man: split the description of sd_bus_error_set()

It was one giant all of text in pseudo-random order. Let's split it into
paragraphs talk about one subject each.

And unfortunately, the description of what happens when the error is not
set was not correct. In general, various functions treat 0/NULL as
not-an-error, and return 0.

1 files changed, 96 insertions, 109 deletions

diff --git a/man/sd_bus_error.xml b/man/sd_bus_error.xml
index bfc5caf4e2..bb8015de5e 100644
--- a/man/sd_bus_error.xml
+++ b/man/sd_bus_error.xml
@@ -147,55 +147,54 @@
   <refsect1>
     <title>Description</title>
 
-    <para>The <structname>sd_bus_error</structname> structure carries
-    information about a D-Bus error condition. The functions described
-    below may be used to set and query fields in this structure. The
-    <structfield>name</structfield> field contains a short identifier
-    of an error. It should follow the rules for error names described
-    in the D-Bus specification, subsection <ulink
-    url="http://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names">Valid
-    Names</ulink>. A number of common, standardized error names are
-    described in
-    <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-    but additional domain-specific errors may be defined by
-    applications. The <structfield>message</structfield> field usually
-    contains a human-readable string describing the details, but might
-    be <constant>NULL</constant>. An unset <structname>sd_bus_error</structname> structure
-    should have both fields initialized to <constant>NULL</constant>. Set an error
-    structure to <constant>SD_BUS_ERROR_NULL</constant> in order to
-    reset both fields to <constant>NULL</constant>. When no longer necessary, resources
-    held by the <structname>sd_bus_error</structname> structure should
-    be destroyed with <function>sd_bus_error_free()</function>.</para>
-
-    <para><function>sd_bus_error_set()</function> sets an error
-    structure to the specified name and message strings. The strings
-    will be copied into internal, newly allocated memory. It is
-    essential to free the error structure again when it is not
-    required anymore (see above). The function will return an
-    <varname>errno</varname>-like negative value (see <citerefentry
+    <para>The <structname>sd_bus_error</structname> structure carries information about a D-Bus error
+    condition, or lack thereof. The functions described below may be used to set and query fields in this
+    structure.
+    <itemizedlist>
+      <listitem><para>The <structfield>name</structfield> field contains a short identifier of an error. It
+      should follow the rules for error names described in the D-Bus specification, subsection <ulink
+      url="http://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names">Valid
+      Names</ulink>. A number of common, standardized error names are described in
+      <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry>, but
+      additional domain-specific errors may be defined by applications.</para></listitem>
+
+      <listitem><para>The <structfield>message</structfield> field usually contains a human-readable string
+      describing the details, but might be <constant>NULL</constant>.</para></listitem>
+    </itemizedlist>
+    An unset <structname>sd_bus_error</structname> structure should have both fields initialized to
+    <constant>NULL</constant>, and signifies lack of an error, i.e. success. Assign
+    <constant>SD_BUS_ERROR_NULL</constant> to the structure in order to initialize both fields to
+    <constant>NULL</constant>. When no longer necessary, resources held by the
+    <structname>sd_bus_error</structname> structure should be destroyed with
+    <function>sd_bus_error_free()</function>.</para>
+
+    <para><function>sd_bus_error_set()</function> sets an error structure to the specified name and message
+    strings. The strings will be copied into internal, newly allocated memory. It is essential to free the
+    contents again when they are not required anymore (see above). Do not use this call on error structures
+    that have already been set. If you intend to reuse an error structure, free the old data stored in it
+    with <function>sd_bus_error_free()</function> first.</para>
+
+    <para><function>sd_bus_error_set()</function> will return an <varname>errno</varname>-like value (see
+    <citerefentry
     project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
-    determined from the specified error name.  Various well-known
-    D-Bus errors are converted to well-known <varname>errno</varname>
-    counterparts, and the other ones to <constant>-EIO</constant>. See
-    <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-    for a list of well-known error names. Additional error mappings
-    may be defined with
-    <citerefentry><refentrytitle>sd_bus_error_add_map</refentrytitle><manvolnum>3</manvolnum></citerefentry>. If
-    <parameter>e</parameter> is <constant>NULL</constant>, no error structure is initialized,
-    but the error is still converted into an
-    <varname>errno</varname>-style error. If
-    <parameter>name</parameter> is <constant>NULL</constant>, it is
-    assumed that no error occurred, and 0 is returned. This means that
-    this function may be conveniently used in a
-    <function>return</function> statement. If
-    <parameter>message</parameter> is <constant>NULL</constant>, no message is set. This
-    call can fail if no memory may be allocated for the name and
-    message strings, in which case an
-    <constant>SD_BUS_ERROR_NO_MEMORY</constant> error might be set
-    instead and -ENOMEM be returned. Do not use this call on error
-    structures that are already initialized. If you intend to reuse an
-    error structure, free the old data stored in it with
-    <function>sd_bus_error_free()</function> first.</para>
+    determined from the specified error name <parameter>name</parameter>. If <parameter>name</parameter> is
+    <constant>NULL</constant>, it is assumed that no error occurred, and <constant>0</constant> is returned.
+    If <parameter>name</parameter> is nonnull, a negative value is always returned. If
+    <parameter>e</parameter> is <constant>NULL</constant>, no error structure is initialized, but
+    <parameter>name</parameter> is still converted into an <varname>errno</varname>-style value.</para>
+
+    <para>Various well-known D-Bus errors are converted to well-known <varname>errno</varname> counterparts,
+    and the other ones to <constant>-EIO</constant>. See
+    <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry> for a
+    list of well-known error names. Additional error mappings may be defined with
+    <citerefentry><refentrytitle>sd_bus_error_add_map</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+    </para>
+
+    <para><function>sd_bus_error_set()</function> is designed to be conveniently used in a
+    <function>return</function> statement. If <parameter>message</parameter> is <constant>NULL</constant>, no
+    message is set. This call can fail if no memory may be allocated for the name and message strings, in
+    which case an <constant>SD_BUS_ERROR_NO_MEMORY</constant> error will be set instead and
+    <constant>-ENOMEM</constant> returned. </para>
 
