/****************************************************************************
* fs/vfs/fs_pread.c
*
* Copyright (C) 2014 Gregory Nutt. All rights reserved.
* Author: Gregory Nutt <gnutt@nuttx.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. Neither the name NuttX nor the names of its contributors may be
* used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
****************************************************************************/
/****************************************************************************
* Included Files
****************************************************************************/
#include <nuttx/config.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <nuttx/fs/fs.h>
/****************************************************************************
* Private Functions
****************************************************************************/
/****************************************************************************
* Public Functions
****************************************************************************/
/****************************************************************************
* Name: file_pread
*
* Description:
* Equivalent to the standard pread function except that is accepts a
* struct file instance instead of a file descriptor. Currently used
* only by aio_read();
*
****************************************************************************/
ssize_t file_pread(FAR struct file *filep, FAR void *buf, size_t nbytes,
off_t offset)
{
off_t savepos;
off_t pos;
ssize_t ret;
int errcode;
/* Perform the seek to the current position. This will not move the
* file pointer, but will return its current setting
*/
savepos = file_seek(filep, 0, SEEK_CUR);
if (savepos == (off_t)-1)
{
/* file_seek might fail if this if the media is not seekable */
return ERROR;
}
/* Then seek to the correct position in the file */
pos = file_seek(filep, offset, SEEK_SET);
if (pos == (off_t)-1)
{
/* This might fail is the offset is beyond the end of file */
return ERROR;
}
/* Then perform the read operation */
ret = file_read(filep, buf, nbytes);
errcode = get_errno();
/* Restore the file position */
pos = file_seek(filep, savepos, SEEK_SET);
if (pos == (off_t)-1 && ret >= 0)
{
/* This really should not fail */
return ERROR;
}
set_errno(errcode);
return ret;
}
/****************************************************************************
* Name: pread
*
* Description:
* The pread() function performs the same action as read(), except that it
* reads from a given position in the file without changing the file
* pointer. The first three arguments to pread() are the same as read()
* with the addition of a fourth argument offset for the desired position
* inside the file. An attempt to perform a pread() on a file that is
* incapable of seeking results in an error.
*
* NOTE: This function could have been wholly implemented within libc but
* it is not. Why? Because if pread were implemented in libc, it would
* require four system calls. If it is implemented within the kernel,
* only three.
*
* Parameters:
* file File structure instance
* buf User-provided to save the data
* nbytes The maximum size of the user-provided buffer
* offset The file offset
*
* Return:
* The positive non-zero number of bytes read on success, 0 on if an
* end-of-file condition, or -1 on failure with errno set appropriately.
* See read() return values
*
****************************************************************************/
ssize_t pread(int fd, FAR void *buf, size_t nbytes, off_t offset)
{
FAR struct file *filep;
/* Get the file structure corresponding to the file descriptor. */
filep = fs_getfilep(fd);
if (!filep)
{
/* The errno value has already been set */
return (ssize_t)ERROR;
}
/* Let file_pread do the real work */
return file_pread(filep, buf, nbytes, offset);
}
