158 lines
4.6 KiB
C
158 lines
4.6 KiB
C
/****************************************************************************
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* fs/vfs/fs_pread.c
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*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership. The
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* ASF licenses this file to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance with the
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* License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*
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****************************************************************************/
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/****************************************************************************
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* Included Files
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****************************************************************************/
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#include <nuttx/config.h>
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#include <sys/types.h>
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#include <unistd.h>
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#include <assert.h>
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#include <errno.h>
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#include <nuttx/cancelpt.h>
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#include <nuttx/fs/fs.h>
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/****************************************************************************
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* Public Functions
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****************************************************************************/
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/****************************************************************************
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* Name: file_pread
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*
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* Description:
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* Equivalent to the standard pread function except that is accepts a
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* struct file instance instead of a file descriptor. Currently used
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* only by aio_read();
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*
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****************************************************************************/
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ssize_t file_pread(FAR struct file *filep, FAR void *buf, size_t nbytes,
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off_t offset)
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{
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off_t savepos;
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off_t pos;
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ssize_t ret;
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/* Perform the seek to the current position. This will not move the
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* file pointer, but will return its current setting
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*/
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savepos = file_seek(filep, 0, SEEK_CUR);
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if (savepos < 0)
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{
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/* file_seek might fail if this if the media is not seekable */
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return (ssize_t)savepos;
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}
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/* Then seek to the correct position in the file */
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pos = file_seek(filep, offset, SEEK_SET);
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if (pos < 0)
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{
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/* This might fail is the offset is beyond the end of file */
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return (ssize_t)pos;
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}
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/* Then perform the read operation */
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ret = file_read(filep, buf, nbytes);
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/* Restore the file position */
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pos = file_seek(filep, savepos, SEEK_SET);
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if (pos < 0 && ret >= 0)
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{
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/* This really should not fail */
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ret = (ssize_t)pos;
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}
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return ret;
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}
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/****************************************************************************
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* Name: pread
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*
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* Description:
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* The pread() function performs the same action as read(), except that it
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* reads from a given position in the file without changing the file
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* pointer. The first three arguments to pread() are the same as read()
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* with the addition of a fourth argument offset for the desired position
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* inside the file. An attempt to perform a pread() on a file that is
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* incapable of seeking results in an error.
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*
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* NOTE: This function could have been wholly implemented within libc but
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* it is not. Why? Because if pread were implemented in libc, it would
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* require four system calls. If it is implemented within the kernel,
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* only three.
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*
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* Input Parameters:
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* file File structure instance
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* buf User-provided to save the data
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* nbytes The maximum size of the user-provided buffer
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* offset The file offset
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*
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* Returned Value:
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* The positive non-zero number of bytes read on success, 0 on if an
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* end-of-file condition, or -1 on failure with errno set appropriately.
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* See read() return values
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*
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****************************************************************************/
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ssize_t pread(int fd, FAR void *buf, size_t nbytes, off_t offset)
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{
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FAR struct file *filep;
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ssize_t ret;
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/* pread() is a cancellation point */
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enter_cancellation_point();
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/* Get the file structure corresponding to the file descriptor. */
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ret = (ssize_t)fs_getfilep(fd, &filep);
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if (ret < 0)
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{
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goto errout;
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}
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/* Let file_pread do the real work */
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ret = file_pread(filep, buf, nbytes, offset);
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fs_putfilep(filep);
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if (ret < 0)
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{
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goto errout;
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}
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leave_cancellation_point();
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return ret;
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errout:
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set_errno((int)-ret);
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leave_cancellation_point();
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return (ssize_t)ERROR;
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}
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