     <para><function>sd_bus_error_setf()</function> is similar to
     <function>sd_bus_error_set()</function>, but takes a <citerefentry
@@ -204,33 +203,25 @@
     <structfield>message</structfield> field.</para>
 
     <para><function>sd_bus_error_set_const()</function> is similar to
-    <function>sd_bus_error_set()</function>, but the string parameters
-    are not copied internally, and must hence remain constant and
-    valid for the lifetime of <parameter>e</parameter>. Use this call
-    to avoid memory allocations when setting error structures. Since
-    this call does not allocate memory, it will not fail with an
-    out-of-memory condition as
-    <function>sd_bus_error_set()</function> can, as described
-    above. Alternatively, the
-    <constant>SD_BUS_ERROR_MAKE_CONST()</constant> macro may be used
-    to generate a literal, constant bus error structure
-    on-the-fly.</para>
-
-    <para><function>sd_bus_error_set_errno()</function> will set
-    <structfield>name</structfield> from an
-    <varname>errno</varname>-like value that is converted to a D-Bus
+    <function>sd_bus_error_set()</function>, but the string parameters are not copied internally, and must
+    hence remain constant and valid for the lifetime of <parameter>e</parameter>. Use this call to avoid
+    memory allocations when setting error structures. Since this call does not allocate memory, it will not
+    fail with an out-of-memory condition as <function>sd_bus_error_set()</function> may, as described
+    above. Alternatively, the <constant>SD_BUS_ERROR_MAKE_CONST()</constant> macro may be used to generate a
+    literal, constant bus error structure on-the-fly.</para>
+
+    <para><function>sd_bus_error_set_errno()</function> will immediately return <constant>0</constant> if the
+    specified error parameter <parameter>error</parameter> is <constant>0</constant>. Otherwise, it will set
+    <structfield>name</structfield> from an <varname>errno</varname>-like value that is converted to a D-Bus
     error. <citerefentry
-    project='die-net'><refentrytitle>strerror_r</refentrytitle><manvolnum>3</manvolnum></citerefentry>
-    will be used to set <structfield>message</structfield>. Well-known
-    D-Bus error names will be used for <structfield>name</structfield>
-    if applicable, otherwise a name in the
-    <literal>System.Error.</literal> namespace will be generated. The
-    sign of the specified error number is ignored. The absolute value
-    is used implicitly. The call always returns a negative value, for
-    convenient usage in <function>return</function> statements. This
-    call might fail due to lack of memory, in which case an
-    <constant>SD_BUS_ERROR_NO_MEMORY</constant> error is set instead,
-    and -ENOMEM is returned.</para>
+    project='die-net'><refentrytitle>strerror_r</refentrytitle><manvolnum>3</manvolnum></citerefentry> will
+    be used to set <structfield>message</structfield>. Well-known D-Bus error names will be used for
+    <structfield>name</structfield> if applicable, otherwise a name in the <literal>System.Error.</literal>
+    namespace will be generated. The sign of the specified error number is ignored and the absolute value is
+    used implicitly. If the specified error <parameter>error</parameter> is non-zero, the call always returns
+    a negative value, for convenient usage in <function>return</function> statements. This call might fail
+    due to lack of memory, in which case an <constant>SD_BUS_ERROR_NO_MEMORY</constant> error is set instead,
+    and <constant>-ENOMEM</constant> is returned.</para>
 
     <para><function>sd_bus_error_set_errnof()</function> is similar to
     <function>sd_bus_error_set_errno()</function>, but in addition to
@@ -246,28 +237,26 @@
     project='man-pages'><refentrytitle>va_arg</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     parameter list.</para>
 
