tools lib traceevent: Man pages for get field value APIs

Create man pages for libtraceevent APIs:

  tep_get_any_field_val(),
  tep_get_common_field_val(),
  tep_get_field_val(),
  tep_get_field_raw()
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/linux-trace-devel/20190503091119.23399-19-tstoyanov@vmware.com
Link: http://lkml.kernel.org/r/20190510200108.885426878@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/Documentation/libtraceevent-field_get_val.txt

+libtraceevent(3)
+================
+
+NAME
+----
+tep_get_any_field_val, tep_get_common_field_val, tep_get_field_val,
+tep_get_field_raw - Get value of a field.
+
+SYNOPSIS
+--------
+[verse]
+--
+*#include <event-parse.h>*
+*#include <trace-seq.h>*
+
+int *tep_get_any_field_val*(struct trace_seq pass:[*]_s_, struct tep_event pass:[*]_event_, const char pass:[*]_name_, struct tep_record pass:[*]_record_, unsigned long long pass:[*]_val_, int _err_);
+int *tep_get_common_field_val*(struct trace_seq pass:[*]_s_, struct tep_event pass:[*]_event_, const char pass:[*]_name_, struct tep_record pass:[*]_record_, unsigned long long pass:[*]_val_, int _err_);
+int *tep_get_field_val*(struct trace_seq pass:[*]_s_, struct tep_event pass:[*]_event_, const char pass:[*]_name_, struct tep_record pass:[*]_record_, unsigned long long pass:[*]_val_, int _err_);
+void pass:[*]*tep_get_field_raw*(struct trace_seq pass:[*]_s_, struct tep_event pass:[*]_event_, const char pass:[*]_name_, struct tep_record pass:[*]_record_, int pass:[*]_len_, int _err_);
+--
+
+DESCRIPTION
+-----------
+These functions can be used to find a field and retrieve its value.
+
+The _tep_get_any_field_val()_ function searches in the _record_ for a field
+with _name_, part of the _event_. If the field is found, its value is stored in
+_val_. If there is an error and _err_ is not zero, then an error string is
+written into _s_.
+
+The _tep_get_common_field_val()_ function does the same as
+_tep_get_any_field_val()_, but searches only in the common fields. This works
+for any event as all events include the common fields.
+
+The _tep_get_field_val()_ function does the same as _tep_get_any_field_val()_,
+but searches only in the event specific fields.
+
+The _tep_get_field_raw()_ function searches in the _record_ for a field with
+_name_, part of the _event_. If the field is found, a pointer to where the field
+exists in the record's raw data is returned. The size of the data is stored in
+_len_. If there is an error and _err_ is not zero, then an error string is
+written into _s_.
+
+RETURN VALUE
+------------
+The _tep_get_any_field_val()_, _tep_get_common_field_val()_ and
+_tep_get_field_val()_ functions return 0 on success, or -1 in case of an error.
+
+The _tep_get_field_raw()_ function returns a pointer to field's raw data, and
+places the length of this data in _len_. In case of an error NULL is returned.
+
+EXAMPLE
+-------
+[source,c]
+--
+#include <event-parse.h>
+#include <trace-seq.h>
+...
+struct tep_handle *tep = tep_alloc();
+...
+struct tep_event *event = tep_find_event_by_name(tep, "kvm", "kvm_exit");
+...
+void process_record(struct tep_record *record)
+{
+	int len;
+	char *comm;
+	struct tep_event_format *event;
+	unsigned long long val;
+
+	event = tep_find_event_by_record(pevent, record);
+	if (event != NULL) {
+		if (tep_get_common_field_val(NULL, event, "common_type",
+					     record, &val, 0) == 0) {
+			/* Got the value of common type field */
+		}
+		if (tep_get_field_val(NULL, event, "pid", record, &val, 0) == 0) {
+			/* Got the value of pid specific field */
+		}
+		comm = tep_get_field_raw(NULL, event, "comm", record, &len, 0);
+		if (comm != NULL) {
+			/* Got a pointer to the comm event specific field */
+		}
+	}
+}
+--
+
+FILES
+-----
+[verse]
+--
+*event-parse.h*
+	Header file to include in order to have access to the library APIs.
+*trace-seq.h*
+	Header file to include in order to have access to trace sequences
+	related APIs. Trace sequences are used to allow a function to call
+	several other functions to create a string of data to use.
+*-ltraceevent*
+	Linker switch to add when building a program that uses the library.
+--
+
+SEE ALSO
+--------
+_libtraceevent(3)_, _trace-cmd(1)_
+
+AUTHOR
+------
+[verse]
+--
+*Steven Rostedt* <rostedt@goodmis.org>, author of *libtraceevent*.
+*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>, author of this man page.
+--
+REPORTING BUGS
+--------------
+Report bugs to  <linux-trace-devel@vger.kernel.org>
+
+LICENSE
+-------
+libtraceevent is Free Software licensed under the GNU LGPL 2.1
+
+RESOURCES
+---------
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git