130 lines
4.5 KiB
C
130 lines
4.5 KiB
C
/****************************************************************************
|
|
* 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>
|
|
|
|
/****************************************************************************
|
|
* Private Functions
|
|
****************************************************************************/
|
|
|
|
/****************************************************************************
|
|
* Public Functions
|
|
****************************************************************************/
|
|
|
|
/****************************************************************************
|
|
* 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)
|
|
{
|
|
off_t savepos;
|
|
off_t pos;
|
|
ssize_t ret;
|
|
int errcode;
|
|
|
|
/* Perform the lseek to the current position. This will not move the
|
|
* file pointer, but will return its current setting
|
|
*/
|
|
|
|
savepos = lseek(fd, 0, SEEK_CUR);
|
|
if (savepos == (off_t)-1)
|
|
{
|
|
/* lseek might fail if this if the media is not seekable */
|
|
|
|
return ERROR;
|
|
}
|
|
|
|
/* Then seek to the correct position in the file */
|
|
|
|
pos = lseek(fd, 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 = read(fd, buf, nbytes);
|
|
errcode = get_errno();
|
|
|
|
/* Restore the file position */
|
|
|
|
pos = lseek(fd, savepos, SEEK_SET);
|
|
if (pos == (off_t)-1 && ret >= 0)
|
|
{
|
|
/* This really should not fail */
|
|
|
|
return ERROR;
|
|
}
|
|
|
|
set_errno(errcode);
|
|
return ret;
|
|
}
|