<!-- Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009, 2013 The President and Fellows of Harvard College. 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 of the University 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 UNIVERSITY 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 UNIVERSITY 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. --> <html> <head> <title>read</title> <body bgcolor=#ffffff> <h2 align=center>read</h2> <h4 align=center>OS/161 Reference Manual</h4> <h3>Name</h3> <p> read - read data from file </p> <h3>Library</h3> <p> Standard C Library (libc, -lc) </p> <h3>Synopsis</h3> <p> <tt>#include <unistd.h></tt><br> <br> <tt>ssize_t</tt><br> <tt>read(int </tt><em>fd</em><tt>, void *</tt><em>buf</em><tt>, size_t </tt><em>buflen</em><tt>);</tt> </p> <h3>Description</h3> <p> <tt>read</tt> reads up to <em>buflen</em> bytes from the file specified by <em>fd</em>, at the location in the file specified by the current seek position of the file, and stores them in the space pointed to by <em>buf</em>. The file must be open for reading. </p> <p> The current seek position of the file is advanced by the number of bytes read. </p> <p> Each read (or <A HREF=write.html>write</A>) operation is atomic relative to other I/O to the same file. Note that the kernel is not obliged to (and generally cannot) make the read atomic with respect to other threads in the same process accessing the I/O buffer during the read. </p> <h3>Return Values</h3> <p> The count of bytes read is returned. This count should be positive. A return value of 0 should be construed as signifying end-of-file. On error, <tt>read</tt> returns -1 and sets <A HREF=errno.html>errno</A> to a suitable error code for the error condition encountered. </p> <p> Note that in some cases, particularly on devices, fewer than <em>buflen</em> (but greater than zero) bytes may be returned. This depends on circumstances and does not necessarily signify end-of-file. </p> <h3>Errors</h3> <p> The following error codes should be returned under the conditions given. Other error codes may be returned for other cases not mentioned here. <table width=90%> <tr><td width=5% rowspan=3> </td> <td width=10% valign=top>EBADF</td> <td><em>fd</em> is not a valid file descriptor, or was not opened for reading.</td></tr> <tr><td valign=top>EFAULT</td> <td>Part or all of the address space pointed to by <em>buf</em> is invalid.</td></tr> <tr><td valign=top>EIO</td> <td>A hardware I/O error occurred reading the data.</td></tr> </table> </p> </body> </html>