PERLCALL(1) Perl Programmers Reference Guide PERLCALL(1)NAMEperlcall - Perl calling conventions from C
DESCRIPTION
The purpose of this document is to show you how to call
Perl subroutines directly from C, i.e., how to write call_
backs.
Apart from discussing the C interface provided by Perl for
writing callbacks the document uses a series of examples
to show how the interface actually works in practice. In
addition some techniques for coding callbacks are covered.
Examples where callbacks are necessary include
An Error Handler
You have created an XSUB interface to an applica
tion's C API.
A fairly common feature in applications is to allow
you to define a C function that will be called when
ever something nasty occurs. What we would like is to
be able to specify a Perl subroutine that will be
called instead.
An Event Driven Program
The classic example of where callbacks are used is
when writing an event driven program like for an X
windows application. In this case you register func
tions to be called whenever specific events occur,
e.g., a mouse button is pressed, the cursor moves
into a window or a menu item is selected.
Although the techniques described here are applicable when
embedding Perl in a C program, this is not the primary
goal of this document. There are other details that must
be considered and are specific to embedding Perl. For
details on embedding Perl in C refer to the perlembed man
page.
Before you launch yourself head first into the rest of
this document, it would be a good idea to have read the
following two documents - the perlxs manpage and the
perlguts manpage.
THE CALL_ FUNCTIONS
Although this stuff is easier to explain using examples,
you first need be aware of a few important definitions.
Perl has a number of C functions that allow you to call
Perl subroutines. They are
I32 call_sv(SV* sv, I32 flags) ;
I32 call_pv(char *subname, I32 flags) ;
I32 call_method(char *methname, I32 flags) ;
I32 call_argv(char *subname, I32 flags, register char **argv) ;
The key function is call_sv. All the other functions are
fairly simple wrappers which make it easier to call Perl
subroutines in special cases. At the end of the day they
will all call call_sv to invoke the Perl subroutine.
All the call_* functions have a "flags" parameter which is
used to pass a bit mask of options to Perl. This bit mask
operates identically for each of the functions. The set
tings available in the bit mask are discussed in the FLAG
VALUES entry elsewhere in this document.
Each of the functions will now be discussed in turn.
call_sv
call_sv takes two parameters, the first, "sv", is an
SV*. This allows you to specify the Perl subroutine
to be called either as a C string (which has first
been converted to an SV) or a reference to a subrou
tine. The section, Using call_sv, shows how you can
make use of call_sv.
call_pv
The function, call_pv, is similar to call_sv except
it expects its first parameter to be a C char* which
identifies the Perl subroutine you want to call,
e.g., "call_pv("fred", 0)". If the subroutine you
want to call is in another package, just include the
package name in the string, e.g., ""pkg::fred"".
call_method
The function call_method is used to call a method
from a Perl class. The parameter "methname" corre
sponds to the name of the method to be called. Note
that the class that the method belongs to is passed
on the Perl stack rather than in the parameter list.
This class can be either the name of the class (for a
static method) or a reference to an object (for a
virtual method). See the perlobj manpage for more
information on static and virtual methods and the
Using call_method entry elsewhere in this document
for an example of using call_method.
call_argv
call_argv calls the Perl subroutine specified by the
C string stored in the "subname" parameter. It also
takes the usual "flags" parameter. The final parame
ter, "argv", consists of a NULL terminated list of C
strings to be passed as parameters to the Perl sub
routine. See Using call_argv.
All the functions return an integer. This is a count of
the number of items returned by the Perl subroutine. The
actual items returned by the subroutine are stored on the
Perl stack.
As a general rule you should always check the return value
from these functions. Even if you are expecting only a
particular number of values to be returned from the Perl
subroutine, there is nothing to stop someone from doing
something unexpected--don't say you haven't been warned.
FLAG VALUES
The "flags" parameter in all the call_* functions is a bit
mask which can consist of any combination of the symbols
defined below, OR'ed together.
G_VOID
Calls the Perl subroutine in a void context.
This flag has 2 effects:
1. It indicates to the subroutine being called that it
is executing in a void context (if it executes wan_
tarray the result will be the undefined value).
2. It ensures that nothing is actually returned from the
subroutine.
The value returned by the call_* function indicates how
many items have been returned by the Perl subroutine - in
this case it will be 0.
G_SCALAR
Calls the Perl subroutine in a scalar context. This is
the default context flag setting for all the call_* func
tions.
