The libxo API¶

This section gives details about the functions in libxo, how to call them, and the actions they perform.

Handles¶

libxo uses “handles” to control its rendering functionality. The handle contains state and buffered data, as well as callback functions to process data.

Handles give an abstraction for libxo that encapsulates the state of a stream of output. Handles have the data type “xo_handle_t” and are opaque to the caller.

The library has a default handle that is automatically initialized. By default, this handle will send text style output (XO_STYLE_TEXT) to standard output. The xo_set_style and xo_set_flags functions can be used to change this behavior.

For the typical command that is generating output on standard output, there is no need to create an explicit handle, but they are available when needed, e.g., for daemons that generate multiple streams of output.

Many libxo functions take a handle as their first parameter; most that do not use the default handle. Any function taking a handle can be passed NULL to access the default handle. For the convenience of callers, the libxo library includes handle-less functions that implicitly use the default handle.

For example, the following are equivalent:

xo_emit("test");
xo_emit_h(NULL, "test");

Handles are created using xo_create and destroy using xo_destroy.

xo_create¶

xo_handle_t *xo_create(xo_style_t style, xo_xof_flags_t flags)¶

The xo_create function allocates a new handle which can be passed to further libxo function calls. The xo_handle_t structure is opaque.

Parameters

style (xo_style_t) – Output style (XO_STYLE_*)
flags (xo_xof_flags_t) – Flags for this handle (XOF_*)

Returns

New libxo handle

Return type

xo_handle_t *

EXAMPLE:
    xo_handle_t *xop = xo_create(XO_STYLE_JSON, XOF_WARN | XOF_PRETTY);
    ....
    xo_emit_h(xop, "testing\n");

xo_create_to_file¶

xo_handle_t *xo_create_to_file(FILE *fp, unsigned style, unsigned flags)¶

The xo_create_to_file function is aconvenience function is provided for situations when output should be written to a different file, rather than the default of standard output.

The XOF_CLOSE_FP flag can be set on the returned handle to trigger a call to fclose() for the FILE pointer when the handle is destroyed, avoiding the need for the caller to perform this task.

Parameters

fp (FILE *) – FILE to use as base for this handle
style (xo_style_t) – Output style (XO_STYLE_*)
flags (xo_xof_flags_t) – Flags for this handle (XOF_*)

Returns

New libxo handle

Return type

xo_handle_t *

xo_set_writer¶

void xo_set_writer(xo_handle_t *xop, void *opaque, xo_write_func_t write_func, xo_close_func_t close_func, xo_flush_func_t flush_func)¶

The xo_set_writer function allows custom functions which can tailor how libxo writes data. The opaque argument is recorded and passed back to the functions, allowing the function to acquire context information. The write_func function writes data to the output stream. The close_func function can release this opaque data and any other resources as needed. The flush_func function is called to flush buffered data associated with the opaque object.

Parameters

xop (xo_handle_t *) – Handle to modify (or NULL for default handle)
opaque (void *) – Pointer to opaque data passed to the given functions
write_func (xo_write_func_t) – New write function
close_func (xo_close_func_t) – New close function
flush_func (xo_flush_func_t) – New flush function

Returns

void

xo_get_style¶

xo_style_t xo_get_style(xo_handle_t *xop)¶

Use the xo_get_style function to find the current output style for a given handle. To use the default handle, pass a NULL handle.

Parameters

xop (xo_handle_t *) – Handle to interrogate (or NULL for default handle)

Returns

Output style (XO_STYLE_*)

Return type

xo_style_t

EXAMPLE::
    style = xo_get_style(NULL);

Output Styles (XO_STYLE_*)¶

The libxo functions accept a set of output styles:

Flag

Description

XO_STYLE_TEXT

Traditional text output

XO_STYLE_XML

XML encoded data

XO_STYLE_JSON

JSON encoded data

XO_STYLE_HTML

HTML encoded data

The “XML”, “JSON”, and “HTML” output styles all use the UTF-8 character encoding. “TEXT” using locale-based encoding.

xo_set_style¶

void xo_set_style(xo_handle_t *xop, xo_style_t style)¶

The xo_set_style function is used to change the output style setting for a handle. To use the default handle, pass a NULL handle.

Parameters

xop (xo_handle_t *) – Handle to modify
style (xo_style_t) – Output style (XO_STYLE_*)

Returns

void

EXAMPLE:
    xo_set_style(NULL, XO_STYLE_XML);

xo_set_style_name¶

int xo_set_style_name(xo_handle_t *xop, const char *style)¶

The xo_set_style_name function can be used to set the style based on a name encoded as a string: The name can be any of the supported styles: “text”, “xml”, “json”, or “html”.

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
style (const char *) – Text name of the style

Returns

zero for success, non-zero for error

Return type

int

EXAMPLE:
    xo_set_style_name(NULL, "html");

xo_set_flags¶

void xo_set_flags(xo_handle_t *xop, xo_xof_flags_t flags)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
flags (xo_xof_flags_t) – Flags to add for the handle

Returns

void

Use the xo_set_flags function to turn on flags for a given libxo handle. To use the default handle, pass a NULL handle.

