getdents (2)
Leading comments
Copyright (C) 1995 Andries Brouwer (aeb@cwi.nl) and Copyright 2008, 2015 Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under th...
NAME
getdents, getdents64 - get directory entriesSYNOPSIS
int getdents(unsigned int fd, struct linux_dirent *dirp,
unsigned int count);
int getdents64(unsigned int fd, struct linux_dirent64 *dirp,
unsigned int count);
Note: There are no glibc wrappers for these system calls; 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 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; /* 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 appropriately.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
Glibc does not provide a wrapper for these system calls; call them using syscall(2). You will need to define the linux_dirent or linux_dirent64 structure yourself. However, you probably want to use readdir(3) instead.These calls supersede readdir(2).
EXAMPLE
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 <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 { 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, nread; char buf[BUF_SIZE]; struct linux_dirent *d; int bpos; 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 (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 %10lld %s\n", d->d_reclen, (long long) d->d_off, d->d_name); bpos += d->d_reclen; } } exit(EXIT_SUCCESS); }