man/sd_bus_error.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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   5
   6 <refentry id="sd_bus_error" xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sd_bus_error</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sd_bus_error</refentrytitle>
  15     <manvolnum>3</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sd_bus_error</refname>
  20     <refname>SD_BUS_ERROR_MAKE_CONST</refname>
  21     <refname>SD_BUS_ERROR_NULL</refname>
  22     <refname>sd_bus_error_free</refname>
  23     <refname>sd_bus_error_set</refname>
  24     <refname>sd_bus_error_setf</refname>
  25     <refname>sd_bus_error_setfv</refname>
  26     <refname>sd_bus_error_set_const</refname>
  27     <refname>sd_bus_error_set_errno</refname>
  28     <refname>sd_bus_error_set_errnof</refname>
  29     <refname>sd_bus_error_set_errnofv</refname>
  30     <refname>sd_bus_error_get_errno</refname>
  31     <refname>sd_bus_error_copy</refname>
  32     <refname>sd_bus_error_move</refname>
  33     <refname>sd_bus_error_is_set</refname>
  34     <refname>sd_bus_error_has_name</refname>
  35     <refname>sd_bus_error_has_names_sentinel</refname>
  36     <refname>sd_bus_error_has_names</refname>
  37
  38     <refpurpose>sd-bus error handling</refpurpose>
  39   </refnamediv>
  40
  41   <refsynopsisdiv>
  42     <funcsynopsis>
  43       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  44
  45       <funcsynopsisinfo>typedef struct {
  46         const char *name;
  47         const char *message;
  48         …
  49 } sd_bus_error;</funcsynopsisinfo>
  50     </funcsynopsis>
  51
  52     <para>
  53       <constant>SD_BUS_ERROR_MAKE_CONST(<replaceable>name</replaceable>, <replaceable>message</replaceable>)</constant>
  54     </para>
  55     <para>
  56       <constant>SD_BUS_ERROR_NULL</constant>
  57     </para>
  58
  59     <funcsynopsis>
  60       <funcprototype>
  61         <funcdef>void <function>sd_bus_error_free</function></funcdef>
  62         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  63       </funcprototype>
  64
  65       <funcprototype>
  66         <funcdef>int <function>sd_bus_error_set</function></funcdef>
  67         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  68         <paramdef>const char *<parameter>name</parameter></paramdef>
  69         <paramdef>const char *<parameter>message</parameter></paramdef>
  70       </funcprototype>
  71
  72       <funcprototype>
  73         <funcdef>int <function>sd_bus_error_setf</function></funcdef>
  74         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  75         <paramdef>const char *<parameter>name</parameter></paramdef>
  76         <paramdef>const char *<parameter>format</parameter></paramdef>
  77         <paramdef>…</paramdef>
  78       </funcprototype>
  79
  80       <funcprototype>
  81         <funcdef>int <function>sd_bus_error_setfv</function></funcdef>
  82         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  83         <paramdef>const char *<parameter>name</parameter></paramdef>
  84         <paramdef>const char *<parameter>format</parameter></paramdef>
  85         <paramdef>va_list <parameter>ap</parameter></paramdef>
  86       </funcprototype>
  87
  88       <funcprototype>
  89         <funcdef>int <function>sd_bus_error_set_const</function></funcdef>
  90         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  91         <paramdef>const char *<parameter>name</parameter></paramdef>
  92         <paramdef>const char *<parameter>message</parameter></paramdef>
  93       </funcprototype>
  94
  95       <funcprototype>
  96         <funcdef>int <function>sd_bus_error_set_errno</function></funcdef>
  97         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
  98         <paramdef>int <parameter>error</parameter></paramdef>
  99       </funcprototype>
 100
 101       <funcprototype>
 102         <funcdef>int <function>sd_bus_error_set_errnof</function></funcdef>
 103         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
 104         <paramdef>int <parameter>error</parameter></paramdef>
 105         <paramdef>const char *<parameter>format</parameter></paramdef>
 106         <paramdef>…</paramdef>
 107       </funcprototype>
 108
 109       <funcprototype>
 110         <funcdef>int <function>sd_bus_error_set_errnofv</function></funcdef>
 111         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
 112         <paramdef>int <parameter>error</parameter></paramdef>