EXAMPLE:
    xo_set_flags(NULL, XOF_PRETTY | XOF_WARN);

Flags (XOF_*)¶

The set of valid flags include:

Flag

Description

XOF_CLOSE_FP

Close file pointer on xo_destroy

XOF_COLOR

Enable color and effects in output

XOF_COLOR_ALLOWED

Allow color/effect for terminal output

XOF_DTRT

Enable “do the right thing” mode

XOF_INFO

Display info data attributes (HTML)

XOF_KEYS

Emit the key attribute (XML)

XOF_NO_ENV

Do not use the LIBXO_OPTIONS env var

XOF_NO_HUMANIZE

Display humanization (TEXT, HTML)

XOF_PRETTY

Make “pretty printed” output

XOF_UNDERSCORES

Replaces hyphens with underscores

XOF_UNITS

Display units (XML, HMTL)

XOF_WARN

Generate warnings for broken calls

XOF_WARN_XML

Generate warnings in XML on stdout

XOF_XPATH

Emit XPath expressions (HTML)

XOF_COLUMNS

Force xo_emit to return columns used

XOF_FLUSH

Flush output after each xo_emit call

The XOF_CLOSE_FP flag will trigger the call of the close_func (provided via xo_set_writer) when the handle is destroyed.

The XOF_COLOR flag enables color and effects in output regardless of output device, while the XOF_COLOR_ALLOWED flag allows color and effects only if the output device is a terminal.

The XOF_PRETTY flag requests “pretty printing”, which will trigger the addition of indentation and newlines to enhance the readability of XML, JSON, and HTML output. Text output is not affected.

The XOF_WARN flag requests that warnings will trigger diagnostic output (on standard error) when the library notices errors during operations, or with arguments to functions. Without warnings enabled, such conditions are ignored.

Warnings allow developers to debug their interaction with libxo. The function xo_failure can used as a breakpoint for a debugger, regardless of whether warnings are enabled.

If the style is XO_STYLE_HTML, the following additional flags can be used:

Flag

Description

XOF_XPATH

Emit “data-xpath” attributes

XOF_INFO

Emit additional info fields

The XOF_XPATH flag enables the emission of XPath expressions detailing the hierarchy of XML elements used to encode the data field, if the XPATH style of output were requested.

The XOF_INFO flag encodes additional informational fields for HTML output. See Field Information (xo_info_t) for details.

If the style is XO_STYLE_XML, the following additional flags can be used:

Flag

Description

XOF_KEYS

Flag “key” fields for XML

The XOF_KEYS flag adds “key” attribute to the XML encoding for field definitions that use the “k” modifier. The key attribute has the value “key”:

  xo_emit("{k:name}", item);

XML:
    <name key="key">truck</name>

xo_clear_flags¶

void xo_clear_flags(xo_handle_t *xop, xo_xof_flags_t flags)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
flags (xo_xof_flags_t) – Flags to clear for the handle

Returns

void

Use the xo_clear_flags function to turn off the given flags in a specific handle. To use the default handle, pass a NULL handle.

xo_set_options¶

int xo_set_options(xo_handle_t *xop, const char *input)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
input (const char *) – string containing options to set

Returns

zero for success, non-zero for error

Return type

int

The xo_set_options function accepts a comma-separated list of output styles and modifier flags and enables them for a specific handle. The options are identical to those listed in Command-line Arguments. To use the default handle, pass a NULL handle.

xo_destroy¶

void xo_destroy(xo_handle_t *xop)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)

Returns

void

The xo_destroy function releases a handle and any resources it is using. Calling xo_destroy with a NULL handle will release any resources associated with the default handle.

Emitting Content (xo_emit)¶

The functions in this section are used to emit output.

The “fmt” argument is a string containing field descriptors as specified in Format Strings. The use of a handle is optional and NULL can be passed to access the internal “default” handle. See Handles.

The remaining arguments to xo_emit and xo_emit_h are a set of arguments corresponding to the fields in the format string. Care must be taken to ensure the argument types match the fields in the format string, since an inappropriate cast can ruin your day. The vap argument to xo_emit_hv points to a variable argument list that can be used to retrieve arguments via va_arg.

xo_ssize_t xo_emit(const char *fmt, ...)¶

Parameters

fmt – The format string, followed by zero or more arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

xo_ssize_t xo_emit_h(xo_handle_t *xop, const char *fmt, ...)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
fmt – The format string, followed by zero or more arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

xo_ssize_t xo_emit_hv(xo_handle_t *xop, const char *fmt, va_list vap)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
fmt – The format string
vap (va_list) – A set of variadic arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

Single Field Emitting Functions (xo_emit_field)¶

The functions in this section can also make output, but only make a single field at a time. These functions are intended to avoid the scenario where one would otherwise need to compose a format descriptors using snprintf. The individual parts of the format descriptor are passed in distinctly.

xo_ssize_t xo_emit_field(const char *rolmod, const char *contents, const char *fmt, const char *efmt, ...)¶

Parameters

rolmod (const char *) – A comma-separated list of field roles and field modifiers
contents (const char *) – The “contents” portion of the field description string
fmt (const char *) – Content format string
efmt (const char *) – Encoding format string, followed by additional arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

EXAMPLE::
    xo_emit_field("T", "Host name is ", NULL, NULL);
    xo_emit_field("V", "host-name", NULL, NULL, host-name);

xo_ssize_t xo_emit_field_h(xo_handle_t *xop, const char *rolmod, const char *contents, const char *fmt, const char *efmt, ...)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
rolmod (const char *) – A comma-separated list of field roles and field modifiers
contents (const char *) – The “contents” portion of the field description string
fmt (const char *) – Content format string
efmt (const char *) – Encoding format string, followed by additional arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

xo_ssize_t xo_emit_field_hv(xo_handle_t *xop, const char *rolmod, const char *contents, const char *fmt, const char *efmt, va_list vap)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)
rolmod (const char *) – A comma-separated list of field roles and field modifiers
contents (const char *) – The “contents” portion of the field description string
fmt (const char *) – Content format string
efmt (const char *) – Encoding format string
vap (va_list) – A set of variadic arguments

Returns

If XOF_COLUMNS is set, the number of columns used; otherwise the number of bytes emitted

Return type

xo_ssize_t

Attributes (xo_attr)¶

The functions in this section emit an XML attribute with the given name and value. This only affects the XML output style.

