channelvariables.tex

asterisk 是一个很有知名度开源软件

\subsection{Functions}

In 1.6 and above, we upgraded the \$[] expressions to handle floating point numbers.
Because of this, folks counting on integer behavior would be disrupted. To make
the same results possible, some rounding and integer truncation functions have been
added to the core of the Expr2 parser. Indeed, dialplan functions can be called from
\$[..] expressions without the \$\{...\} operators. The only trouble might be in the fact that
the arguments to these functions must be specified with a comma. If you try to call
the MATH function, for example, and try to say 3 + MATH(7*8), the expression parser will
evaluate 7*8 for you into 56, and the MATH function will most likely complain that its
input doesn't make any sense.

We also provide access to most of the floating point functions in the C library. (but not all of them).

While we don't expect someone to want to do Fourier analysis in the dialplan, we
don't want to preclude it, either.

Here is a list of the 'builtin' functions in Expr2. All other dialplan functions
are available by simply calling them (read-only). In other words, you don't need to
surround function calls in \$[...] expressions with \$\{...\}. Don't jump to conclusions,
though! -- you still need to wrap variable names in curly braces!

\begin{enumerate}
\item COS(x) x is in radians. Results vary from -1 to 1.
\item SIN(x) x is in radians. Results vary from -1 to 1.
\item TAN(x) x is in radians.
\item ACOS(x) x should be a value between -1 and 1.
\item ASIN(x) x should be a value between -1 and 1.
\item ATAN(x) returns the arc tangent in radians; between -PI/2 and PI/2.
\item ATAN2(x,y) returns a result resembling y/x, except that the signs of both args are used to determine the quadrant of the result. Its result is in radians, between -PI and PI.
\item POW(x,y) returns the value of x raised to the power of y.
\item SQRT(x) returns the square root of x.
\item FLOOR(x) rounds x down to the nearest integer.
\item CEIL(x) rounds x up to the nearest integer.
\item ROUND(x) rounds x to the nearest integer, but round halfway cases away from zero.
\item RINT(x) rounds x to the nearest integer, rounding halfway cases to the nearest even integer.
\item TRUNC(x) rounds x to the nearest integer not larger in absolute value.
\item REMAINDER(x,y) computes the remainder of dividing x by y. The return value is x - n*y, where n is the value x/y, rounded to the nearest integer. If this quotient is 1/2, it is rounded to the nearest even number.
\item EXP(x) returns e to the x power.
\item EXP2(x) returns 2 to the x power.
\item LOG(x) returns the natural logarithm of x.
\item LOG2(x) returns the base 2 log of x.
\item LOG10(x) returns the base 10 log of x.
\end{enumerate}

\subsection{Examples}

\begin{astlisting}
\begin{verbatim}
 "One Thousand Five Hundred" =~ "(T[^ ]+)"
   returns: Thousand

 "One Thousand Five Hundred" =~ "T[^ ]+"
   returns: 8

 "One Thousand Five Hundred" : "T[^ ]+"
   returns: 0

 "8015551212" : "(...)"
   returns: 801

 "3075551212":"...(...)"
   returns: 555

 ! "One Thousand Five Hundred" =~ "T[^ ]+"
   returns: 0 (because it applies to the string, which is non-null,
                    which it turns to "0", and then looks for the pattern
                    in the "0", and doesn't find it)

 !( "One Thousand Five Hundred" : "T[^ ]+" )
   returns: 1  (because the string doesn't start with a word starting
                     with T, so the match evals to 0, and the ! operator
                     inverts it to 1 ).

 2 + 8 / 2
   returns 6. (because of operator precedence; the division is done first, then the addition).

 2+8/2
   returns 6. Spaces aren't necessary.

(2+8)/2
   returns 5, of course.

(3+8)/2
   returns 5.5 now.

TRUNC((3+8)/2)
   returns 5.

FLOOR(2.5)
   returns 2

FLOOR(-2.5)
   returns -3

CEIL(2.5)
   returns 3.