 113         <paramdef>const char *<parameter>format</parameter></paramdef>
 114         <paramdef>va_list <parameter>ap</parameter></paramdef>
 115       </funcprototype>
 116
 117       <funcprototype>
 118         <funcdef>int <function>sd_bus_error_get_errno</function></funcdef>
 119         <paramdef>const sd_bus_error *<parameter>e</parameter></paramdef>
 120       </funcprototype>
 121
 122       <funcprototype>
 123         <funcdef>int <function>sd_bus_error_copy</function></funcdef>
 124         <paramdef>sd_bus_error *<parameter>dst</parameter></paramdef>
 125         <paramdef>const sd_bus_error *<parameter>e</parameter></paramdef>
 126       </funcprototype>
 127
 128       <funcprototype>
 129         <funcdef>int <function>sd_bus_error_move</function></funcdef>
 130         <paramdef>sd_bus_error *<parameter>dst</parameter></paramdef>
 131         <paramdef>sd_bus_error *<parameter>e</parameter></paramdef>
 132       </funcprototype>
 133
 134       <funcprototype>
 135         <funcdef>int <function>sd_bus_error_is_set</function></funcdef>
 136         <paramdef>const sd_bus_error *<parameter>e</parameter></paramdef>
 137       </funcprototype>
 138
 139       <funcprototype>
 140         <funcdef>int <function>sd_bus_error_has_name</function></funcdef>
 141         <paramdef>const sd_bus_error *<parameter>e</parameter></paramdef>
 142         <paramdef>const char *<parameter>name</parameter></paramdef>
 143       </funcprototype>
 144
 145       <funcprototype>
 146         <funcdef>int <function>sd_bus_error_has_names_sentinel</function></funcdef>
 147         <paramdef>const sd_bus_error *<parameter>e</parameter></paramdef>
 148         <paramdef>...</paramdef>
 149       </funcprototype>
 150     </funcsynopsis>
 151
 152     <para>
 153       &#35;define sd_bus_error_has_names(e, ...) sd_bus_error_has_names_sentinel(e, ..., NULL)
 154     </para>
 155
 156   </refsynopsisdiv>
 157
 158   <refsect1>
 159     <title>Description</title>
 160
 161     <para>The <structname>sd_bus_error</structname> structure carries information about a D-Bus error
 162     condition, or lack thereof. The functions described below may be used to set and query fields in this
 163     structure.
 164     <itemizedlist>
 165       <listitem><para>The <structfield>name</structfield> field contains a short identifier of an error. It
 166       should follow the rules for error names described in the D-Bus specification, subsection <ulink
 167       url="https://dbus.freedesktop.org/doc/dbus-specification.html#message-protocol-names">Valid
 168       D-Bus Names</ulink>. A number of common, standardized error names are described in
 169       <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry>, but
 170       additional domain-specific errors may be defined by applications.</para></listitem>
 171
 172       <listitem><para>The <structfield>message</structfield> field usually contains a human-readable string
 173       describing the details, but might be <constant>NULL</constant>.</para></listitem>
 174     </itemizedlist>
 175     An unset <structname>sd_bus_error</structname> structure should have both fields initialized to
 176     <constant>NULL</constant>, and signifies lack of an error, i.e. success. Assign
 177     <constant>SD_BUS_ERROR_NULL</constant> to the structure in order to initialize both fields to
 178     <constant>NULL</constant>. When no longer necessary, resources held by the
 179     <structname>sd_bus_error</structname> structure should be destroyed with
 180     <function>sd_bus_error_free()</function>.</para>
 181
 182     <para><function>sd_bus_error_set()</function> sets an error structure to the specified name and message
 183     strings. The strings will be copied into internal, newly allocated memory. It is essential to free the
 184     contents again when they are not required anymore (see above). Do not use this call on error structures
 185     that have already been set. If you intend to reuse an error structure, free the old data stored in it
 186     with <function>sd_bus_error_free()</function> first.</para>
 187
 188     <para><function>sd_bus_error_set()</function> will return an <varname>errno</varname>-like value (see
 189     <citerefentry
 190     project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
 191     determined from the specified error name <parameter>name</parameter>. If <parameter>name</parameter> is
 192     <constant>NULL</constant>, it is assumed that no error occurred, and <constant>0</constant> is returned.