The name parameter give the name of the attribute to be encoded. The fmt parameter gives a printf-style format string used to format the value of the attribute using any remaining arguments, or the vap parameter passed to xo_attr_hv.

All attributes recorded via xo_attr are placed on the next container, instance, leaf, or leaf list that is emitted.

Since attributes are only emitted in XML, their use should be limited to meta-data and additional or redundant representations of data already emitted in other form.

xo_ssize_t xo_attr(const char *name, const char *fmt, ...)¶

Parameters

name (const char *) – Attribute name
fmt (const char *) – Attribute value, as variadic arguments

Returns

-1 for error, or the number of bytes in the formatted attribute value

Return type

xo_ssize_t

EXAMPLE:
    xo_attr("seconds", "%ld", (unsigned long) login_time);
    struct tm *tmp = localtime(login_time);
    strftime(buf, sizeof(buf), "%R", tmp);
    xo_emit("Logged in at {:login-time}\n", buf);
XML:
    <login-time seconds="1408336270">00:14</login-time>

xo_ssize_t xo_attr_h(xo_handle_t *xop, const char *name, const char *fmt, ...)¶

Parameters

xop (xo_handle_t *) – Handle for modify (or NULL for default handle)

The xo_attr_h function follows the conventions of xo_attr but adds an explicit libxo handle.

xo_ssize_t xo_attr_hv(xo_handle_t *xop, const char *name, const char *fmt, va_list vap)¶: The xo_attr_h function follows the conventions of xo_attr_h but replaced the variadic list with a variadic pointer.

Flushing Output (xo_flush)¶

xo_ssize_t xo_flush(void)¶

Returns: -1 for error, or the number of bytes generated
Return type: xo_ssize_t

libxo buffers data, both for performance and consistency, but also to allow for the proper function of various advanced features. At various times, the caller may wish to flush any data buffered within the library. The xo_flush call is used for this.

Calling xo_flush also triggers the flush function associated with the handle. For the default handle, this is equivalent to “fflush(stdio);”.

xo_ssize_t xo_flush_h(xo_handle_t *xop)¶

Parameters

xop (xo_handle_t *) – Handle for flush (or NULL for default handle)

Returns

-1 for error, or the number of bytes generated

Return type

xo_ssize_t

The xo_flush_h function follows the conventions of xo_flush, but adds an explicit libxo handle.

Finishing Output (xo_finish)¶

When the program is ready to exit or close a handle, a call to xo_finish or xo_finish_h is required. This flushes any buffered data, closes open libxo constructs, and completes any pending operations.

Calling this function is vital to the proper operation of libxo, especially for the non-TEXT output styles.

xo_ssize_t xo_finish(void)¶

Returns: -1 on error, or the number of bytes flushed
Return type: xo_ssize_t

xo_ssize_t xo_finish_h(xo_handle_t *xop)¶

Parameters

xop (xo_handle_t *) – Handle for finish (or NULL for default handle)

Returns

-1 on error, or the number of bytes flushed

Return type

xo_ssize_t

void xo_finish_atexit(void)¶: The xo_finish_atexit function is suitable for use with atexit(3) to ensure that xo_finish is called on the default handle when the application exits.

Emitting Hierarchy¶

libxo represents two types of hierarchy: containers and lists. A container appears once under a given parent where a list consists of instances that can appear multiple times. A container is used to hold related fields and to give the data organization and scope.

YANG Terminology

libxo uses terminology from YANG (RFC 7950), the data modeling language for NETCONF: container, list, leaf, and leaf-list.

For XML and JSON, individual fields appear inside hierarchies which provide context and meaning to the fields. Unfortunately, these encoding have a basic disconnect between how lists is similar objects are represented.

XML encodes lists as set of sequential elements:

<user>phil</user>
<user>pallavi</user>
<user>sjg</user>

JSON encodes lists using a single name and square brackets:

"user": [ "phil", "pallavi", "sjg" ]

This means libxo needs three distinct indications of hierarchy: one for containers of hierarchy appear only once for any specific parent, one for lists, and one for each item in a list.

Containers¶

A “container” is an element of a hierarchy that appears only once under any specific parent. The container has no value, but serves to contain and organize other nodes.

To open a container, call xo_open_container() or xo_open_container_h(). The former uses the default handle and the latter accepts a specific handle. To close a level, use the xo_close_container() or xo_close_container_h() functions.

Each open call must have a matching close call. If the XOF_WARN flag is set and the name given does not match the name of the currently open container, a warning will be generated.

xo_ssize_t xo_open_container(const char *name)¶

Parameters

name (const char *) – Name of the container

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The name parameter gives the name of the container, encoded in UTF-8. Since ASCII is a proper subset of UTF-8, traditional C strings can be used directly.

xo_ssize_t xo_open_container_h(xo_handle_t *xop, const char *name)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_open_container_h function adds a handle parameter.

xo_ssize_t xo_close_container(const char *name)¶

Parameters

name (const char *) – Name of the container

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

xo_ssize_t xo_close_container_h(xo_handle_t *xop, const char *name)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_close_container_h function adds a handle parameter.

Use the XOF_WARN flag to generate a warning if the name given on the close does not match the current open container.

For TEXT and HTML output, containers are not rendered into output text, though for HTML they are used to record an XPath value when the XOF_XPATH flag is set.

