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-<h1>
-User service daemon and client specification
-<br>Chapter 3 - Execution environment of the service program
-</h1>
-
-
-<hr>
-
-
-<p>
-The daemon which is handling the service user side of things will read
-configuration files to decide what to do. If it decides to allow the service
-to be provided it will fork a subprocess to execute the service.
-
-<p>
-The service will have no controlling terminal, but it will be a process group
-leader.
-
-<p>
-If the client is killed or times out or a file or descriptor being read or
-written by the client process gets an error then the service will be
-disconnected from the client. The client will return an exit status of 255 and
-some the service's pipes may be closed at the other end. The service will
-become a child of <code>init</code>. The service may well not notice the
-disconnection, though writing to a pipe after this may produce a
-<code>SIGPIPE</code> and the facility exists to have a <code>SIGHUP</code> sent
-to the service on disconnection.
-
-<hr>
-
-<a name="s3.1"></a>
-<h2>3.1 File descriptors</h2>
-
-<p>
-The service program's standard filedescriptors, and possibly other file
-descriptors, will be connected to pipes or to <code>/dev/null</code>. The
-<code>userv</code> client/daemon pair will arrange that data is copied between
-the files or file descriptors specified to to the client by the caller and
-these these pipes.
-
-<p>
-Pipes which may be written to will be closed if a write error occurs on the
-corresponding client-side file or descriptor, which may result in a
-<code>SIGPIPE</code> in the service program; pipes open for reading will get
-<code>EOF</code> if the client-side file descriptor gets <code>EOF</code> or an
-error.
-
-<p>
-If the service closes one of its reading file descriptors the writing end of
-the corresponding pipe will generate a <code>SIGPIPE</code> when attempts are
-made by the client/daemon pair to write to it. This will not be considered an
-error; rather, the relevant pipe will be discarded and the corresponding file
-or file descriptor held by the client will be closed.
-
-<p>
-Likewise, if one of the file descriptors held by the client for writing by the
-service is a pipe whose other end is closed by the caller then the
-client/daemon pair will see an error when trying to copy data provided by the
-service. This too will not be considered an error; rather, the pipe
-correspondong to that descriptor will be closed and any further writes will
-cause the service to get a <code>SIGPIPE</code>.
-
-<p>
-Note that not all write errors or broken pipes on file descriptors may be
-visible to the service, since buffered data may be discarded by the operating
-system and there will be a finite interval between the error happening and the
-service being disconnected from the client or the next write causing a
-<code>SIGPIPE</code>.
-
-<p>
-Read errors on file descriptors (and disconnection) will only be visible to the
-service and distinguishable from normal end of file if
-<code>disconnect-hup</code> is in effect.
-
-<p>
-Read and write errors (other than broken pipes, as described above) will always
-be visible to the caller; they are system errors, and will therefore cause the
-client to print an error message to stderr and return with an exit status of
-255.
-
-<p>
-If the main service program process exits while it still has running children
-any file descriptors held by those children can remain open, depending on the
-use of <samp>wait</samp>, <samp>nowait</samp> or <samp>close</samp> for the
-relevant file descriptor in the client's arguments. By default writing
-filedescriptors remain open and the client will wait for them to be closed at
-the service end, and reading file descriptors are closed immediately. These
-leftover child processes will not get a any <code>SIGHUP</code> even if a read
-or write error occurs or the client disconnects before then.
-
-<hr>
-
-<a name="s3.2"></a>
-<h2>3.2 Environment</h2>
-
-<p>
-The service will have some information in environment variables:
-<dl>
-<dt><samp>USERV_USER</samp></dt>
-<dd>
-The login name of the calling user. If the <code>LOGNAME</code> variable is
-set (or, if that is unset, if the <code>USER</code> variable is set) in the
-environment passed to the client by the caller then the password entry for that
-login name will be looked up; if that password entry's uid is the same as that
-of the calling process then that login name will be used, otherwise (or if
-neither <code>LOGNAME</code> nor <code>USER</code> is set) the calling
-process's uid will be looked up to determine their login name (and if this
-lookup fails then the service will not be invoked).
-</dd>
-<dt><samp>USERV_UID</samp></dt>
-<dd>
-The uid of the calling process.
-</dd>
-<dt><samp>USERV_GID</samp></dt>
-<dd>
-The gid and supplementary group list of the calling process: first the group in
-gid and then those in the supplementary group list, in decimal, separated by
-spaces.
-</dd>
-<dt><samp>USERV_GROUP</samp></dt>
-<dd>
-The group names of the calling process, listed in the same way as the ids are
-in <code>USERV_GID</code>. If no name can be found for any of the calling
-process's group(s) then the service will not be invoked.
-</dd>
-<dt><samp>USERV_CWD</samp></dt>
-<dd>
-The client's current working directory name (this directory may not be
-accessible to the service). If it could not be determined or the
-<code>--hidecwd</code> flag was used then this variable will be set to an empty
-string (this is not considered an error).
-</dd>
-<dt><samp>USERV_SERVICE</samp></dt>
-<dd>
-The service name requested by the caller.
-</dd>
-<dt><samp>USERV_U_<var>name</var></samp></dt>
-<dd>
-The value supplied to the client by the caller using -D<var>name</var>.
-</dd>
-</dl>
-
-<p>
-<code>HOME</code>, <code>PATH</code>, <code>SHELL</code>, <code>LOGNAME</code>
-and <code>USER</code> will be set appropriately (according to the details of
-the service user).
-
-<hr>
-
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-[ <a href="index.html#contents">Contents</a> ]
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-[ 3 ]
-[ <a href="ch-config.html">4</a> ]
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-
-<hr>
-
-<p>
-User service daemon and client specification
-
-<address>
-1.0.3<br>
-Ian Jackson <code><a href="mailto:ian@davenant.greenend.org.uk">ian@davenant.greenend.org.uk</a></code>
-</address>
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