CEIL(-2.5)
   returns -2.

ROUND(2.5)
   returns 3.

ROUND(3.5)
   returns 4.

ROUND(-2.5)
   returns -3

RINT(2.5)
   returns 2.

RINT(3.5)
   returns 4.

RINT(-2.5)
   returns -2.

RINT(-3.5)
   returns -4.

TRUNC(2.5)
   returns 2.

TRUNC(3.5)
   returns 3.

TRUNC(-3.5)
   returns -3.
\end{verbatim}
\end{astlisting}

Of course, all of the above examples use constants, but would work the
same if any of the numeric or string constants were replaced with a
variable reference \$\{CALLERID(num)\}, for instance.


\subsection{Numbers Vs. Strings}

Tokens consisting only of numbers are converted to 'long double' if possible, which
are from 80 bits to 128 bits depending on the OS, compiler, and hardware.
This means that overflows can occur when the
numbers get above 18 digits (depending on the number of bits involved).  Warnings will appear in the logs in this
case.

\subsection{Conditionals}

There is one conditional application - the conditional goto :
\begin{astlisting}
\begin{verbatim}
  exten => 1,2,GotoIf(condition?label1:label2)
\end{verbatim}
\end{astlisting}

If condition is true go to label1, else go to label2. Labels are interpreted
exactly as in the normal goto command.

"condition" is just a string. If the string is empty or "0", the condition
is considered to be false, if it's anything else, the condition is true.
This is designed to be used together with the expression syntax described
above, eg :

\begin{astlisting}
\begin{verbatim}
  exten => 1,2,GotoIf($[${CALLERID(all)} = 123456]?2,1:3,1)
\end{verbatim}
\end{astlisting}

Example of use :
\begin{astlisting}
\begin{verbatim}
exten => s,2,Set(vara=1)
exten => s,3,Set(varb=$[${vara} + 2])
exten => s,4,Set(varc=$[${varb} * 2])
exten => s,5,GotoIf($[${varc} = 6]?99,1:s,6)
\end{verbatim}
\end{astlisting}

\subsection{Parse Errors}

Syntax errors are now output with 3 lines.

If the extensions.conf file contains a line like:

\begin{astlisting}
\begin{verbatim}
exten => s,6,GotoIf($[ "${CALLERID(num)}"  = "3071234567" & &  "${CALLERID(name)}" : "Privacy Manager" ]?callerid-liar,s,1:s,7)
\end{verbatim}
\end{astlisting}

You may see an error in \path{/var/log/asterisk/messages} like this:
\begin{astlisting}
\begin{verbatim}
Jul 15 21:27:49 WARNING[1251240752]: ast_yyerror(): syntax error: parse error, unexpected TOK_AND, expecting TOK_MINUS or TOK_LP or TOKEN; Input:
"3072312154"  = "3071234567" & & "Steves Extension" : "Privacy Manager"
               ^
\end{verbatim}
\end{astlisting}

The log line tells you that a syntax error was encountered. It now
also tells you (in grand standard bison format) that it hit an "AND"
(\&) token unexpectedly, and that was hoping for for a MINUS (-), LP
(left parenthesis), or a plain token (a string or number).

The next line shows the evaluated expression, and the line after
that, the position of the parser in the expression when it became confused,
marked with the "\^" character.

\subsection{NULL Strings}
Testing to see if a string is null can be done in one of two different ways:
\begin{astlisting}
\begin{verbatim}
  exten => _XX.,1,GotoIf($["${calledid}" != ""]?3)
    or
  exten => _XX.,1,GotoIf($[foo${calledid} != foo]?3)
\end{verbatim}
\end{astlisting}

The second example above is the way suggested by the WIKI. It will
work as long as there are no spaces in the evaluated value.

The first way should work in all cases, and indeed, might now
be the safest way to handle this situation.