EXAMPLE:
    xo_open_container("top");
    xo_open_container("system");
    xo_emit("{:host-name/%s%s%s}", hostname,
            domainname ? "." : "", domainname ?: "");
    xo_close_container("system");
    xo_close_container("top");
TEXT:
    my-host.example.org
XML:
    <top>
      <system>
          <host-name>my-host.example.org</host-name>
      </system>
    </top>
JSON:
    "top" : {
      "system" : {
          "host-name": "my-host.example.org"
      }
    }
HTML:
    <div class="data"
         data-tag="host-name">my-host.example.org</div>

Lists and Instances¶

A “list” is set of one or more instances that appear under the same parent. The instances contain details about a specific object. One can think of instances as objects or records. A call is needed to open and close the list, while a distinct call is needed to open and close each instance of the list.

The name given to all calls must be identical, and it is strongly suggested that the name be singular, not plural, as a matter of style and usage expectations:

EXAMPLE:
    xo_open_list("item");

    for (ip = list; ip->i_title; ip++) {
        xo_open_instance("item");
        xo_emit("{L:Item} '{:name/%s}':\n", ip->i_title);
        xo_close_instance("item");
    }

    xo_close_list("item");

Getting the list and instance calls correct is critical to the proper generation of XML and JSON data.

Opening Lists¶

xo_ssize_t xo_open_list(const char *name)¶

Parameters

name (const char *) – Name of the list

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_open_list function open a list of instances.

xo_ssize_t xo_open_list_h(xo_handle_t *xop, const char *name)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

Closing Lists¶

xo_ssize_t xo_close_list(const char *name)¶

Parameters

name (const char *) – Name of the list

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_close_list function closes a list of instances.

xo_ssize_t xo_close_list_h(xo_handle_t *xop, const char *name)¶

Parameters

xop – Handle to use (or NULL for default handle)

Opening Instances¶

xo_ssize_t xo_open_instance(const char *name)¶

Parameters

name (const char *) – Name of the instance (same as the list name)

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_open_instance function open a single instance.

xo_ssize_t xo_open_instance_h(xo_handle_t *xop, const char *name)¶

Parameters

xop – Handle to use (or NULL for default handle)

Closing Instances¶

xo_ssize_t xo_close_instance(const char *name)¶

Parameters

name (const char *) – Name of the instance

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_close_instance function closes an open instance.

xo_ssize_t xo_close_instance_h(xo_handle_t *xop, const char *name)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_close_instance_h function adds a handle parameter.

EXAMPLE:
    xo_open_list("user");
    for (i = 0; i < num_users; i++) {
        xo_open_instance("user");
        xo_emit("{k:name}:{:uid/%u}:{:gid/%u}:{:home}\n",
                pw[i].pw_name, pw[i].pw_uid,
                pw[i].pw_gid, pw[i].pw_dir);
        xo_close_instance("user");
    }
    xo_close_list("user");
TEXT:
    phil:1001:1001:/home/phil
    pallavi:1002:1002:/home/pallavi
XML:
    <user>
        <name>phil</name>
        <uid>1001</uid>
        <gid>1001</gid>
        <home>/home/phil</home>
    </user>
    <user>
        <name>pallavi</name>
        <uid>1002</uid>
        <gid>1002</gid>
        <home>/home/pallavi</home>
    </user>
JSON:
    user: [
        {
            "name": "phil",
            "uid": 1001,
            "gid": 1001,
            "home": "/home/phil",
        },
        {
            "name": "pallavi",
            "uid": 1002,
            "gid": 1002,
            "home": "/home/pallavi",
        }
    ]

Markers¶

Markers are used to protect and restore the state of open hierarchy constructs (containers, lists, or instances). While a marker is open, no other open constructs can be closed. When a marker is closed, all constructs open since the marker was opened will be closed.

Markers use names which are not user-visible, allowing the caller to choose appropriate internal names.

In this example, the code whiffles through a list of fish, calling a function to emit details about each fish. The marker “fish-guts” is used to ensure that any constructs opened by the function are closed properly:

EXAMPLE:
    for (i = 0; fish[i]; i++) {
        xo_open_instance("fish");
        xo_open_marker("fish-guts");
        dump_fish_details(i);
        xo_close_marker("fish-guts");
    }

xo_ssize_t xo_open_marker(const char *name)¶

Parameters

name (const char *) – Name of the instance

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_open_marker function records the current state of open tags in order for xo_close_marker to close them at some later point.

xo_ssize_t xo_open_marker_h(const char *name)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_open_marker_h function adds a handle parameter.

xo_ssize_t xo_close_marker(const char *name)¶

Parameters

name (const char *) – Name of the instance

Returns

-1 on error, or the number of bytes generated

Return type

xo_ssize_t

The xo_close_marker function closes any open containers, lists, or instances as needed to return to the state recorded when xo_open_marker was called with the matching name.

xo_ssize_t xo_close_marker(const char *name)

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_close_marker_h function adds a handle parameter.

DTRT Mode¶

Some users may find tracking the names of open containers, lists, and instances inconvenient. libxo offers a “Do The Right Thing” mode, where libxo will track the names of open containers, lists, and instances so the close function can be called without a name. To enable DTRT mode, turn on the XOF_DTRT flag prior to making any other libxo output:

xo_set_flags(NULL, XOF_DTRT);

Each open and close function has a version with the suffix “_d”, which will close the open container, list, or instance:

xo_open_container_d("top");
...
xo_close_container_d();

This also works for lists and instances:

xo_open_list_d("item");
for (...) {
    xo_open_instance_d("item");
    xo_emit(...);
    xo_close_instance_d();
}
xo_close_list_d();

Note that the XOF_WARN flag will also cause libxo to track open containers, lists, and instances. A warning is generated when the name given to the close function and the name recorded do not match.

