https://homeostasis.scs.carleton.ca/wiki/index.php?action=history&feed=atom&title=Operating_Systems_2022F_Lecture_7 2026-04-05T22:41:23Z Revision history for this page on the wiki MediaWiki 1.42.1 https://homeostasis.scs.carleton.ca/wiki/index.php?title=Operating_Systems_2022F_Lecture_7&diff=24082&oldid=prev 2022-09-29T20:21:15Z

<p>Created page with &quot;==Video== Video from the lecture given on September 29, 2022 is now available: * [https://homeostasis.scs.carleton.ca/~soma/os-2022f/lectures/comp3000-2022f-lec07-20220929.m4v video] * [https://homeostasis.scs.carleton.ca/~soma/os-2022f/lectures/comp3000-2022f-lec07-20220929.cc.vtt auto-generated captions] Video is also available through Brightspace (Resources-&gt;Zoom meeting-&gt;Cloud Recordings tab) ==Notes== &lt;pre&gt; Lecture 7 --------- Covered A1 solutions, walked throug...&quot;</p> <p><b>New page</b></p><div>==Video==<br /> <br /> Video from the lecture given on September 29, 2022 is now available:<br /> * [https://homeostasis.scs.carleton.ca/~soma/os-2022f/lectures/comp3000-2022f-lec07-20220929.m4v video]<br /> * [https://homeostasis.scs.carleton.ca/~soma/os-2022f/lectures/comp3000-2022f-lec07-20220929.cc.vtt auto-generated captions]<br /> Video is also available through Brightspace (Resources-&gt;Zoom meeting-&gt;Cloud Recordings tab)<br /> <br /> ==Notes==<br /> <br /> &lt;pre&gt;<br /> Lecture 7<br /> ---------<br /> <br /> Covered A1 solutions, walked through gdb a bit<br /> <br /> File descriptors<br /> ----------------<br /> <br /> When you run &quot;printf&quot;, what is that function doing?<br /> - it is writing to &quot;standard out&quot;<br /> - stdin, stdout, stderr are C concepts, supported on all platforms<br /> C runs on<br /> - but what is stdin, stdout, stderr on UNIX?<br /> (FILE * is not a thing on UNIX, it is a C thing)<br /> - but on UNIX, we have file descriptors<br /> - small numbers associated with open files<br /> <br /> the system call open returns a file descriptor, and open, read, write, close<br /> all take a file descriptor as an argument<br /> <br /> So what is stdin, stdout, and stderr in terms of file descriptors?<br /> <br /> 0 = stdin<br /> 1 = stdout<br /> 2 = stderr<br /> <br /> That&#039;s it<br /> <br /> Note that a file descriptor is a reference to an OPEN file. So someone has to open it.<br /> <br /> When a process starts on UNIX, by convention file descriptors 0, 1, and 2 are valid. 0 can be read from, and 1 and 2 can be written to. What they point to...can be any file!<br /> <br /> (So what this means is that before you do an execve you make sure file descriptors 0, 1, and 2 point to where you want them for the program you&#039;re exec&#039;ing)<br /> <br /> If you don&#039;t do anything, the new program will just get the same file descriptors that you had (memory is wiped but files that were open stay open)<br /> <br /> There is a maximum, it is defined by the kernel<br /> <br /> Files are accessed through file descriptors<br /> Most other I/O is also accessed through file descriptors, they just are<br /> file descriptors referring to &quot;special files&quot; (i.e., device files)<br /> <br /> /dev/tty, /dev/pts/0, /dev/null, etc are all special files<br /> <br /> Normally open just opens the file on the next available file descriptor. But you can use dup2 to copy it to another file descriptor (like 0, 1, or 2)<br /> &lt;/pre&gt;</div>

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