man-pages/man2/getdents.2

.\" Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright 2008, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Written 11 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 22 July 1995 by Michael Chastain <mec@duracef.shout.net>:
.\"   Derived from 'readdir.2'.
.\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\"
.TH GETDENTS 2  2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
getdents, getdents64 \- get directory entries
.SH SYNOPSIS
.nf
.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
.B #include <unistd.h>
.PP
.BI "long syscall(SYS_getdents, unsigned int " fd \
", struct linux_dirent *" dirp ,
.BI "             unsigned int " count );
.PP
.BR "#define _GNU_SOURCE" "           /* See feature_test_macros(7) */"
.BR "#include <dirent.h>"
.PP
.BI "ssize_t getdents64(int " fd ", void *" dirp ", size_t " count );
.fi
.PP
.IR Note :
glibc provides no wrapper for
.BR getdents (),
necessitating the use of
.BR syscall (2).
.PP
.IR Note :
There is no definition of
.I struct linux_dirent
in glibc; see NOTES.
.SH DESCRIPTION
These are not the interfaces you are interested in.
Look at
.BR readdir (3)
for the POSIX-conforming C library interface.
This page documents the bare kernel system call interfaces.
.SS getdents()
The system call
.BR getdents ()
reads several
.I linux_dirent
structures from the directory
referred to by the open file descriptor
.I fd
into the buffer pointed to by
.IR dirp .
The argument
.I count
specifies the size of that buffer.
.PP
The
.I linux_dirent
structure is declared as follows:
.PP
.in +4n
.EX
struct linux_dirent {
    unsigned long  d_ino;     /* Inode number */
    unsigned long  d_off;     /* Offset to next \fIlinux_dirent\fP */
    unsigned short d_reclen;  /* Length of this \fIlinux_dirent\fP */
    char           d_name[];  /* Filename (null\-terminated) */
                      /* length is actually (d_reclen \- 2 \-
                         offsetof(struct linux_dirent, d_name)) */
    /*
    char           pad;       // Zero padding byte
    char           d_type;    // File type (only since Linux
                              // 2.6.4); offset is (d_reclen \- 1)
    */
}
.EE
.in
.PP
.I d_ino
is an inode number.
.I d_off
is the distance from the start of the directory to the start of the next
.IR linux_dirent .
.I d_reclen
is the size of this entire
.IR linux_dirent .
.I d_name
is a null-terminated filename.
.PP
.I d_type
is a byte at the end of the structure that indicates the file type.
It contains one of the following values (defined in
.IR <dirent.h> ):
.TP 12
.B DT_BLK
This is a block device.
.TP
.B DT_CHR
This is a character device.
.TP
.B DT_DIR
This is a directory.
.TP
.B DT_FIFO
This is a named pipe (FIFO).
.TP
.B DT_LNK
This is a symbolic link.
.TP
.B DT_REG
This is a regular file.
.TP
.B DT_SOCK
This is a UNIX domain socket.
.TP
.B DT_UNKNOWN
The file type is unknown.
.PP
The
.I d_type
field is implemented since Linux 2.6.4.
It occupies a space that was previously a zero-filled padding byte in the
.IR linux_dirent
structure.
Thus, on kernels up to and including 2.6.3,
attempting to access this field always provides the value 0
.RB ( DT_UNKNOWN ).
.PP
Currently,
.\" kernel 2.6.27
.\" The same sentence is in readdir.2
only some filesystems (among them: Btrfs, ext2, ext3, and ext4)
have full support for returning the file type in
.IR d_type .
All applications must properly handle a return of
.BR DT_UNKNOWN .
.SS getdents64()
The original Linux
.BR getdents ()
system call did not handle large filesystems and large file offsets.
Consequently, Linux 2.4 added
.BR getdents64 (),
with wider types for the
.I d_ino
and
.I d_off
fields.
In addition,
.BR getdents64 ()
supports an explicit
.I d_type
field.
.PP
The
.BR getdents64 ()
system call is like
.BR getdents (),
except that its second argument is a pointer to a buffer containing
structures of the following type:
.PP
.in +4n
.EX
struct linux_dirent64 {
    ino64_t        d_ino;    /* 64\-bit inode number */
    off64_t        d_off;    /* 64\-bit offset to next structure */
    unsigned short d_reclen; /* Size of this dirent */
    unsigned char  d_type;   /* File type */
    char           d_name[]; /* Filename (null\-terminated) */
};
.EE
.in
.SH RETURN VALUE
On success, the number of bytes read is returned.
On end of directory, 0 is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
Invalid file descriptor
.IR fd .
.TP
.B EFAULT
Argument points outside the calling process's address space.
.TP
.B EINVAL
Result buffer is too small.
.TP
.B ENOENT
No such directory.
.TP
.B ENOTDIR
File descriptor does not refer to a directory.
.SH CONFORMING TO
SVr4.
.\" SVr4 documents additional ENOLINK, EIO error conditions.
.SH NOTES
Library support for
.BR getdents64 ()
was added in glibc 2.30;
Glibc does not provide a wrapper for
.BR getdents ();
call
.BR getdents ()
(or
.BR getdents64 ()
on earlier glibc versions) using
.BR syscall (2).
In that case you will need to define the
.I linux_dirent
or
.I linux_dirent64
structure yourself.
.PP
Probably, you want to use
.BR readdir (3)
instead of these system calls.
.PP
These calls supersede
.BR readdir (2).
.SH EXAMPLES
.\" FIXME The example program needs to be revised, since it uses the older
.\" getdents() system call and the structure with smaller field widths.
The program below demonstrates the use of
.BR getdents ().
The following output shows an example of what we see when running this
program on an ext2 directory:
.PP
.in +4n
.EX
.RB "$" " ./a.out /testfs/"
-\-\-\-\-\-\-\-\-\-\-\-\-\-\- nread=120 \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-
inode#    file type  d_reclen  d_off   d_name
       2  directory    16         12  .
       2  directory    16         24  ..
      11  directory    24         44  lost+found
      12  regular      16         56  a
  228929  directory    16         68  sub
   16353  directory    16         80  sub2
  130817  directory    16       4096  sub3
.EE
.in
.SS Program source
\&
.EX
#define _GNU_SOURCE
#include <dirent.h>     /* Defines DT_* constants */
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>