-    <para><function>sd_bus_error_get_errno()</function> converts the
-    <structfield>name</structfield> field of an error structure to an
-    <varname>errno</varname>-like (positive) value using the same
-    rules as <function>sd_bus_error_set()</function>.  If
-    <parameter>e</parameter> is <constant>NULL</constant>, 0 will be
-    returned.</para>
-
-    <para><function>sd_bus_error_copy()</function> will initialize
-    <parameter>dst</parameter> using the values in
-    <parameter>e</parameter>. If the strings in
-    <parameter>e</parameter> were set using
-    <function>sd_bus_error_set_const()</function>, they will be shared.
-    Otherwise, they will be copied. Returns a converted
-    <varname>errno</varname>-like, negative error code.</para>
-
-    <para><function>sd_bus_error_move()</function> is similar to <function>sd_bus_error_copy()</function>, but will
-    move any error information from <parameter>e</parameter> into <parameter>dst</parameter>, resetting the
-    former. This function cannot fail, as no new memory is allocated. Note that if <parameter>e</parameter> is not set
-    (or <constant>NULL</constant>) <parameter>dst</parameter> is initializated to
-    <constant>SD_BUS_ERROR_NULL</constant>. Moreover, if <parameter>dst</parameter> is <constant>NULL</constant> no
-    operation is executed on it and resources held by <parameter>e</parameter> are freed and reset. Returns a
-    converted <varname>errno</varname>-like, negative error code.</para>
+    <para><function>sd_bus_error_get_errno()</function> converts the <structfield>name</structfield> field of
+    an error structure to an <varname>errno</varname>-like (positive) value using the same rules as
+    <function>sd_bus_error_set()</function>.  If <parameter>e</parameter> is <constant>NULL</constant>,
+    <constant>0</constant> will be returned.</para>
+
+    <para><function>sd_bus_error_copy()</function> will initialize <parameter>dst</parameter> using the
+    values in <parameter>e</parameter>, if <parameter>e</parameter> has been set with an error value before.
+    Otherwise, it will return immediately. If the strings in <parameter>e</parameter> were set using
+    <function>sd_bus_error_set_const()</function>, they will be shared. Otherwise, they will be
+    copied. Returns a converted <varname>errno</varname>-like, negative error code or <constant>0</constant>.
+    Before this call, <parameter>dst</parameter> must be unset, i.e. either freshly initialized with
+    <constant>NULL</constant> or reset using <function>sd_bus_error_free()</function>.</para>
+
+    <para><function>sd_bus_error_move()</function> is similar to <function>sd_bus_error_copy()</function>,
+    but will move any error information from <parameter>e</parameter> into <parameter>dst</parameter>,
+    resetting the former. This function cannot fail, as no new memory is allocated. Note that if
+    <parameter>e</parameter> is not set, <parameter>dst</parameter> is initializated to
+    <constant>SD_BUS_ERROR_NULL</constant>. Moreover, if <parameter>dst</parameter> is
+    <constant>NULL</constant> no operation is executed on it and resources held by <parameter>e</parameter>
+    are freed and reset. Returns a converted <varname>errno</varname>-like, non-positive error value.</para>
 
     <para><function>sd_bus_error_is_set()</function> will return a
     non-zero value if <parameter>e</parameter> is
@@ -300,25 +289,23 @@
   <refsect1>
     <title>Return Value</title>
 
-    <para>The functions <function>sd_bus_error_set()</function>,
-    <function>sd_bus_error_setf()</function>, and
-    <function>sd_bus_error_set_const()</function>, when successful,
-    return the negative errno value corresponding to the
-    <parameter>name</parameter> parameter. The functions
-    <function>sd_bus_error_set_errno()</function>,
-    <function>sd_bus_error_set_errnof()</function> and
-    <function>sd_bus_error_set_errnofv()</function>, when successful,
-    return the negative value of the <parameter>error</parameter>
-    parameter. If an error occurs, one of the negative error values
-    listed below will be returned.</para>
+    <para>The functions <function>sd_bus_error_set()</function>, <function>sd_bus_error_setf()</function>,
+    and <function>sd_bus_error_set_const()</function> always return <constant>0</constant> when the specified
+    error value is <constant>NULL</constant>, and a negative errno-like value corresponding to the
+    <parameter>name</parameter> parameter otherwise. The functions
+    <function>sd_bus_error_set_errno()</function>, <function>sd_bus_error_set_errnof()</function> and
+    <function>sd_bus_error_set_errnofv()</function>, return <constant>0</constant> when the specified error
+    value is <constant>0</constant>, and a a negative errno-like value corresponding to the
+    <parameter>error</parameter> parameter otherwise. If an error occurs internally, one of the negative
+    error values listed below will be returned.</para>
 
     <para><function>sd_bus_error_get_errno()</function> returns
     <constant>false</constant> when <parameter>e</parameter> is
     <constant>NULL</constant>, and a positive errno value mapped from
     <parameter>e-&gt;name</parameter> otherwise.</para>
 
-    <para><function>sd_bus_error_copy()</function> and <function>sd_bus_error_move()</function> return 0 or a positive
-    integer on success, and a negative error value converted from the error name otherwise.</para>
+    <para><function>sd_bus_error_copy()</function> and <function>sd_bus_error_move()</function> return a
+    negative error value converted from the source error, and zero if the error has not been set.</para>
 
     <para><function>sd_bus_error_is_set()</function> returns a
     non-zero value when <parameter>e</parameter> and the