This flag has 2 effects:
1. It indicates to the subroutine being called that it
is executing in a scalar context (if it executes wan_
tarray the result will be false).
2. It ensures that only a scalar is actually returned
from the subroutine. The subroutine can, of course,
ignore the wantarray and return a list anyway. If so,
then only the last element of the list will be
returned.
The value returned by the call_* function indicates how
many items have been returned by the Perl subroutine - in
this case it will be either 0 or 1.
If 0, then you have specified the G_DISCARD flag.
If 1, then the item actually returned by the Perl subrou
tine will be stored on the Perl stack - the section
Returning a Scalar shows how to access this value on the
stack. Remember that regardless of how many items the
Perl subroutine returns, only the last one will be acces
sible from the stack - think of the case where only one
value is returned as being a list with only one element.
Any other items that were returned will not exist by the
time control returns from the call_* function. The sec
tion Returning a list in a scalar context shows an example
of this behavior.
G_ARRAY
Calls the Perl subroutine in a list context.
As with G_SCALAR, this flag has 2 effects:
1. It indicates to the subroutine being called that it
is executing in a list context (if it executes wan_
tarray the result will be true).
2. It ensures that all items returned from the subrou
tine will be accessible when control returns from the
call_* function.
The value returned by the call_* function indicates how
many items have been returned by the Perl subroutine.
If 0, then you have specified the G_DISCARD flag.
If not 0, then it will be a count of the number of items
returned by the subroutine. These items will be stored on
the Perl stack. The section Returning a list of values
gives an example of using the G_ARRAY flag and the mechan
ics of accessing the returned items from the Perl stack.
G_DISCARD
By default, the call_* functions place the items returned
from by the Perl subroutine on the stack. If you are not
interested in these items, then setting this flag will
make Perl get rid of them automatically for you. Note
that it is still possible to indicate a context to the
Perl subroutine by using either G_SCALAR or G_ARRAY.
If you do not set this flag then it is very important that
you make sure that any temporaries (i.e., parameters
passed to the Perl subroutine and values returned from the
subroutine) are disposed of yourself. The section Return_
ing a Scalar gives details of how to dispose of these tem
poraries explicitly and the section Using Perl to dispose
of temporaries discusses the specific circumstances where
you can ignore the problem and let Perl deal with it for
you.
G_NOARGS
Whenever a Perl subroutine is called using one of the
call_* functions, it is assumed by default that parameters
are to be passed to the subroutine. If you are not pass
ing any parameters to the Perl subroutine, you can save a
bit of time by setting this flag. It has the effect of
not creating the "@_" array for the Perl subroutine.
Although the functionality provided by this flag may seem
straightforward, it should be used only if there is a good
reason to do so. The reason for being cautious is that
even if you have specified the G_NOARGS flag, it is still
possible for the Perl subroutine that has been called to
think that you have passed it parameters.
In fact, what can happen is that the Perl subroutine you
have called can access the "@_" array from a previous Perl
subroutine. This will occur when the code that is execut
ing the call_* function has itself been called from
another Perl subroutine. The code below illustrates this
sub fred
{ print "@_\n" }
sub joe
{ &fred }
&joe(1,2,3) ;
This will print
1 2 3
What has happened is that "fred" accesses the "@_" array
which belongs to "joe".
G_EVAL
It is possible for the Perl subroutine you are calling to
terminate abnormally, e.g., by calling die explicitly or
by not actually existing. By default, when either of
these events occurs, the process will terminate immedi
ately. If you want to trap this type of event, specify
the G_EVAL flag. It will put an eval { } around the sub
routine call.
Whenever control returns from the call_* function you need
to check the "$@" variable as you would in a normal Perl
script.
The value returned from the call_* function is dependent
on what other flags have been specified and whether an
error has occurred. Here are all the different cases that
can occur:
If the call_* function returns normally, then the
value returned is as specified in the previous sec
tions.
If G_DISCARD is specified, the return value will
always be 0.
If G_ARRAY is specified and an error has occurred,
the return value will always be 0.
If G_SCALAR is specified and an error has occurred,
the return value will be 1 and the value on the top
of the stack will be undef. This means that if you
have already detected the error by checking "$@" and
you want the program to continue, you must remember
to pop the undef from the stack.
See Using G_EVAL for details on using G_EVAL.
G_KEEPERR
You may have noticed that using the G_EVAL flag described
above will always clear the "$@" variable and set it to a
string describing the error iff there was an error in the
called code. This unqualified resetting of "$@" can be
problematic in the reliable identification of errors using
the "eval {}" mechanism, because the possibility exists
that perl will call other code (end of block processing
code, for example) between the time the error causes "$@"
to be set within "eval {}", and the subsequent statement
which checks for the value of "$@" gets executed in the
user's script.