Support Functions¶

Parsing Command-line Arguments (xo_parse_args)¶

int xo_parse_args(int argc, char **argv)¶

Parameters

argc (int) – Number of arguments
argv – Array of argument strings

Returns

-1 on error, or the number of remaining arguments

Return type

int

The xo_parse_args function is used to process a program’s arguments. libxo-specific options are processed and removed from the argument list so the calling application does not need to process them. If successful, a new value for argc is returned. On failure, a message is emitted and -1 is returned:

argc = xo_parse_args(argc, argv);
if (argc < 0)
    exit(EXIT_FAILURE);

Following the call to xo_parse_args, the application can process the remaining arguments in a normal manner. See Command-line Arguments for a description of valid arguments.

xo_set_program¶

void xo_set_program(const char *name)¶

Parameters

name (const char *) – Name to use as the program name

Returns

void

The xo_set_program function sets the name of the program as reported by functions like xo_failure, xo_warn, xo_err, etc. The program name is initialized by xo_parse_args, but subsequent calls to xo_set_program can override this value:

EXAMPLE:
    xo_set_program(argv[0]);

Note that the value is not copied, so the memory passed to xo_set_program (and xo_parse_args) must be maintained by the caller.

xo_set_version¶

void xo_set_version(const char *version)¶

Parameters

name (const char *) – Value to use as the version string

Returns

void

The xo_set_version function records a version number to be emitted as part of the data for encoding styles (XML and JSON). This version number is suitable for tracking changes in the content, allowing a user of the data to discern which version of the data model is in use.

void xo_set_version_h(xo_handle_t *xop, const char *version)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)

The xo_set_version function adds a handle parameter.

Field Information (xo_info_t)¶

HTML data can include additional information in attributes that begin with “data-“. To enable this, three things must occur:

First the application must build an array of xo_info_t structures, one per tag. The array must be sorted by name, since libxo uses a binary search to find the entry that matches names from format instructions.

Second, the application must inform libxo about this information using the xo_set_info call:

typedef struct xo_info_s {
    const char *xi_name;    /* Name of the element */
    const char *xi_type;    /* Type of field */
    const char *xi_help;    /* Description of field */
} xo_info_t;

void xo_set_info (xo_handle_t *xop, xo_info_t *infop, int count);

Like other libxo calls, passing NULL for the handle tells libxo to use the default handle.

If the count is -1, libxo will count the elements of infop, but there must be an empty element at the end. More typically, the number is known to the application:

xo_info_t info[] = {
    { "in-stock", "number", "Number of items in stock" },
    { "name", "string", "Name of the item" },
    { "on-order", "number", "Number of items on order" },
    { "sku", "string", "Stock Keeping Unit" },
    { "sold", "number", "Number of items sold" },
};
int info_count = (sizeof(info) / sizeof(info[0]));
...
xo_set_info(NULL, info, info_count);

Third, the emission of info must be triggered with the XOF_INFO flag using either the xo_set_flags function or the “--libxo=info” command line argument.

The type and help values, if present, are emitted as the “data-type” and “data-help” attributes:

<div class="data" data-tag="sku" data-type="string"
     data-help="Stock Keeping Unit">GRO-000-533</div>

void xo_set_info(xo_handle_t *xop, xo_info_t *infop, int count)¶

Parameters

xop (xo_handle_t *) – Handle to use (or NULL for default handle)
infop (xo_info_t *) – Array of information structures

Returns

void

Memory Allocation¶

The xo_set_allocator function allows libxo to be used in environments where the standard realloc(3) and free(3) functions are not appropriate.

void xo_set_allocator(xo_realloc_func_t realloc_func, xo_free_func_t free_func)¶

Parameters

realloc_func (xo_realloc_func_t) – Allocation function
free_func (xo_free_func_t) – Free function

realloc_func should expect the same arguments as realloc(3) and return a pointer to memory following the same convention. free_func will receive the same argument as free(3) and should release it, as appropriate for the environment.

By default, the standard realloc(3) and free(3) functions are used.

LIBXO_OPTIONS¶

The environment variable “LIBXO_OPTIONS” can be set to a subset of libxo options, including:

color
flush
flush-line
no-color
no-humanize
no-locale
no-retain
pretty
retain
underscores
warn

For example, warnings can be enabled by:

% env LIBXO_OPTIONS=warn my-app

Since environment variables are inherited, child processes will have the same options, which may be undesirable, making the use of the “--libxo” command-line option preferable in most situations.

Errors, Warnings, and Messages¶

Many programs make use of the standard library functions err(3) and warn(3) to generate errors and warnings for the user. libxo wants to pass that information via the current output style, and provides compatible functions to allow this:

void xo_warn (const char *fmt, ...);
void xo_warnx (const char *fmt, ...);
void xo_warn_c (int code, const char *fmt, ...);
void xo_warn_hc (xo_handle_t *xop, int code,
                 const char *fmt, ...);
void xo_err (int eval, const char *fmt, ...);
void xo_errc (int eval, int code, const char *fmt, ...);
void xo_errx (int eval, const char *fmt, ...);

void xo_message (const char *fmt, ...);
void xo_message_c (int code, const char *fmt, ...);
void xo_message_hc (xo_handle_t *xop, int code,
                    const char *fmt, ...);
void xo_message_hcv (xo_handle_t *xop, int code,
                     const char *fmt, va_list vap);

These functions display the program name, a colon, a formatted message based on the arguments, and then optionally a colon and an error message associated with either errno or the code parameter:

EXAMPLE:
    if (open(filename, O_RDONLY) < 0)
        xo_err(1, "cannot open file '%s'", filename);

xo_error¶

void xo_error(const char *fmt, ...)¶

Parameters

fmt (const char *) – Format string

Returns

void

void xo_error_h(xo_handle_t *xop, const char *fmt, ...)¶

Parameters

xop (xo_handle_t *) – libxo handle pointer
fmt (const char *) – Format string

Returns

void

void xo_error_hv(xo_handle_t *xop, const char *fmt, va_list vap)¶

Parameters

xop (va_list) – libxo handle pointer
fmt (const char *) – Format string
vap – variadic arguments

