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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getdents(2) System Calls Manual getdents(2)
getdents, getdents64 - get directory entries
Standard C library (libc, -lc)
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
long syscall(SYS_getdents, unsigned int fd, struct linux_dirent *dirp,
unsigned int count);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <dirent.h>
ssize_t getdents64(int fd, void dirp[.count], size_t count);
Note: glibc provides no wrapper for getdents(), necessitating the
use of syscall(2).
Note: There is no definition of struct linux_dirent in glibc; see
NOTES.
These are not the interfaces you are interested in. Look at
readdir(3) for the POSIX-conforming C library interface. This
page documents the bare kernel system call interfaces.
getdents()
The system call getdents() reads several linux_dirent structures
from the directory referred to by the open file descriptor fd
into the buffer pointed to by dirp. The argument count specifies
the size of that buffer.
The linux_dirent structure is declared as follows:
struct linux_dirent {
unsigned long d_ino; /* Inode number */
unsigned long d_off; /* Not an offset; see below */
unsigned short d_reclen; /* Length of this linux_dirent */
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)
*/
}
d_ino is an inode number. d_off is a filesystem-specific value
with no specific meaning to user space, though on older
filesystems it used to be the distance from the start of the
directory to the start of the next linux_dirent; see readdir(3).
d_reclen is the size of this entire linux_dirent. d_name is a
null-terminated filename.
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
<dirent.h>):
DT_BLK This is a block device.
DT_CHR This is a character device.
DT_DIR This is a directory.
DT_FIFO
This is a named pipe (FIFO).
DT_LNK This is a symbolic link.
DT_REG This is a regular file.
DT_SOCK
This is a UNIX domain socket.
DT_UNKNOWN
The file type is unknown.
The d_type field is implemented since Linux 2.6.4. It occupies a
space that was previously a zero-filled padding byte in the
linux_dirent structure. Thus, on kernels up to and including
Linux 2.6.3, attempting to access this field always provides the
value 0 (DT_UNKNOWN).
Currently, only some filesystems (among them: Btrfs, ext2, ext3,
and ext4) have full support for returning the file type in
d_type. All applications must properly handle a return of
DT_UNKNOWN.
getdents64()
The original Linux getdents() system call did not handle large
filesystems and large file offsets. Consequently, Linux 2.4
added getdents64(), with wider types for the d_ino and d_off
fields. In addition, getdents64() supports an explicit d_type
field.
The getdents64() system call is like getdents(), except that its
second argument is a pointer to a buffer containing structures of
the following type:
struct linux_dirent64 {
ino64_t d_ino; /* 64-bit inode number */
off64_t d_off; /* Not an offset; see getdents() */
unsigned short d_reclen; /* Size of this dirent */
unsigned char d_type; /* File type */
char d_name[]; /* Filename (null-terminated) */
};
On success, the number of bytes read is returned. On end of
directory, 0 is returned. On error, -1 is returned, and errno is
set to indicate the error.
EBADF Invalid file descriptor fd.
EFAULT Argument points outside the calling process's address
space.
EINVAL Result buffer is too small.
ENOENT No such directory.
ENOTDIR
File descriptor does not refer to a directory.
None.
SVr4.
getdents64()
glibc 2.30.
glibc does not provide a wrapper for getdents(); call getdents()
using syscall(2). In that case you will need to define the
linux_dirent or linux_dirent64 structure yourself.
Probably, you want to use readdir(3) instead of these system
calls.
These calls supersede readdir(2).
The program below demonstrates the use of getdents(). The
following output shows an example of what we see when running
this program on an ext2 directory:
$ ./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
Program source
#define _GNU_SOURCE
#include <dirent.h> /* Defines DT_* constants */
#include <err.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
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;
char d_type;
char buf[BUF_SIZE];
long nread;
struct linux_dirent *d;
fd = open(argc > 1 ? argv[1] : ".", O_RDONLY | O_DIRECTORY);
if (fd == -1)
err(EXIT_FAILURE, "open");
for (;;) {
nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
if (nread == -1)
err(EXIT_FAILURE, "getdents");
if (nread == 0)
break;
printf("--------------- nread=%ld ---------------\n", nread);
printf("inode# file type d_reclen d_off d_name\n");
for (size_t bpos = 0; bpos < nread;) {
d = (struct linux_dirent *) (buf + bpos);
printf("%8lu ", 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\n", d->d_reclen,
(intmax_t) d->d_off, d->d_name);
bpos += d->d_reclen;
}
}
exit(EXIT_SUCCESS);
}
readdir(2), readdir(3), inode(7)
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Linux man-pages 6.9.1 2024-06-15 getdents(2)
Pages that refer to this page: readdir(2), syscalls(2), readdir(3), proc(5), proc_pid_task(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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