX} requirements, or for ftw(), the specified depth is less than 2, or for nftw(), the specified depth is less than 2 and FTW_CHDIR is not set. When ftw() and nftw() return, they close any file descriptors they have opened; they do not close any file descriptors that might have been opened by fn.

Return Values

If the tree is exhausted, ftw() and nftw() return 0. If the function pointed to by fn returns a non-zero value, ftw() and nftw() stop their tree traversal and return whatever value was returned by the function pointed to by fn. If ftw() and nftw() detect an error, they return −1 and set errno to indicate the error.

If ftw() and nftw() encounter an error other than EACCES (see FTW_DNR and FTW_NS above), they return −1 and set errno to indicate the error. The external variable errno can contain any error value that is possible when a directory is opened or when one of the stat functions is executed on a directory or file.

Errors

The ftw() and nftw() functions will fail if:

ELOOP

A loop exists in symbolic links encountered during resolution of the path argument

ENAMETOOLONG

The length of the path name pointed to by the path argument exceeds {PATH_MAX}, or a path name component is longer than {NAME_MAX}.

ENOENT

A component of path does not name an existing file or path is an empty string.

ENOTDIR

A component of path is not a directory.

EOVERFLOW

A field in the stat structure cannot be represented correctly in the current programming environment for one or more files found in the file hierarchy.

The ftw() function will fail if:

EACCES

Search permission is denied for any component of path or read permission is denied for path.

ENAMETOOLONG

The ftw() function has descended to a path that exceeds {PATH_MAX} and the depth argument specified by the application is less than 2 and FTW_CHDIR is not set.

The nftw() function will fail if:

EACCES

Search permission is denied for any component of path or read permission is denied for path, or fn() returns −1 and does not reset errno.

The nftw() and ftw() functions may fail if:

ELOOP

Too many symbolic links were encountered during resolution of the path argument.

ENAMETOOLONG

Pathname resolution of a symbolic link in the path name pointed to by the path argument produced an intermediate result whose length exceeds {PATH_MAX}.

The ftw() function may fail if:

EINVAL

The value of the depth argument is invalid.

The nftw() function may fail if:

EMFILE

There are {OPEN_MAX} file descriptors currently open in the calling process.

ENFILE

Too many files are currently open in the system.

If the function pointed to by fn encounters system errors, errno may be set accordingly.

Examples

Example 1 Walk a directory structure using ftw().

The following example walks the current directory structure, calling the fn() function for every directory entry, using at most 10 file descriptors:

#include <ftw.h>
...
if (ftw(".", fn, 10) != 0) {
       perror("ftw"); exit(2);
}
Example 2 Walk a directory structure using nftw().

The following example walks the /tmp directory and its subdirectories, calling the nftw() function for every directory entry, to a maximum of 5 levels deep.

#include <ftw.h>  
...
int nftwfunc(const char *, const struct stat *, int, struct FTW *);
int nftwfunc(const char *filename, const struct stat *statptr,
      int fileflags, struct FTW *pfwt)
{
      return 0;
}
...
char *startpath = "/tmp";
int depth = 5;
int flags = FTW_CHDIR | FTW_DEPTH | FTW_MOUNT;
int ret;
ret = nftw(startpath, nftwfunc, depth, flags);

Usage

Because ftw() and nftw() are recursive, they can terminate with a memory fault when applied by a thread with a small stack to very deep file structures.

The ftw() and nftw() functions allocate resources (memory, file descriptors) during their operation. If ftw() they are forcibly terminated, such as by longjmp(3C) being executed by fn or an interrupt routine, they will not have a chance to free those resources, so they remain permanently allocated. A safe way to handle interrupts is to store the fact that an interrupt has occurred and arrange to have fn return a non-zero value at its next invocation.

The ftw() and nftw() functions have transitional interfaces for 64-bit file offsets. See lf64(5).

The ftw() function is safe in multithreaded applications. The nftw() function is safe in multithreaded applications when the FTW_CHDIR flag is not set.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe with exceptions
Standard

See Also

stat(2), longjmp(3C), attributes(5), lf64(5), standards(5)