Returns

void

void xo_errorn(const char *fmt, ...)¶

Parameters

fmt (const char *) – Format string

Returns

void

void xo_errorn_h(xo_handle_t *xop, const char *fmt, ...)¶

Parameters

xop (xo_handle_t *) – libxo handle pointer
fmt (const char *) – Format string

Returns

void

void xo_errorn_hv(xo_handle_t *xop, int need_newline, const char *fmt, va_list vap)¶

Parameters

xop (va_list) – libxo handle pointer
need_newline (int) – boolean indicating need for trailing newline
fmt (const char *) – Format string
vap – variadic arguments

Returns

void

The xo_error function can be used for generic errors that should be reported over the handle, rather than to stderr. The xo_error function behaves like xo_err for TEXT and HTML output styles, but puts the error into XML or JSON elements:

EXAMPLE::
    xo_error("Does not %s", "compute");
XML::
    <error><message>Does not compute</message></error>
JSON::
    "error": { "message": "Does not compute" }

The xo_error_h and xo_error_hv add a handle object and a variadic-ized parameter to the signature, respectively.

The xo_errorn function supplies a newline at the end the error message if the format string does not include one. The xo_errorn_h and xo_errorn_hv functions add a handle object and a variadic-ized parameter to the signature, respectively. The xo_errorn_hv function also adds a boolean to indicate the need for a trailing newline.

xo_no_setlocale¶

void xo_no_setlocale(void)¶: libxo automatically initializes the locale based on setting of the environment variables LC_CTYPE, LANG, and LC_ALL. The first of this list of variables is used and if none of the variables, the locale defaults to “UTF-8”. The caller may wish to avoid this behavior, and can do so by calling the xo_no_setlocale function.

Emitting syslog Messages¶

syslog is the system logging facility used throughout the unix world. Messages are sent from commands, applications, and daemons to a hierarchy of servers, where they are filtered, saved, and forwarded based on configuration behaviors.

syslog is an older protocol, originally documented only in source code. By the time RFC 3164 published, variation and mutation left the leading “<pri>” string as only common content. RFC 5424 defines a new version (version 1) of syslog and introduces structured data into the messages. Structured data is a set of name/value pairs transmitted distinctly alongside the traditional text message, allowing filtering on precise values instead of regular expressions.

These name/value pairs are scoped by a two-part identifier; an enterprise identifier names the party responsible for the message catalog and a name identifying that message. Enterprise IDs are defined by IANA, the Internet Assigned Numbers Authority.

Use the xo_set_syslog_enterprise_id function to set the Enterprise ID, as needed.

The message name should follow the conventions in What makes a good field name?, as should the fields within the message:

/* Both of these calls are optional */
xo_set_syslog_enterprise_id(32473);
xo_open_log("my-program", 0, LOG_DAEMON);

/* Generate a syslog message */
xo_syslog(LOG_ERR, "upload-failed",
          "error <%d> uploading file '{:filename}' "
          "as '{:target/%s:%s}'",
          code, filename, protocol, remote);

xo_syslog(LOG_INFO, "poofd-invalid-state",
          "state {:current/%u} is invalid {:connection/%u}",
          state, conn);

The developer should be aware that the message name may be used in the future to allow access to further information, including documentation. Care should be taken to choose quality, descriptive names.

Priority, Facility, and Flags¶

The xo_syslog, xo_vsyslog, and xo_open_log functions accept a set of flags which provide the priority of the message, the source facility, and some additional features. These values are OR’d together to create a single integer argument:

xo_syslog(LOG_ERR | LOG_AUTH, "login-failed",
         "Login failed; user '{:user}' from host '{:address}'",
         user, addr);

These values are defined in <syslog.h>.

The priority value indicates the importance and potential impact of each message:

Priority

Description

LOG_EMERG

A panic condition, normally broadcast to all users

LOG_ALERT

A condition that should be corrected immediately

LOG_CRIT

Critical conditions

LOG_ERR

Generic errors

LOG_WARNING

Warning messages

LOG_NOTICE

Non-error conditions that might need special handling

LOG_INFO

Informational messages

LOG_DEBUG

Developer-oriented messages

The facility value indicates the source of message, in fairly generic terms:

Facility

Description

LOG_AUTH

The authorization system (e.g. login(1))

LOG_AUTHPRIV

As LOG_AUTH, but logged to a privileged file

LOG_CRON

The cron daemon: cron(8)

LOG_DAEMON

System daemons, not otherwise explicitly listed

LOG_FTP

The file transfer protocol daemons

LOG_KERN

Messages generated by the kernel

LOG_LPR

The line printer spooling system

LOG_MAIL

The mail system

LOG_NEWS

The network news system

LOG_SECURITY

Security subsystems, such as ipfw(4)

LOG_SYSLOG

Messages generated internally by syslogd(8)

LOG_USER

Messages generated by user processes (default)

LOG_UUCP

The uucp system

LOG_LOCAL0..7

Reserved for local use

In addition to the values listed above, xo_open_log accepts a set of addition flags requesting specific logging behaviors:

Flag

Description

LOG_CONS

If syslogd fails, attempt to write to /dev/console

LOG_NDELAY

Open the connection to syslogd(8) immediately

LOG_PERROR

Write the message also to standard error output

LOG_PID

Log the process id with each message

xo_syslog¶

void xo_syslog(int pri, const char *name, const char *fmt, ...)¶

Parameters

pri (int) – syslog priority
name (const char *) – Name of the syslog event
fmt (const char *) – Format string, followed by arguments

Returns

void

Use the xo_syslog function to generate syslog messages by calling it with a log priority and facility, a message name, a format string, and a set of arguments. The priority/facility argument are discussed above, as is the message name.

