1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter�14.�Tracing samba system calls</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.71.0"><link rel="start" href="index.html" title="SAMBA Developers Guide"><link rel="up" href="pt04.html" title="Part�IV.�Debugging and tracing"><link rel="prev" href="pt04.html" title="Part�IV.�Debugging and tracing"><link rel="next" href="devprinting.html" title="Chapter�15.�Samba Printing Internals"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter�14.�Tracing samba system calls</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="pt04.html">Prev</a>�</td><th width="60%" align="center">Part�IV.�Debugging and tracing</th><td width="20%" align="right">�<a accesskey="n" href="devprinting.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="tracing"></a>Chapter�14.�Tracing samba system calls</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Andrew</span> <span class="surname">Tridgell</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span></div></div></div></div></div><p> 2This file describes how to do a system call trace on Samba to work out 3what its doing wrong. This is not for the faint of heart, but if you 4are reading this then you are probably desperate. 5</p><p> 6Actually its not as bad as the the above makes it sound, just don't 7expect the output to be very pretty :-) 8</p><p> 9Ok, down to business. One of the big advantages of unix systems is 10that they nearly all come with a system trace utility that allows you 11to monitor all system calls that a program is making. This is 12extremely using for debugging and also helps when trying to work out 13why something is slower than you expect. You can use system tracing 14without any special compilation options. 15</p><p> 16The system trace utility is called different things on different 17systems. On Linux systems its called strace. Under SunOS 4 its called 18trace. Under SVR4 style systems (including solaris) its called 19truss. Under many BSD systems its called ktrace. 20</p><p> 21The first thing you should do is read the man page for your native 22system call tracer. In the discussion below I'll assume its called 23strace as strace is the only portable system tracer (its available for 24free for many unix types) and its also got some of the nicest 25features. 26</p><p> 27Next, try using strace on some simple commands. For example, <code class="literal">strace 28ls</code> or <code class="literal">strace echo hello</code>. 29</p><p> 30You'll notice that it produces a LOT of output. It is showing you the 31arguments to every system call that the program makes and the 32result. Very little happens in a program without a system call so you 33get lots of output. You'll also find that it produces a lot of 34"preamble" stuff showing the loading of shared libraries etc. Ignore 35this (unless its going wrong!) 36</p><p> 37For example, the only line that really matters in the <code class="literal">strace echo 38hello</code> output is: 39</p><pre class="programlisting"> 40write(1, "hello\n", 6) = 6 41</pre><p>all the rest is just setting up to run the program.</p><p> 42Ok, now you're familiar with strace. To use it on Samba you need to 43strace the running smbd daemon. The way I tend ot use it is to first 44login from my Windows PC to the Samba server, then use smbstatus to 45find which process ID that client is attached to, then as root I do 46<code class="literal">strace -p PID</code> to attach to that process. I normally redirect the 47stderr output from this command to a file for later perusal. For 48example, if I'm using a csh style shell: 49</p><p><code class="literal">strace -f -p 3872 >& strace.out</code></p><p>or with a sh style shell:</p><p><code class="literal">strace -f -p 3872 > strace.out 2>&1</code></p><p> 50Note the "-f" option. This is only available on some systems, and 51allows you to trace not just the current process, but any children it 52forks. This is great for finding printing problems caused by the 53"print command" being wrong. 54</p><p> 55Once you are attached you then can do whatever it is on the client 56that is causing problems and you will capture all the system calls 57that smbd makes. 58</p><p> 59So how do you interpret the results? Generally I search through the 60output for strings that I know will appear when the problem 61happens. For example, if I am having touble with permissions on a file 62I would search for that files name in the strace output and look at 63the surrounding lines. Another trick is to match up file descriptor 64numbers and "follow" what happens to an open file until it is closed. 65</p><p> 66Beyond this you will have to use your initiative. To give you an idea 67of what you are looking for here is a piece of strace output that 68shows that <code class="filename">/dev/null</code> is not world writeable, which 69causes printing to fail with Samba: 70</p><pre class="programlisting"> 71[pid 28268] open("/dev/null", O_RDWR) = -1 EACCES (Permission denied) 72[pid 28268] open("/dev/null", O_WRONLY) = -1 EACCES (Permission denied) 73</pre><p> 74The process is trying to first open <code class="filename">/dev/null</code> read-write 75then read-only. Both fail. This means <code class="filename">/dev/null</code> has 76incorrect permissions. 77</p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pt04.html">Prev</a>�</td><td width="20%" align="center"><a accesskey="u" href="pt04.html">Up</a></td><td width="40%" align="right">�<a accesskey="n" href="devprinting.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part�IV.�Debugging and tracing�</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">�Chapter�15.�Samba Printing Internals</td></tr></table></div></body></html> 78