Add pread() and pwrite()

fs/vfs/Make.defs

@@ -43,6 +43,10 @@ ifneq ($(CONFIG_NSOCKET_DESCRIPTORS),0)
 
 CSRCS += fs_close.c fs_read.c fs_write.c fs_ioctl.c fs_poll.c fs_select.c
 
+# Support for positional file access
+
+CSRCS += fs_pread.c fs_pwrite.c
+
 # Support for network access using streams
 
 ifneq ($(CONFIG_NFILE_STREAMS),0)
@@ -70,6 +74,10 @@ CSRCS += fs_fsync.c fs_ioctl.c fs_lseek.c fs_mkdir.c fs_open.c fs_poll.c
 CSRCS += fs_read.c fs_rename.c fs_rmdir.c fs_stat.c fs_statfs.c fs_select.c
 CSRCS += fs_unlink.c fs_write.c
 
+# Support for positional file access
+
+CSRCS += fs_pread.c fs_pwrite.c
+
 # Stream support
 
 ifneq ($(CONFIG_NFILE_STREAMS),0)

fs/vfs/fs_pread.c

@@ -0,0 +1,129 @@
+/****************************************************************************
+ * 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_CUR);
+  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_CUR);
+  if (pos == (off_t)-1 && ret >= 0)
+    {
+      /* This really should not fail */
+
+      return ERROR;
+    }
+
+  set_errno(errcode);
+  return ret;
+}

fs/vfs/fs_pwrite.c

@@ -0,0 +1,127 @@
+/****************************************************************************
+ * fs/vfs/fs_pwrite.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: pwrite
+ *
+ * Description:
+ *   The pwrite() function performs the same action as write(), except that
+ *   it writes into a given position without changing the file pointer. The
+ *   first three arguments to pwrite() are the same as write() with the
+ *   addition of a fourth argument offset for the desired position inside
+ *   the file.
+ *
+ *   NOTE: This function could have been wholly implemented within libc but
+ *   it is not.  Why?  Because if pwrite were implemented in libc, it would
+ *   require four system calls.  If it is implemented within the kernel,
+ *   only three.
+ *
+ * Parameters:
+ *   fd       file descriptor (or socket descriptor) to write to
+ *   buf      Data to write
+ *   nbytes   Length of data to write
+ *
+ * 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 write() return values
+ *
+ ****************************************************************************/
+
+ssize_t pwrite(int fd, FAR const 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_CUR);
+  if (pos == (off_t)-1)
+    {
+      /* This might fail is the offset is beyond the end of file */
+
+      return ERROR;
+    }
+
+  /* Then perform the write operation */
+
+  ret = write(fd, buf, nbytes);
+  errcode = get_errno();
+
+  /* Restore the file position */
+
+  pos = lseek(fd, savepos, SEEK_CUR);
+  if (pos == (off_t)-1 && ret >= 0)
+    {
+      /* This really should not fail */
+
+      return ERROR;
+    }
+
+  set_errno(errcode);
+  return ret;
+}

fs/vfs/fs_read.c

@@ -69,7 +69,7 @@
  *
  * Return:
  *   The positive non-zero number of bytes read on success, 0 on if an
- *   end-of-file condition, or -1 on failure withi errno set appropriately.
+ *   end-of-file condition, or -1 on failure with errno set appropriately.
  *
  ****************************************************************************/
 
@@ -101,7 +101,7 @@ ssize_t file_read(FAR struct file *filep, FAR void *buf, size_t nbytes)
   else if (inode && inode->u.i_ops && inode->u.i_ops->read)
     {
       /* Yes.. then let it perform the read.  NOTE that for the case of the
-       * mountpoint, we depend on the read methods bing identical in
+       * mountpoint, we depend on the read methods being identical in
        * signature and position in the operations vtable.
        */
 
@@ -135,7 +135,7 @@ ssize_t file_read(FAR struct file *filep, FAR void *buf, size_t nbytes)
  *
  * Return:
  *   The positive non-zero number of bytes read on success, 0 on if an
- *   end-of-file condition, or -1 on failure withi errno set appropriately.
+ *   end-of-file condition, or -1 on failure with errno set appropriately.
  *
  ****************************************************************************/
 

include/unistd.h

@@ -146,6 +146,8 @@ int     fsync(int fd);
 off_t   lseek(int fd, off_t offset, int whence);
 ssize_t read(int fd, FAR void *buf, size_t nbytes);
 ssize_t write(int fd, FAR const void *buf, size_t nbytes);
+ssize_t pread(int fd, FAR void *buf, size_t nbytes, off_t offset);
+ssize_t pwrite(int fd, FAR const void *buf, size_t nbytes, off_t offset);
 
 /* Memory management */