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CentOS 7.0 - man page for getdents (centos section 2)

GETDENTS(2)			    Linux Programmer's Manual			      GETDENTS(2)

NAME
       getdents - get directory entries

SYNOPSIS
       int getdents(unsigned int fd, struct linux_dirent *dirp,
		    unsigned int count);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       This is not the function you are interested in.	Look at readdir(3) for the POSIX conform-
       ing C library interface.  This page documents the bare kernel system call interface.

       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 argu-
       ment 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 previ-
       ously a zero-filled padding byte in the linux_dirent structure.	Thus, on  kernels  before
       2.6.3, attempting to access this field always provides the value 0 (DT_UNKNOWN).

       Currently, only some file systems (among them: Btrfs, ext2, ext3, and ext4) have full sup-
       port for returning the file type in d_type.   All  applications	must  properly	handle	a
       return of DT_UNKNOWN.

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 this system call; call it using syscall(2).  You will
       need  to  define  the  linux_dirent structure yourself.	However, you probably want to use
       readdir(3) instead.

       This call supersedes readdir(2).

       The original Linux getdents() system call did not handle large file systems and large file
       offsets.   Consequently,  Linux 2.4 added getdents64(), with wider types for the d_ino and
       d_off fields employed in the linux_dirent structure.

EXAMPLE
       The program below demonstrates the use of getdents().  The following output shows an exam-
       ple of what we see when running this program on an ext2 directory:

	   $ ./a.out /testfs/
	   --------------- nread=120 ---------------
	   i-node#  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("i-node#	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);
       }

SEE ALSO
       readdir(2), readdir(3)

COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2012-08-03				      GETDENTS(2)


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