\subsection{Warning}

If you need to do complicated things with strings, asterisk expressions
is most likely NOT the best way to go about it. AGI scripts are an
excellent option to this need, and make available the full power of
whatever language you desire, be it Perl, C, C++, Cobol, RPG, Java,
Snobol, PL/I, Scheme, Common Lisp, Shell scripts, Tcl, Forth, Modula,
Pascal, APL, assembler, etc.

\subsection{Incompatabilities}

The asterisk expression parser has undergone some evolution. It is hoped
that the changes will be viewed as positive.

The "original" expression parser had a simple, hand-written scanner,
and a simple bison grammar. This was upgraded to a more involved bison
grammar, and a hand-written scanner upgraded to allow extra spaces,
and to generate better error diagnostics. This upgrade required bison
1.85, and part of the user community felt the pain of having to
upgrade their bison version.

The next upgrade included new bison and flex input files, and the makefile
was upgraded to detect current version of both flex and bison, conditionally
compiling and linking the new files if the versions of flex and bison would
allow it.

If you have not touched your extensions.conf files in a year or so, the
above upgrades may cause you some heartburn in certain circumstances, as
several changes have been made, and these will affect asterisk's behavior on
legacy extension.conf constructs.  The changes have been engineered
to minimize these conflicts, but there are bound to be problems.

The following list gives some (and most likely, not all) of areas
of possible concern with "legacy" extension.conf files:

\begin{enumerate}
\item Tokens separated by space(s).
   Previously, tokens were separated by spaces. Thus, ' 1 + 1 ' would evaluate
   to the value '2', but '1+1' would evaluate to the string '1+1'. If this
   behavior was depended on, then the expression evaluation will break. '1+1'
   will now evaluate to '2', and something is not going to work right.
   To keep such strings from being evaluated, simply wrap them in double
   quotes: '  "1+1" '

\item The colon operator. In versions previous to double quoting, the
   colon operator takes the right hand string, and using it as a
   regex pattern, looks for it in the left hand string. It is given
   an implicit \^ operator at the beginning, meaning the pattern
   will match only at the beginning of the left hand string.
   If the pattern or the matching string had double quotes around
   them, these could get in the way of the pattern match. Now,
   the wrapping double quotes are stripped from both the pattern
   and the left hand string before applying the pattern. This
   was done because it recognized that the new way of
   scanning the expression doesn't use spaces to separate tokens,
   and the average regex expression is full of operators that
   the scanner will recognize as expression operators. Thus, unless
   the pattern is wrapped in double quotes, there will be trouble.
   For instance,      \$\{VAR1\} : (Who$|$What*)+
   may have have worked before, but unless you wrap the pattern
   in double quotes now, look out for trouble! This is better:
         "\$\{VAR1\}" : "(Who$|$What*)+"
   and should work as previous.

\item Variables and Double Quotes
   Before these changes, if a variable's value contained one or more double
   quotes, it was no reason for concern. It is now!

\item LE, GE, NE operators removed. The code supported these operators,
   but they were not documented. The symbolic operators, $<$=, $>$=, and !=
   should be used instead.

\item  Added the unary '-' operator. So you can 3+ -4 and get -1.

\item  Added the unary '!' operator, which is a logical complement.
    Basically, if the string or number is null, empty, or '0',
    a '1' is returned. Otherwise a '0' is returned.

\item  Added the '=~' operator, just in case someone is just looking for
    match anywhere in the string. The only diff with the ':' is that
    match doesn't have to be anchored to the beginning of the string.

\item  Added the conditional operator  'expr1 ? true\_expr : false\_expr'
    First, all 3 exprs are evaluated, and if expr1 is false, the 'false\_expr'
    is returned as the result. See above for details.

\item  Unary operators '-' and '!' were made right associative.
\end{enumerate}

\subsection{Debugging Hints}

There are two utilities you can build to help debug the \$[ ] in
your extensions.conf file.

The first, and most simplistic, is to issue the command:
\begin{astlisting}
\begin{verbatim}
make testexpr2
\end{verbatim}
\end{astlisting}