#define handle_error(msg) \e
        do { perror(msg); exit(EXIT_FAILURE); } while (0)

struct linux_dirent {
    unsigned long  d_ino;
    off_t          d_off;
    unsigned short d_reclen;
    char           d_name[];
};

#define BUF_SIZE 1024

int
main(int argc, char *argv[])
{
    int fd;
    long nread;
    char buf[BUF_SIZE];
    struct linux_dirent *d;
    char d_type;

    fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
    if (fd == \-1)
        handle_error("open");

    for (;;) {
        nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
        if (nread == \-1)
            handle_error("getdents");

        if (nread == 0)
            break;

        printf("\-\-\-\-\-\-\-\-\-\-\-\-\-\-\- nread=%d \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\en", nread);
        printf("inode#    file type  d_reclen  d_off   d_name\en");
        for (long bpos = 0; bpos < nread;) {
            d = (struct linux_dirent *) (buf + bpos);
            printf("%8ld  ", d\->d_ino);
            d_type = *(buf + bpos + d\->d_reclen \- 1);
            printf("%\-10s ", (d_type == DT_REG) ?  "regular" :
                             (d_type == DT_DIR) ?  "directory" :
                             (d_type == DT_FIFO) ? "FIFO" :
                             (d_type == DT_SOCK) ? "socket" :
                             (d_type == DT_LNK) ?  "symlink" :
                             (d_type == DT_BLK) ?  "block dev" :
                             (d_type == DT_CHR) ?  "char dev" : "???");
            printf("%4d %10jd  %s\en", d\->d_reclen,
                    (intmax_t) d\->d_off, d\->d_name);
            bpos += d\->d_reclen;
        }
    }

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR readdir (2),
.BR readdir (3),
.BR inode (7)