 193     If <parameter>name</parameter> is nonnull, a negative value is always returned. If
 194     <parameter>e</parameter> is <constant>NULL</constant>, no error structure is initialized, but
 195     <parameter>name</parameter> is still converted into an <varname>errno</varname>-style value.</para>
 196
 197     <para>Various well-known D-Bus errors are converted to well-known <varname>errno</varname> counterparts,
 198     and the other ones to <constant>-EIO</constant>. See
 199     <citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry> for a
 200     list of well-known error names. Additional error mappings may be defined with
 201     <citerefentry><refentrytitle>sd_bus_error_add_map</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 202     </para>
 203
 204     <para><function>sd_bus_error_set()</function> is designed to be conveniently used in a
 205     <function>return</function> statement. If <parameter>message</parameter> is <constant>NULL</constant>, no
 206     message is set. This call can fail if no memory may be allocated for the name and message strings, in
 207     which case an <constant>SD_BUS_ERROR_NO_MEMORY</constant> error will be set instead and
 208     <constant>-ENOMEM</constant> returned. </para>
 209
 210     <para><function>sd_bus_error_setf()</function> and <function>sd_bus_error_setfv()</function> are similar
 211     to <function>sd_bus_error_set()</function>, but take a <citerefentry
 212     project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry> format
 213     string and corresponding arguments to generate the <structfield>message</structfield> field.
 214     <function>sd_bus_error_setf()</function> uses variadic arguments, and
 215     <function>sd_bus_error_setfv()</function> accepts the arguments as a
 216     <citerefentry
 217     project='man-pages'><refentrytitle>va_arg</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 218     parameter list.</para>
 219
 220     <para><function>sd_bus_error_set_const()</function> is similar to
 221     <function>sd_bus_error_set()</function>, but the string parameters are not copied internally, and must
 222     hence remain constant and valid for the lifetime of <parameter>e</parameter>. Use this call to avoid
 223     memory allocations when setting error structures. Since this call does not allocate memory, it will not
 224     fail with an out-of-memory condition as <function>sd_bus_error_set()</function> may, as described
 225     above. Alternatively, the <constant>SD_BUS_ERROR_MAKE_CONST()</constant> macro may be used to generate a
 226     literal, constant bus error structure on-the-fly.</para>
 227
 228     <para><function>sd_bus_error_set_errno()</function> will immediately return <constant>0</constant> if the
 229     specified error parameter <parameter>error</parameter> is <constant>0</constant>. Otherwise, it will set
 230     <structfield>name</structfield> from an <varname>errno</varname>-like value that is converted to a D-Bus
 231     error. <citerefentry
 232     project='die-net'><refentrytitle>strerror_r</refentrytitle><manvolnum>3</manvolnum></citerefentry> will
 233     be used to set <structfield>message</structfield>. Well-known D-Bus error names will be used for
 234     <structfield>name</structfield> if applicable, otherwise a name in the <literal>System.Error.</literal>
 235     namespace will be generated. The sign of the specified error number is ignored and the absolute value is
 236     used implicitly. If the specified error <parameter>error</parameter> is non-zero, the call always returns
 237     a negative value, for convenient usage in <function>return</function> statements. This call might fail
 238     due to lack of memory, in which case an <constant>SD_BUS_ERROR_NO_MEMORY</constant> error is set instead,
 239     and <constant>-ENOMEM</constant> is returned.</para>
 240
 241     <para><function>sd_bus_error_set_errnof()</function> and <function>sd_bus_error_set_errnof()</function>
 242     are similar to <function>sd_bus_error_set_errno()</function>, but in addition to
 243     <parameter>error</parameter>, take a <citerefentry
 244     project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry> format
 245     string and corresponding arguments. The <structfield>message</structfield> field will be generated from
 246     <parameter>format</parameter> and the arguments.
 247     <function>sd_bus_error_set_errnof()</function> uses variadic arguments, and
 248     <function>sd_bus_error_set_errnofv()</function> accepts the arguments as a
 249     <citerefentry
 250     project='man-pages'><refentrytitle>va_arg</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 251     parameter list.</para>
 252
 253     <para><function>sd_bus_error_get_errno()</function> converts the <structfield>name</structfield> field of
 254     an error structure to an <varname>errno</varname>-like (positive) value using the same rules as
 255     <function>sd_bus_error_set()</function>.  If <parameter>e</parameter> is <constant>NULL</constant>,
 256     <constant>0</constant> will be returned.</para>
 257
 258     <para><function>sd_bus_error_copy()</function> will initialize <parameter>dst</parameter> using the
 259     values in <parameter>e</parameter>, if <parameter>e</parameter> has been set with an error value before.