The format string follows the same conventions as xo_emit’s format string, with each field being rendered as an SD-PARAM pair:

xo_syslog(LOG_ERR, "poofd-missing-file",
          "'{:filename}' not found: {:error/%m}", filename);

... [poofd-missing-file@32473 filename="/etc/poofd.conf"
      error="Permission denied"] '/etc/poofd.conf' not
      found: Permission denied

Support functions¶

xo_vsyslog¶

void xo_vsyslog(int pri, const char *name, const char *fmt, va_list vap)¶

Parameters

pri (int) – syslog priority
name (const char *) – Name of the syslog event
fmt (const char *) – Format string
vap (va_list) – Variadic argument list

Returns

void

xo_vsyslog is identical in function to xo_syslog, but takes the set of arguments using a va_list:

EXAMPLE:
    void
    my_log (const char *name, const char *fmt, ...)
    {
        va_list vap;
        va_start(vap, fmt);
        xo_vsyslog(LOG_ERR, name, fmt, vap);
        va_end(vap);
    }

xo_open_log¶

void xo_open_log(const char *ident, int logopt, int facility)¶

Parameters

indent (const char *) –
logopt (int) – Bit field containing logging options
facility (int) –

Returns

void

xo_open_log functions similar to openlog(3), allowing customization of the program name, the log facility number, and the additional option flags described in Priority, Facility, and Flags.

xo_close_log¶

void xo_close_log(void)¶: The xo_close_log function is similar to closelog(3), closing the log file and releasing any associated resources.

xo_set_logmask¶

int xo_set_logmask(int maskpri)¶

Parameters

maskpri (int) – the log priority mask

Returns

The previous log priority mask

The xo_set_logmask function is similar to setlogmask(3), restricting the set of generated log event to those whose associated bit is set in maskpri. Use LOG_MASK(pri) to find the appropriate bit, or LOG_UPTO(toppri) to create a mask for all priorities up to and including toppri:

EXAMPLE:
    setlogmask(LOG_UPTO(LOG_WARN));

xo_set_syslog_enterprise_id¶

void xo_set_syslog_enterprise_id(unsigned short eid)¶

Use the xo_set_syslog_enterprise_id to supply a platform- or application-specific enterprise id. This value is used in any future syslog messages.

Ideally, the operating system should supply a default value via the “kern.syslog.enterprise_id” sysctl value. Lacking that, the application should provide a suitable value.

Enterprise IDs are administered by IANA, the Internet Assigned Number Authority. The complete list is EIDs on their web site:

https://www.iana.org/assignments/enterprise-numbers/enterprise-numbers

New EIDs can be requested from IANA using the following page:

http://pen.iana.org/pen/PenApplication.page

Each software development organization that defines a set of syslog messages should register their own EID and use that value in their software to ensure that messages can be uniquely identified by the combination of EID + message name.

Creating Custom Encoders¶

The number of encoding schemes in current use is staggering, with new and distinct schemes appearing daily. While libxo provide XML, JSON, HMTL, and text natively, there are requirements for other encodings.

Rather than bake support for all possible encoders into libxo, the API allows them to be defined externally. libxo can then interfaces with these encoding modules using a simplistic API. libxo processes all functions calls, handles state transitions, performs all formatting, and then passes the results as operations to a customized encoding function, which implements specific encoding logic as required. This means your encoder doesn’t need to detect errors with unbalanced open/close operations but can rely on libxo to pass correct data.

By making a simple API, libxo internals are not exposed, insulating the encoder and the library from future or internal changes.

The three elements of the API are:

loading
initialization
operations

The following sections provide details about these topics.

libxo source contains an encoder for Concise Binary Object Representation, aka CBOR (RFC 7049), which can be used as an example for the API for other encoders.

Loading Encoders¶

Encoders can be registered statically or discovered dynamically. Applications can choose to call the xo_encoder_register function to explicitly register encoders, but more typically they are built as shared libraries, placed in the libxo/extensions directory, and loaded based on name. libxo looks for a file with the name of the encoder and an extension of “.enc”. This can be a file or a symlink to the shared library file that supports the encoder:

% ls -1 lib/libxo/extensions/*.enc
lib/libxo/extensions/cbor.enc
lib/libxo/extensions/test.enc

Encoder Initialization¶

Each encoder must export a symbol used to access the library, which must have the following signature:

int xo_encoder_library_init (XO_ENCODER_INIT_ARGS);

XO_ENCODER_INIT_ARGS is a macro defined in “xo_encoder.h” that defines an argument called “arg”, a pointer of the type xo_encoder_init_args_t. This structure contains two fields:

xei_version is the version number of the API as implemented within libxo. This version is currently as 1 using XO_ENCODER_VERSION. This number can be checked to ensure compatibility. The working assumption is that all versions should be backward compatible, but each side may need to accurately know the version supported by the other side. xo_encoder_library_init can optionally check this value, and must then set it to the version number used by the encoder, allowing libxo to detect version differences and react accordingly. For example, if version 2 adds new operations, then libxo will know that an encoding library that set xei_version to 1 cannot be expected to handle those new operations.
xei_handler must be set to a pointer to a function of type xo_encoder_func_t, as defined in “xo_encoder.h”. This function takes a set of parameters: - xop is a pointer to the opaque xo_handle_t structure - op is an integer representing the current operation - name is a string whose meaning differs by operation - value is a string whose meaning differs by operation - private is an opaque structure provided by the encoder

Additional arguments may be added in the future, so handler functions should use the XO_ENCODER_HANDLER_ARGS macro. An appropriate “extern” declaration is provided to help catch errors.

