Initial Spring 2016 commit.
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Geoffrey Challen
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man/syscall/open.html
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man/syscall/open.html
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<html>
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<head>
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<title>open</title>
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<link rel="stylesheet" type="text/css" media="all" href="../man.css">
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</head>
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<body bgcolor=#ffffff>
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<h2 align=center>open</h2>
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<h4 align=center>OS/161 Reference Manual</h4>
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<h3>Name</h3>
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<p>
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open - open a file
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</p>
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<h3>Library</h3>
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<p>
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Standard C Library (libc, -lc)
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</p>
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<h3>Synopsis</h3>
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<p>
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<tt>#include <unistd.h></tt><br>
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<tt>#include <fcntl.h></tt><br>
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<br>
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<tt>int</tt><br>
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<tt>open(const char *</tt><em>filename</em><tt>,
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int </tt><em>flags</em><tt>);</tt><br>
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<tt>int</tt><br>
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<tt>open(const char *</tt><em>filename</em><tt>, int </tt><em>flags</em><tt>,
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mode_t </tt><em>mode</em><tt>);</tt><br>
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</p>
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<h3>Description</h3>
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<p>
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<tt>open</tt> opens the file, device, or other kernel object named by
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the pathname <em>filename</em>. The <em>flags</em> argument specifies
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how to open the file. The optional <em>mode</em> argument provides the
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file permissions to use and is only meaningful in Unix, or if you
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choose to implement Unix-style security later on. it can be ignored in
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OS/161.
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</p>
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<p>
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The flags argument should consist of one of
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<table width=90%>
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<tr><td width=5% rowspan=3> </td>
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<td width=30%>O_RDONLY</td> <td>Open for reading only.</td></tr>
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<tr><td>O_WRONLY</td> <td>Open for writing only.</td></tr>
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<tr><td>O_RDWR</td> <td>Open for reading and writing.</td></tr>
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</table>
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</p>
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<p>
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It may also have any of the following flags OR'd in:
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<table width=90%>
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<tr><td width=5% rowspan=4> </td>
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<td width=20%>O_CREAT</td>
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<td>Create the file if it doesn't exist.</td></tr>
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<tr><td>O_EXCL</td> <td>Fail if the file already exists.</td></tr>
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<tr><td>O_TRUNC</td> <td>Truncate the file to length 0 upon open.</td></tr>
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<tr><td>O_APPEND</td> <td>Open the file in append mode.</td></tr>
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</table>
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O_EXCL is only meaningful if O_CREAT is also used.
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</p>
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<p>
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O_APPEND causes all writes to the file to occur at the end of file, no
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matter what gets written to the file by whoever else, including
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concurrently. (This functionality may be optional; consult your
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course's assignments.)
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</p>
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<p>
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<tt>open</tt> returns a file handle suitable for passing to
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<A HREF=read.html>read</A>,
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<A HREF=write.html>write</A>,
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<A HREF=close.html>close</A>,
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etc. This file handle must be greater than or equal to zero. Note
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that file handles 0 (STDIN_FILENO), 1 (STDOUT_FILENO), and 2
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(STDERR_FILENO) are used in special ways and are typically assumed by
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user-level code to always be open.
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</p>
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<p>
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If you are implementing symbolic links, there are some additional
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points to take note of. If <em>filename</em> refers to a symbolic
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link, that link does not point to an existing object, and O_CREAT is
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specified, a new file is created <i>at the name the link points
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to</i>. However, if in this case both O_CREAT and O_EXCL are
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specified, <tt>open</tt> fails with EEXIST. These semantics are a
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nuisance to implement but important for correct functioning.
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</p>
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<p>
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<tt>open</tt> (like all system calls) should be atomic. It is
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important for the handling of O_EXCL in the destination directory to
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be atomic. Note, however, that in practice looking up directories that
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contain <tt>..</tt> is usually not quite atomic.
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</p>
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<h3>Return Values</h3>
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<p>
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On success, <tt>open</tt> returns a nonnegative file handle. On error,
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-1 is returned, and <A HREF=errno.html>errno</A> is set according to
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the error encountered.
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</p>
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<h3>Errors</h3>
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<p>
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The following error codes should be returned under the conditions
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given. Other error codes may be returned for other cases not
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mentioned here.
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<table width=90%>
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<tr><td width=5% rowspan=15> </td>
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<td width=10% valign=top>ENODEV</td>
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<td>The device prefix of <em>filename</em> did
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not exist.</td></tr>
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<tr><td valign=top>ENOTDIR</td> <td>A non-final component of <em>filename</em>
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was not a directory.</td></tr>
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<tr><td valign=top>ENOENT</td> <td>A non-final component of <em>filename</em>
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did not exist.</td></tr>
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<tr><td valign=top>ENOENT</td> <td>The named file does not exist, and O_CREAT
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was not specified.</td></tr>
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<tr><td valign=top>EEXIST</td> <td>The named file exists, and O_EXCL was
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specified.</td></tr>
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<tr><td valign=top>EISDIR</td> <td>The named object is a directory, and it
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was to be opened for writing.</td></tr>
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<tr><td valign=top>EMFILE</td> <td>The process's file table was full, or a
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process-specific limit on open files
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was reached.</td></tr>
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<tr><td valign=top>ENFILE</td> <td>The system file table is full, if such a
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thing exists, or a system-wide limit
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on open files was reached.</td></tr>
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<tr><td valign=top>ENXIO</td> <td>The named object is a block device with no
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filesystem mounted on it.</td></tr>
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<tr><td valign=top>ENOSPC</td> <td>The file was to be created, and the
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filesystem involved is full.</td></tr>
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<tr><td valign=top>EINVAL</td> <td><em>flags</em> contained invalid
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values.</td></tr>
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<tr><td valign=top>EIO</td> <td>A hard I/O error occurred.</td></tr>
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<tr><td valign=top>EFAULT</td> <td><em>filename</em> was an invalid
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pointer.</td></tr>
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</table>
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</p>
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</body>
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</html>
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