<!-- Copyright (c) 2014 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>semfs</title> <body bgcolor=#ffffff> <h2 align=center>semfs</h2> <h4 align=center>OS/161 Reference Manual</h4> <h3>Name</h3> <p> semfs - semaphore filesystem </p> <h3>Synopsis</h3> <p> options semfs </p> <h3>Description</h3> <p> semfs is a simple "fake" (memory-only) file system that provides synchronization facilities to userland in the form of counting semaphores. There is one semfs instance, called "sem:", which is created and mounted during system boot. </p> <p> Semaphores in semfs appear as files in "sem:". To create a semaphore, open such a file using <tt>O_CREAT</tt>. To destroy it, remove it with <tt>remove()</tt>. </p> <p> To use the semaphore, write one byte to increase the semaphore count by one (the "V" operation) and write one byte to decrease the semaphore count by one (the "P" operation). The count will not decrease below zero; attempts to do so will block until other concurrent write operations raise the count again. You can also set the count explicitly using <tt>ftruncate()</tt>; this is useful for initializing a semaphore to a nonzero value. Note that no data is actually transferred by the read and write calls; it is acceptable to pass NULL as the data pointer. </p> <p> You can create as many semaphores as you want (until memory runs out or the directory reaches 2^32 entries); however, semfs does not support subdirectories, hard links of semaphores, or renaming. </p> <h3>Files</h3> <p> <tt>sem:</tt> </p> </body> </html>