Once the encoder initialization function has completed processing, it should return zero to indicate that no error has occurred. A non-zero return code will cause the handle initialization to fail.

Operations¶

The encoder API defines a set of operations representing the processing model of libxo. Content is formatted within libxo, and callbacks are made to the encoder’s handler function when data is ready to be processed:

Operation

Meaning (Base function)

XO_OP_CREATE

Called when the handle is created

XO_OP_OPEN_CONTAINER

Container opened (xo_open_container)

XO_OP_CLOSE_CONTAINER

Container closed (xo_close_container)

XO_OP_OPEN_LIST

List opened (xo_open_list)

XO_OP_CLOSE_LIST

List closed (xo_close_list)

XO_OP_OPEN_LEAF_LIST

Leaf list opened (xo_open_leaf_list)

XO_OP_CLOSE_LEAF_LIST

Leaf list closed (xo_close_leaf_list)

XO_OP_OPEN_INSTANCE

Instance opened (xo_open_instance)

XO_OP_CLOSE_INSTANCE

Instance closed (xo_close_instance)

XO_OP_STRING

Field with Quoted UTF-8 string

XO_OP_CONTENT

Field with content

XO_OP_FINISH

Finish any pending output

XO_OP_FLUSH

Flush any buffered output

XO_OP_DESTROY

Clean up resources

XO_OP_ATTRIBUTE

An attribute name/value pair

XO_OP_VERSION

A version string

For all the open and close operations, the name parameter holds the name of the construct. For string, content, and attribute operations, the name parameter is the name of the field and the value parameter is the value. “string” are differentiated from “content” to allow differing treatment of true, false, null, and numbers from real strings, though content values are formatted as strings before the handler is called. For version operations, the value parameter contains the version.

All strings are encoded in UTF-8.

Flag	Description
XO_STYLE_TEXT	Traditional text output
XO_STYLE_XML	XML encoded data
XO_STYLE_JSON	JSON encoded data
XO_STYLE_HTML	HTML encoded data

Flag	Description
XOF_CLOSE_FP	Close file pointer on `xo_destroy`
XOF_COLOR	Enable color and effects in output
XOF_COLOR_ALLOWED	Allow color/effect for terminal output
XOF_DTRT	Enable “do the right thing” mode
XOF_INFO	Display info data attributes (HTML)
XOF_KEYS	Emit the key attribute (XML)
XOF_NO_ENV	Do not use the LIBXO_OPTIONS env var
XOF_NO_HUMANIZE	Display humanization (TEXT, HTML)
XOF_PRETTY	Make “pretty printed” output
XOF_UNDERSCORES	Replaces hyphens with underscores
XOF_UNITS	Display units (XML, HMTL)
XOF_WARN	Generate warnings for broken calls
XOF_WARN_XML	Generate warnings in XML on stdout
XOF_XPATH	Emit XPath expressions (HTML)
XOF_COLUMNS	Force xo_emit to return columns used
XOF_FLUSH	Flush output after each `xo_emit` call

Flag	Description
XOF_XPATH	Emit “data-xpath” attributes
XOF_INFO	Emit additional info fields

Priority	Description
LOG_EMERG	A panic condition, normally broadcast to all users
LOG_ALERT	A condition that should be corrected immediately
LOG_CRIT	Critical conditions
LOG_ERR	Generic errors
LOG_WARNING	Warning messages
LOG_NOTICE	Non-error conditions that might need special handling
LOG_INFO	Informational messages
LOG_DEBUG	Developer-oriented messages

Facility	Description
LOG_AUTH	The authorization system (e.g. login(1))
LOG_AUTHPRIV	As LOG_AUTH, but logged to a privileged file
LOG_CRON	The cron daemon: cron(8)
LOG_DAEMON	System daemons, not otherwise explicitly listed
LOG_FTP	The file transfer protocol daemons
LOG_KERN	Messages generated by the kernel
LOG_LPR	The line printer spooling system
LOG_MAIL	The mail system
LOG_NEWS	The network news system
LOG_SECURITY	Security subsystems, such as ipfw(4)
LOG_SYSLOG	Messages generated internally by syslogd(8)
LOG_USER	Messages generated by user processes (default)
LOG_UUCP	The uucp system
LOG_LOCAL0..7	Reserved for local use

Flag	Description
LOG_CONS	If syslogd fails, attempt to write to /dev/console
LOG_NDELAY	Open the connection to syslogd(8) immediately
LOG_PERROR	Write the message also to standard error output
LOG_PID	Log the process id with each message

Operation	Meaning (Base function)
XO_OP_CREATE	Called when the handle is created
XO_OP_OPEN_CONTAINER	Container opened (xo_open_container)
XO_OP_CLOSE_CONTAINER	Container closed (xo_close_container)
XO_OP_OPEN_LIST	List opened (xo_open_list)
XO_OP_CLOSE_LIST	List closed (xo_close_list)
XO_OP_OPEN_LEAF_LIST	Leaf list opened (xo_open_leaf_list)
XO_OP_CLOSE_LEAF_LIST	Leaf list closed (xo_close_leaf_list)
XO_OP_OPEN_INSTANCE	Instance opened (xo_open_instance)
XO_OP_CLOSE_INSTANCE	Instance closed (xo_close_instance)
XO_OP_STRING	Field with Quoted UTF-8 string
XO_OP_CONTENT	Field with content
XO_OP_FINISH	Finish any pending output
XO_OP_FLUSH	Flush any buffered output
XO_OP_DESTROY	Clean up resources
XO_OP_ATTRIBUTE	An attribute name/value pair
XO_OP_VERSION	A version string