getdents.c

它通过提供glibc兼容使得应用程序移植到较小的c 库时相当得容易. 它能够应用到带虚拟存储的Linux和uClinux上.在大多数带MMU部件的平台上为使它更加紧凑,它也能够编译成共享库.uClib

/* Copyright (C) 1993, 1995-2002 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

#include <alloca.h>
#include <assert.h>
#include <errno.h>
#include <dirent.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>
#include <sys/param.h>
#include <sys/types.h>
#include <sysdep.h>
#include <sys/syscall.h>


#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

struct kernel_dirent
{
    long		d_ino;
    __kernel_off_t	d_off;
    unsigned short	d_reclen;
    char		d_name[256];
};

#define __NR___syscall_getdents __NR_getdents
static inline _syscall3(int, __syscall_getdents, int, fd, unsigned char *, kdirp, size_t, count);


ssize_t __getdents (int fd, char *buf, size_t nbytes)
{
    struct dirent *dp;
    off_t last_offset = -1;
    ssize_t retval;
    size_t red_nbytes;
    struct kernel_dirent *skdp, *kdp;
    const size_t size_diff = (offsetof (struct dirent, d_name)
	    - offsetof (struct kernel_dirent, d_name));

    red_nbytes = MIN (nbytes - ((nbytes / 
		    (offsetof (struct dirent, d_name) + 14)) * size_diff), 
	    nbytes - size_diff);

    dp = (struct dirent *) buf;
    skdp = kdp = alloca (red_nbytes);

    retval = __syscall_getdents(fd, (char *)kdp, red_nbytes);
    if (retval == -1)
	return -1;

    while ((char *) kdp < (char *) skdp + retval) {
	const size_t alignment = __alignof__ (struct dirent);
	/* Since kdp->d_reclen is already aligned for the kernel structure
	   this may compute a value that is bigger than necessary.  */
	size_t new_reclen = ((kdp->d_reclen + size_diff + alignment - 1)
		& ~(alignment - 1));
	if ((char *) dp + new_reclen > buf + nbytes) {
	    /* Our heuristic failed.  We read too many entries.  Reset
	       the stream.  */
	    assert (last_offset != -1);
	    lseek(fd, last_offset, SEEK_SET);

	    if ((char *) dp == buf) {
		/* The buffer the user passed in is too small to hold even
		   one entry.  */
		__set_errno (EINVAL);
		return -1;
	    }
	    break;
	}

	last_offset = kdp->d_off;
	dp->d_ino = kdp->d_ino;
	dp->d_off = kdp->d_off;
	dp->d_reclen = new_reclen;
	dp->d_type = DT_UNKNOWN;
	memcpy (dp->d_name, kdp->d_name,
		kdp->d_reclen - offsetof (struct kernel_dirent, d_name));
	dp = (struct dirent *) ((char *) dp + new_reclen);
	kdp = (struct kernel_dirent *) (((char *) kdp) + kdp->d_reclen);
    }
    return (char *) dp - buf;
}