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GETDENTS(2) Linux Programmer's Manual GETDENTS(2)
NAME
getdents, getdents64 - get directory entries
SYNOPSIS
#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, 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.
DESCRIPTION
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; /* Offset to next linux_dirent */
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 the distance from the start of the directory to the start of the next linux_dirent. 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 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 prop‐
erly 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; /* 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) */
};
RETURN VALUE
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.
ERRORS
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.
CONFORMING TO
SVr4.
NOTES
Library support for getdents64() was added in glibc 2.30; Glibc does not provide a wrapper for getdents(); call getdents() (or getdents64() on earlier glibc ver‐
sions) 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).
EXAMPLES
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 <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) \
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 ---------------\n", nread);
printf("inode# file type d_reclen d_off d_name\n");
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\n", d->d_reclen,
(intmax_t) d->d_off, d->d_name);
bpos += d->d_reclen;
}
}
exit(EXIT_SUCCESS);
}
SEE ALSO
readdir(2), readdir(3), inode(7)
Linux 2021-03-22 GETDENTS(2)