NFTW(3P)

💅 NFTW(3P) — walk a file tree 💅

okay so bestie, we're reading NFTW(3P) from the POSIX Programmer's Manual today, and honestly? she ate. 🦋 let's go.

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🌸 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.

like, for real — if you're on Linux, go check the Linux page too, because bestie might be acting a little different over there. 🙈

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💖 NAME 💖

nftw — walk a file tree. that's it, that's the whole girl. main character energy, no cap. ✨

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🎀 SYNOPSIS 🎀

#include <ftw.h>

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

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🌈 DESCRIPTION 🌈

okay so nftw() recursively descends the directory hierarchy rooted in path. she's basically ftw()'s cooler older sister, except she takes one extra argument: flags, which is a bitwise-inclusive OR of zero or more of these absolute icons:

• FTW_CHDIR — if set, nftw() shall change the current working directory to each directory as it reports files in that directory. if clear, nftw() shall not change the current working directory. (she stays put, respectfully)
• FTW_DEPTH — if set, nftw() shall report all files in a directory before reporting the directory itself. if clear, nftw() shall report any directory before reporting the files in that directory.
• FTW_MOUNT — if set, nftw() shall only report files in the same file system as path. if clear, nftw() shall report all files encountered during the walk. (no hopping the fence, bestie)
• FTW_PHYS — if set, nftw() shall perform a physical walk and shall not follow symbolic links.

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and now the combo lore, because this part is important:

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

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

1. The first argument is the pathname of the object.
2. The second argument is a pointer to the stat buffer containing information on the object, filled in as if fstatat(), stat(), or lstat() had been called to retrieve the information.
3. The third argument is an integer giving additional information. Its value is one of the following:
   • FTW_D — the object is a directory.
   • FTW_DNR — the object is a directory that cannot be read. The fn function shall not be called for any of its descendants.
   • FTW_DP — the object is a directory and subdirectories have been visited. (this condition shall only occur if the FTW_DEPTH flag is included in flags.)
   • FTW_F — the object is a non-directory file.
   • 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() shall return -1. (yes, undefined. be scared.)
   • FTW_SL — the object is a symbolic link. (this condition shall only occur if the FTW_PHYS flag is included in flags.)
   • FTW_SLN — the object is a symbolic link that does not name an existing file. (this condition shall only occur if the FTW_PHYS flag is not included in flags.)
4. The fourth argument is a pointer to an FTW structure. The value of base is the offset of the object's filename in the pathname passed as the first argument to fn. The value of level indicates depth relative to the root of the walk, where the root level is 0.

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⚠️ bestie listen. The results are unspecified if the application-supplied fn function does not preserve the current working directory. so like… don't do that. 💀

The argument fd_limit sets the maximum number of file descriptors that shall be used by nftw() while traversing the file tree. At most one file descriptor shall be used for each directory level. (budget queen 🎀)

The nftw() function need not be thread-safe. ✨

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⭐ RETURN VALUE ⭐

nftw() shall continue until the first of the following conditions occurs:

• An invocation of fn shall return a non-zero value, in which case nftw() shall return that value.
• The nftw() function detects an error other than [EACCES] (see FTW_DNR and FTW_NS above), in which case nftw() shall return -1 and set errno to indicate the error.
• The tree is exhausted, in which case nftw() shall return 0.

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🫶 ERRORS 🫶

The nftw() function shall 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.
• ELOOP — a loop exists in symbolic links encountered during resolution of the path argument.
• ENAMETOOLONG — the length of a component of a pathname 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 names an existing file that is neither a directory nor a symbolic link to 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 nftw() function may fail if:

• ELOOP — more than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path 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.

In addition, errno may be set if the function pointed to by fn causes errno to be set. (yes bestie, your callback can leak chaos into errno. she's messy like that. 💅)

The following sections are informative.

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🍓 EXAMPLES 🍓

The following program traverses the directory tree under the path named in its first command-line argument, or under the current directory if no argument is supplied. It displays various information about each file. The second command-line argument can be used to specify characters that control the value assigned to the flags argument when calling nftw().

#include <ftw.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

static int
display_info(const char *fpath, const struct stat *sb,
    int tflag, struct FTW *ftwbuf)
{
    printf("%-3s %2d %7jd   %-40s %d %s\n",
        (tflag == FTW_D) ?   "d"   : (tflag == FTW_DNR) ? "dnr" :
        (tflag == FTW_DP) ?  "dp"  : (tflag == FTW_F) ?
            (S_ISBLK(sb->st_mode) ? "f b" :
             S_ISCHR(sb->st_mode) ? "f c" :
             S_ISFIFO(sb->st_mode) ? "f p" :
             S_ISREG(sb->st_mode) ? "f r" :
             S_ISSOCK(sb->st_mode) ? "f s" : "f ?") :
        (tflag == FTW_NS) ?  "ns"  : (tflag == FTW_SL) ?  "sl" :
        (tflag == FTW_SLN) ? "sln" : "?",
        ftwbuf->level, (intmax_t) sb->st_size,
        fpath, ftwbuf->base, fpath + ftwbuf->base);
    return 0;           /* To tell nftw() to continue */
}

int
main(int argc, char *argv[])
{
    int flags = 0;

    if (argc > 2 && strchr(argv[2], 'd') != NULL)
        flags |= FTW_DEPTH;
    if (argc > 2 && strchr(argv[2], 'p') != NULL)
        flags |= FTW_PHYS;

    if (nftw((argc < 2) ? "." : argv[1], display_info, 20, flags) == -1)
    {
        perror("nftw");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}

look at her go. passing d in that second arg gets you FTW_DEPTH, p gets you FTW_PHYS, and 20 is the fd_limit. periodt.

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💜 APPLICATION USAGE 💜

The nftw() function may allocate dynamic storage during its operation. If nftw() is forcibly terminated, such as by longjmp() or siglongjmp() being executed by the function pointed to by fn or an interrupt routine, nftw() does not have a chance to free that storage, so it remains permanently allocated. 😭

a safe way to handle interrupts is to store the fact that an interrupt has occurred, and arrange to have the function pointed to by fn return a non-zero value at its next invocation. do that instead, bestie. your memory will thank you. 🫶

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🧚 RATIONALE 🧚

None. (she said nothing, your honor.)

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🌸 FUTURE DIRECTIONS 🌸

None. (and she plans to keep saying nothing.)

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🦋 SEE ALSO 🦋

• fdopendir()
• fstatat()
• readdir()

The Base Definitions volume of POSIX.1‐2017, <ftw.h> ⭐

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👑 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 .

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and that's the whole page, bestie!! 🎀 mwah. this one came to you from IEEE/The Open Group, 2017 — yeah, the NFTW(3P) of it all. stay recursive, stay slay. 💅✨🦋