\lt_var{EXPR} can contain references to the payload fields
of \lt_var{E} and to the current
\link #lttng_event_context_type context\endlink fields.

The expected syntax of \lt_var{EXPR} is similar to the syntax
of a C language conditional expression (an expression
which an \c if statement can evaluate), but there are a few
differences:

- A <code><em>NAME</em></code> expression identifies an event
  payload field named <code><em>NAME</em></code> (a
  C&nbsp;identifier).

  Use the C&nbsp;language dot and square bracket notations to
  access nested structure and array/sequence fields. You can
  only use a constant, positive integer number within square
  brackets. If the index is out of bounds, then
  \lt_var{EXPR} is \b false.

  The value of an enumeration field is an integer.

  When a field expression doesn't exist, \lt_var{EXPR} is
  \b false.

  Examples: <code>my_field</code>, <code>target_cpu</code>,
  <code>seq[7]</code>, <code>msg.user[1].data[2][17]</code>.

- A <code>$ctx.<em>TYPE</em></code> expression identifies the
  statically-known context field having the type
  <code><em>TYPE</em></code> (a C&nbsp;identifier).

  <code><em>TYPE</em></code> can be any of the
  statically known names in the “Field name” column
  of the table of #lttng_event_context_type (that is, excluding the
  #LTTNG_EVENT_CONTEXT_PERF_CPU_COUNTER,
  #LTTNG_EVENT_CONTEXT_PERF_THREAD_COUNTER,
  and #LTTNG_EVENT_CONTEXT_APP_CONTEXT rows).

  When a field expression doesn't exist, \lt_var{EXPR} is \b
  false.

  Examples: <code>$ctx.prio</code>,
  <code>$ctx.gid</code>,
  <code>$ctx.preemptible</code>.

- A <code>$app.<em>PROVIDER</em>:<em>TYPE</em></code>
  expression identifies the application-specific context field
  having the type <code><em>TYPE</em></code> (a
  C&nbsp;identifier) from the provider
  <code><em>PROVIDER</em></code> (a C&nbsp;identifier).

  When a field expression doesn't exist, \lt_var{EXPR} is \b
  false.

  Example: <code>$app.server:cur_user</code>.

- Compare strings, either string fields or string literals
  (double-quoted), with the <code>==</code> and
  <code>!=</code> operators.

  When comparing to a string literal, the <code>*</code>
  character means “match anything”. To match a literal
  <code>*</code> character, use <code>\\*</code>.

  Examples: <code>my_field&nbsp;==&nbsp;"user34"</code>,
  <code>my_field&nbsp;==&nbsp;my_other_field</code>,
  <code>my_field&nbsp;==&nbsp;"192.168.*"</code>.

- The
  <a href="https://en.wikipedia.org/wiki/Order_of_operations">precedence table</a>
  of the operators which are supported in
  \lt_var{EXPR} is as follows. In this table, the highest
  precedence is&nbsp;1:

  <table>
    <tr>
      <th>Precedence
      <th>Operator
      <th>Description
      <th>Associativity
    <tr>
      <td>1
      <td><code>-</code>
      <td>Unary minus
      <td>Right-to-left
    <tr>
      <td>1
      <td><code>+</code>
      <td>Unary plus
      <td>Right-to-left
    <tr>
      <td>1
      <td><code>!</code>
      <td>Logical NOT
      <td>Right-to-left
    <tr>
      <td>1
      <td><code>~</code>
      <td>Bitwise NOT
      <td>Right-to-left
    <tr>
      <td>2
      <td><code>&lt;&lt;</code>
      <td>Bitwise left shift
      <td>Left-to-right
    <tr>
      <td>2
      <td><code>&gt;&gt;</code>
      <td>Bitwise right shift
      <td>Left-to-right
    <tr>
      <td>3
      <td><code>&amp;</code>
      <td>Bitwise AND
      <td>Left-to-right
    <tr>
      <td>4
      <td><code>^</code>
      <td>Bitwise XOR
      <td>Left-to-right
    <tr>
      <td>5
      <td><code>|</code>
      <td>Bitwise OR
      <td>Left-to-right
    <tr>
      <td>6
      <td><code>&lt;</code>
      <td>Less than
      <td>Left-to-right
    <tr>
      <td>6
      <td><code>&lt;=</code>
      <td>Less than or equal to
      <td>Left-to-right
    <tr>
      <td>6
      <td><code>&gt;</code>
      <td>Greater than
      <td>Left-to-right
    <tr>
      <td>6
      <td><code>&gt;=</code>
      <td>Greater than or equal to
      <td>Left-to-right
    <tr>
      <td>7
      <td><code>==</code>
      <td>Equal to
      <td>Left-to-right
    <tr>
      <td>7
      <td><code>!=</code>
      <td>Not equal to
      <td>Left-to-right
    <tr>
      <td>8
      <td><code>&amp;&amp;</code>
      <td>Logical AND
      <td>Left-to-right
    <tr>
      <td>9
      <td><code>||</code>
      <td>Logical OR
      <td>Left-to-right
  </table>

  Parentheses are supported to bypass the default order.

  @attention
      Unlike the C&nbsp;language, the bitwise AND and OR
      operators (<code>&amp;</code> and <code>|</code>) in
      \lt_var{EXPR} take precedence over relational
      operators (<code>&lt;&lt;</code>, <code>&lt;=</code>,
      <code>&gt;</code>, <code>&gt;=</code>, <code>==</code>,
      and <code>!=</code>). This means the expression
      <code>2&nbsp;&&nbsp;2&nbsp;==&nbsp;2</code>
      is \b true while the equivalent C&nbsp;expression
      is \b false.

  The arithmetic operators are :not: supported.

  LTTng first casts all integer constants and fields to signed
  64-bit integers. The representation of negative integers is
  two's complement. This means that, for example, the signed
  8-bit integer field 0xff (-1) becomes 0xffffffffffffffff
  (still&nbsp;-1) once casted.

  Before a bitwise operator is applied, LTTng casts all its
  operands to unsigned 64-bit integers, and then casts the
  result back to a signed 64-bit integer. For the bitwise NOT
  operator, it's the equivalent of this C&nbsp;expression:

  @code
  (int64_t) ~((uint64_t) val)
  @endcode

  For the binary bitwise operators, it's the equivalent of those
  C&nbsp;expressions:

  @code
  (int64_t) ((uint64_t) lhs >> (uint64_t) rhs)
  (int64_t) ((uint64_t) lhs << (uint64_t) rhs)
  (int64_t) ((uint64_t) lhs & (uint64_t) rhs)
  (int64_t) ((uint64_t) lhs ^ (uint64_t) rhs)
  (int64_t) ((uint64_t) lhs | (uint64_t) rhs)
  @endcode

  If the right-hand side of a bitwise shift operator
  (<code>&lt;&lt;</code> and <code>&gt;&gt;</code>) is not in
  the [0,&nbsp;63] range, then \lt_var{EXPR} is \b false.

@note
    See \ref api_proc_filter to allow or disallow processes to
    record LTTng events based on their attributes
    instead of using equivalent statically-known context
    fields in \lt_var{EXPR} like <code>$ctx.pid</code>.
    The former method is much more efficient.

\lt_var{EXPR} examples:

@code{.unparsed}
msg_id == 23 && size >= 2048
@endcode

@code{.unparsed}
$ctx.procname == "lttng*" && (!flag || poel < 34)
@endcode

@code{.unparsed}
$app.my_provider:my_context == 17.34e9 || some_enum >= 14
@endcode

@code{.unparsed}
$ctx.cpu_id == 2 && filename != "*.log"
@endcode

@code{.unparsed}
eax_reg & 0xff7 == 0x240 && x[4] >> 12 <= 0x1234
@endcode