 260     Otherwise, it will return immediately. If the strings in <parameter>e</parameter> were set using
 261     <function>sd_bus_error_set_const()</function>, they will be shared. Otherwise, they will be
 262     copied. Before this call, <parameter>dst</parameter> must be unset, i.e. either freshly initialized with
 263     <constant>NULL</constant> or reset using <function>sd_bus_error_free()</function>.</para>
 264
 265     <para><function>sd_bus_error_copy()</function> generally returns <constant>0</constant> or a negative
 266     <varname>errno</varname>-like value based on the input parameter <parameter>e</parameter>:
 267     <constant>0</constant> if it was unset and a negative integer if it was set to some error, similarly to
 268     <function>sd_bus_error_set()</function>. It may however also return an error generated internally, for
 269     example <constant>-ENOMEM</constant> if a memory allocation fails.</para>
 270
 271     <para><function>sd_bus_error_move()</function> is similar to <function>sd_bus_error_copy()</function>,
 272     but will move any error information from <parameter>e</parameter> into <parameter>dst</parameter>,
 273     resetting the former. This function cannot fail, as no new memory is allocated. Note that if
 274     <parameter>e</parameter> is not set, <parameter>dst</parameter> is initialized to
 275     <constant>SD_BUS_ERROR_NULL</constant>. Moreover, if <parameter>dst</parameter> is
 276     <constant>NULL</constant> no operation is executed on it and resources held by <parameter>e</parameter>
 277     are freed and reset. Returns a converted <varname>errno</varname>-like, non-positive error value.</para>
 278
 279     <para><function>sd_bus_error_is_set()</function> will return a
 280     non-zero value if <parameter>e</parameter> is
 281     non-<constant>NULL</constant> and an error has been set,
 282     <constant>false</constant> otherwise.</para>
 283
 284     <para><function>sd_bus_error_has_name()</function> will return a
 285     non-zero value if <parameter>e</parameter> is
 286     non-<constant>NULL</constant> and an error with the same
 287     <parameter>name</parameter> has been set,
 288     <constant>false</constant> otherwise.</para>
 289
 290     <para><function>sd_bus_error_has_names_sentinel()</function> is similar to
 291     <function>sd_bus_error_has_name()</function>, but takes multiple names to check against. The list must be
 292     terminated with <constant>NULL</constant>. <function>sd_bus_error_has_names()</function>
 293     is a macro wrapper around <function>sd_bus_error_has_names_sentinel()</function> that adds the
 294     <constant>NULL</constant> sentinel automatically.</para>
 295
 296     <para><function>sd_bus_error_free()</function> will destroy
 297     resources held by <parameter>e</parameter>. The parameter itself
 298     will not be deallocated, and must be <citerefentry
 299     project='man-pages'><refentrytitle>free</refentrytitle><manvolnum>3</manvolnum></citerefentry>d
 300     by the caller if necessary. The function may also be called safely
 301     on unset errors (error structures with both fields set to <constant>NULL</constant>),
 302     in which case it performs no operation. This call will reset the
 303     error structure after freeing the data, so that all fields are set
 304     to <constant>NULL</constant>. The structure may be reused afterwards.</para>
 305   </refsect1>
 306
 307   <refsect1>
 308     <title>Reference ownership</title>
 309
 310     <para><structname>sd_bus_error</structname> is not reference-counted. Users should destroy resources held
 311     by it by calling <function>sd_bus_error_free()</function>. Usually, error structures are allocated on the
 312     stack or passed in as function parameters, but they may also be allocated dynamically, in which case it
 313     is the duty of the caller to <citerefentry
 314     project='man-pages'><refentrytitle>free</refentrytitle><manvolnum>3</manvolnum></citerefentry> the memory
 315     held by the structure itself after freeing its contents with
 316     <function>sd_bus_error_free()</function>.</para>
 317   </refsect1>
 318
 319   <refsect1>
 320     <title>Return Value</title>
 321
 322     <para>The functions <function>sd_bus_error_set()</function>, <function>sd_bus_error_setf()</function>,
 323     and <function>sd_bus_error_set_const()</function> always return <constant>0</constant> when the specified
 324     error value is <constant>NULL</constant>, and a negative errno-like value corresponding to the
 325     <parameter>name</parameter> parameter otherwise. The functions
 326     <function>sd_bus_error_set_errno()</function>, <function>sd_bus_error_set_errnof()</function> and
 327     <function>sd_bus_error_set_errnofv()</function>, return <constant>0</constant> when the specified error
 328     value is <constant>0</constant>, and a negative errno-like value corresponding to the
 329     <parameter>error</parameter> parameter otherwise. If an error occurs internally, one of the negative
 330     error values listed below will be returned. This allows those functions to be conveniently used in a
