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