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<title>User service daemon and client specification - Information passed through the client/daemon combination</title>
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<h1>
-User service daemon and client specification - chapter 5<br>
+User service daemon and client specification - Chapter 5<br>
Information passed through the client/daemon combination
</h1>
+<hr>
+
+<p>
The information described below is the only information which passes
between the caller and the service.
<ul>
-<li>The service name supplied by the caller is available in the
+<p><li>The service name supplied by the caller is available in the
configuration language for deciding whether and which service program
-to invoke, in the <kbd>service</kbd> parameter, and is used by the
-<kbd>execute-from-directory</kbd> and <kbd>execute-from-path</kbd>
+to invoke, in the <code>service</code> parameter, and is used by the
+<code>execute-from-directory</code> and <code>execute-from-path</code>
configuration directives. It is usually used to select which service
program to invoke. It is also passed to the service program in the
-<kbd>USERV_SERVICE</kbd> environment variable.
+<code>USERV_SERVICE</code> environment variable.
+
<p><li>File descriptors specified by the client and allowed according to the
configuration language will be connected. Each file descriptor is
opened for reading or writing. Communication is via pipes, one end of
each pipe being open on the appropriate file descriptor in the service
program (when it is invoked) and the other end being held by the
client process, which will read and write files it opens on behalf of
-its caller or file descriptors it is passed by its caller.<P>
+its caller or file descriptors it is passed by its caller.
Data may be passed into the service through reading pipes and out of
it through writing pipes. These pipes can remain open only until the
service and client have terminated, or can be made to stay open after
the client has terminated and (if the service program forks) the main
service process has exited; the behaviour is controlled by options
-passed to the client by its caller.<P>
+passed to the client by its caller.
The caller can arrange that a writing pipe be connected to a pipe or
similar object and cause attempts to write to that descriptor by the
-service to generate a <kbd>SIGPIPE</kbd> (or <kbd>EPIPE</kbd> if
-<kbd>SIGPIPE</kbd> is caught or ignored) in the service.<P>
+service to generate a <code>SIGPIPE</code> (or <code>EPIPE</code> if
+<code>SIGPIPE</code> is caught or ignored) in the service.
Likewise, the service can close filedescriptors specified for reading,
which will cause the corresponding filedescriptors passed by the
caller to be closed, so that if these are pipes processes which write
-to them will receive <kbd>SIGPIPE</kbd> or <kbd>EPIPE</kbd>.
-<p><li>If <kbd>no-suppress-args</kbd> is set then arguments passed to the client
+to them will receive <code>SIGPIPE</code> or <code>EPIPE</code>.
+
+<p><li>If <code>no-suppress-args</code> is set then arguments passed to the client
by its caller will be passed on, verbatim, to the service.
+
<p><li>Fatal signals and system call failures experienced by the client will
result in the disconnection of the service from the client and
possibly some of the communication file descriptors described above;
-if <kbd>disconnect-hup</kbd> is set then the service will also be sent a
-<kbd>SIGHUP</kbd>.
-<p><li>The value of the <kbd>LOGNAME</kbd> (or <kbd>USER</kbd>) environment variable
+if <code>disconnect-hup</code> is set then the service will also be sent a
+<code>SIGHUP</code>.
+
+<p><li>The value of the <code>LOGNAME</code> (or <code>USER</code>) environment variable
as passed to the client will be used as the login name of the calling
user if the uid of the calling process matches the uid corresponding
to that login name. Otherwise the calling uid's password entry will
-be used to determine the calling user's login name.<P>
+be used to determine the calling user's login name.
This login name and the calling uid are available in the configuration
-language in the <kbd>calling-user</kbd> parameter and are passed to the
-service program in environment variables <kbd>USERV_USER</kbd> and
-<kbd>USERV_UID</kbd>.<P>
+language in the <code>calling-user</code> parameter and are passed to the
+service program in environment variables <code>USERV_USER</code> and
+<code>USERV_UID</code>.
The shell corresponding to that login name (according to the password
entry) is available as in the configuration language's
-<kbd>calling-user-shell</kbd> parameter.<P>
+<code>calling-user-shell</code> parameter.
If no relevant password entry can be found then no service will be
invoked.
+
<p><li>The numeric values and textual names for calling gid and supplementary
group list are available in the configuration language in the
-<kbd>calling-group</kbd> parameter and are passed to the service in
-environment variables.<P>
+<code>calling-group</code> parameter and are passed to the service in
+environment variables.
If no name can be found for a numeric group to which the calling
process belongs then no service will be invoked.
+
<p><li>The name of the current working directory in which the client was
-invoked is passed, if available and not hidden using <kbd>--hidecwd</kbd>,
-to the service program in the <kbd>USERV_CWD</kbd> variable. This grants no
+invoked is passed, if available and not hidden using <code>--hidecwd</code>,
+to the service program in the <code>USERV_CWD</code> variable. This grants no
special access to that directory unless it is a subdirectory of a
directory which is executable (searchable) but not readable by the
service user.
-<p><li>Settings specified by the caller using the <code>--defvar
-</code><var>name</var><code>=</code><var>value</var><code></code> option to the client are available in the
-configuration language as the corresponding <code>u-</code><var>name</var><code></code>
+
+<p><li>Settings specified by the caller using the <samp>--defvar
+<var>name</var>=<var>value</var></samp> option to the client are available in the
+configuration language as the corresponding <samp>u-<var>name</var></samp>
parameters and are passed to the service program in environment
-variables <code>USERV_U_</code><var>name</var><code></code>.
+variables <samp>USERV_U_<var>name</var></samp>.
+
<p><li>If the calling user is root or the same as the service user then
options may be given to the client which bypass the usual security
features; in this case other information may pass between the caller
and the service.
-<p></ul>
+
+</ul>
+</p>
<hr>
-User service daemon and client specification
-- <A href="index.html#copyright"><kbd>userv</kbd> is Copyright 1996-1999 Ian Jackson.</A>
-<br>
-<A href="index.html#toc">Contents</A>; <A href="index.html#abstract">abstract</A>; <A href="ch-notes.html">next</A>; <A href="ch-config.html">back</A>.
-<br>
-<address>0.61.4<br>
-Ian Jackson <A href="mailto:ian@davenant.greenend.org.uk">ian@davenant.greenend.org.uk</A></address>
-</body></html>
+
+[<a href="ch-config.html">back</a>]
+ [<a href="index.html#abstract">Abstract</a>]
+ [<a href="index.html#copyright">Copyright Notice</a>]
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+ [<a href="ch-notes.html">next</a>]
+
+<hr>
+
+User service daemon and client specification<br>
+
+<address>
+1.0.0<br>
+Ian Jackson <a href="mailto:ian@davenant.greenend.org.uk">ian@davenant.greenend.org.uk</a>
+</address>
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