 331     <constant>return</constant> statement, see the example below.</para>
 332
 333     <para><function>sd_bus_error_get_errno()</function> returns
 334     <constant>false</constant> when <parameter>e</parameter> is
 335     <constant>NULL</constant>, and a positive errno value mapped from
 336     <parameter>e-&gt;name</parameter> otherwise.</para>
 337
 338     <para><function>sd_bus_error_copy()</function> and <function>sd_bus_error_move()</function> return a
 339     negative error value converted from the source error, and zero if the error has not been set.  This
 340     allows those functions to be conveniently used in a <constant>return</constant> statement, see the
 341     example below.</para>
 342
 343     <para><function>sd_bus_error_is_set()</function> returns a
 344     non-zero value when <parameter>e</parameter> and the
 345     <structfield>name</structfield> field are
 346     non-<constant>NULL</constant>, zero otherwise.</para>
 347
 348     <para><function>sd_bus_error_has_name()</function>, <function>sd_bus_error_has_names()</function>, and
 349     <function>sd_bus_error_has_names_sentinel()</function> return a non-zero value when <parameter>e</parameter> is
 350     non-<constant>NULL</constant> and the <structfield>name</structfield> field is equal to one of the given
 351     names, zero otherwise.</para>
 352
 353     <refsect2>
 354       <title>Errors</title>
 355
 356       <para>Return value may indicate the following problems in the invocation of the function itself:</para>
 357
 358       <variablelist>
 359         <varlistentry>
 360           <term><constant>-EINVAL</constant></term>
 361
 362           <listitem><para>Error was already set in the <structname>sd_bus_error</structname> structure when
 363           one the error-setting functions was called.</para></listitem>
 364         </varlistentry>
 365
 366         <varlistentry>
 367           <term><constant>-ENOMEM</constant></term>
 368
 369           <listitem><para>Memory allocation failed.</para></listitem>
 370         </varlistentry>
 371       </variablelist>
 372
 373       <para>On success, <function>sd_bus_error_set()</function>, <function>sd_bus_error_setf()</function>,
 374       <function>sd_bus_error_set_const()</function>, <function>sd_bus_error_set_errno()</function>,
 375       <function>sd_bus_error_set_errnof()</function>, <function>sd_bus_error_set_errnofv()</function>,
 376       <function>sd_bus_error_copy()</function>, and <function>sd_bus_error_move()</function> will return a
 377       negative converted <varname>errno</varname>-style value, or <constant>0</constant> if the error
 378       parameter is <constant>NULL</constant> or unset. D-Bus errors are converted to the integral
 379       <varname>errno</varname>-style value, and the mapping mechanism is extensible, see the discussion
 380       above. This effectively means that almost any negative <varname>errno</varname>-style value can be
 381       returned.</para>
 382     </refsect2>
 383   </refsect1>
 384
 385   <refsect1>
 386     <title>Examples</title>
 387
 388     <example>
 389       <title>Using the negative return value to propagate an error</title>
 390
 391       <programlisting><xi:include href="sd_bus_error-example.c" parse="text" /></programlisting>
 392     </example>
 393   </refsect1>
 394
 395   <xi:include href="libsystemd-pkgconfig.xml" />
 396
 397   <refsect1>
 398     <title>History</title>
 399     <para><function>sd_bus_error_free()</function>,
 400     <function>sd_bus_error_set()</function>,
 401     <function>sd_bus_error_setf()</function>,
 402     <function>sd_bus_error_set_const()</function>,
 403     <function>sd_bus_error_set_errno()</function>,
 404     <function>sd_bus_error_set_errnof()</function>,
 405     <function>sd_bus_error_get_errno()</function>,
 406     <function>sd_bus_error_copy()</function>,
 407     <function>sd_bus_error_is_set()</function>, and
 408     <function>sd_bus_error_has_name()</function> were added in version 209.</para>
 409     <para><function>sd_bus_error_set_errnofv()</function> was added in version 223.</para>
 410     <para><function>sd_bus_error_move()</function> was added in version 240.</para>
 411     <para><function>sd_bus_error_has_names_sentinel()</function> was added in version 247.</para>
 412     <para><function>sd_bus_error_setfv()</function> was added in version 252.</para>
 413   </refsect1>
 414
 415   <refsect1>
 416     <title>See Also</title>
 417
 418     <para><simplelist type="inline">
 419       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 420       <member><citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 421       <member><citerefentry><refentrytitle>sd-bus-errors</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 422       <member><citerefentry><refentrytitle>sd_bus_error_add_map</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 423       <member><citerefentry project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 424       <member><citerefentry project='man-pages'><refentrytitle>strerror_r</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 425     </simplelist></para>
 426   </refsect1>
 427
 428 </refentry>