This scenario will mostly be applicable to code that is
meant to be called from within destructors, asynchronous
callbacks, signal handlers, "__DIE__" or "__WARN__" hooks,
and "tie" functions. In such situations, you will not
want to clear "$@" at all, but simply to append any new
errors to any existing value of "$@".
The G_KEEPERR flag is meant to be used in conjunction with
G_EVAL in call_* functions that are used to implement such
code. This flag has no effect when G_EVAL is not used.
When G_KEEPERR is used, any errors in the called code will
be prefixed with the string "\t(in cleanup)", and appended
to the current value of "$@".
The G_KEEPERR flag was introduced in Perl version 5.002.
See Using G_KEEPERR for an example of a situation that
warrants the use of this flag.
Determining the Context
As mentioned above, you can determine the context of the
currently executing subroutine in Perl with wantarray.
The equivalent test can be made in C by using the
"GIMME_V" macro, which returns "G_ARRAY" if you have been
called in a list context, "G_SCALAR" if in a scalar con
text, or "G_VOID" if in a void context (i.e. the return
value will not be used). An older version of this macro
is called "GIMME"; in a void context it returns "G_SCALAR"
instead of "G_VOID". An example of using the "GIMME_V"
macro is shown in section Using GIMME_V.
KNOWN PROBLEMS
This section outlines all known problems that exist in the
call_* functions.
1. If you are intending to make use of both the G_EVAL
and G_SCALAR flags in your code, use a version of
Perl greater than 5.000. There is a bug in version
5.000 of Perl which means that the combination of
these two flags will not work as described in the
section FLAG VALUES.
Specifically, if the two flags are used when calling
a subroutine and that subroutine does not call die,
the value returned by call_* will be wrong.
2. In Perl 5.000 and 5.001 there is a problem with using
call_* if the Perl sub you are calling attempts to
trap a die.
The symptom of this problem is that the called Perl
sub will continue to completion, but whenever it
attempts to pass control back to the XSUB, the pro
gram will immediately terminate.
For example, say you want to call this Perl sub
sub fred
{
eval { die "Fatal Error" ; }
print "Trapped error: $@\n"
if $@ ;
}
via this XSUB
void
Call_fred()
CODE:
PUSHMARK(SP) ;
call_pv("fred", G_DISCARD|G_NOARGS) ;
fprintf(stderr, "back in Call_fred\n") ;
When "Call_fred" is executed it will print
Trapped error: Fatal Error
As control never returns to "Call_fred", the ""back
in Call_fred"" string will not get printed.
To work around this problem, you can either upgrade
to Perl 5.002 or higher, or use the G_EVAL flag with
call_* as shown below
void
Call_fred()
CODE:
PUSHMARK(SP) ;
call_pv("fred", G_EVAL|G_DISCARD|G_NOARGS) ;
fprintf(stderr, "back in Call_fred\n") ;
EXAMPLES
Enough of the definition talk, let's have a few examples.
Perl provides many macros to assist in accessing the Perl
stack. Wherever possible, these macros should always be
used when interfacing to Perl internals. We hope this
should make the code less vulnerable to any changes made
to Perl in the future.
Another point worth noting is that in the first series of
examples I have made use of only the call_pv function.
This has been done to keep the code simpler and ease you
into the topic. Wherever possible, if the choice is
between using call_pv and call_sv, you should always try
to use call_sv. See Using call_sv for details.
No Parameters, Nothing returned
This first trivial example will call a Perl subroutine,
PrintUID, to print out the UID of the process.
sub PrintUID
{
print "UID is $<\n" ;
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP ;
PUSHMARK(SP) ;
call_pv("PrintUID", G_DISCARD|G_NOARGS) ;
}
Simple, eh.
A few points to note about this example.
1. Ignore "dSP" and "PUSHMARK(SP)" for now. They will be
discussed in the next example.
2. We aren't passing any parameters to PrintUID so
G_NOARGS can be specified.
3. We aren't interested in anything returned from Print_
UID, so G_DISCARD is specified. Even if PrintUID was
changed to return some value(s), having specified
G_DISCARD will mean that they will be wiped by the
time control returns from call_pv.
4. As call_pv is being used, the Perl subroutine is
specified as a C string. In this case the subroutine
name has been 'hard-wired' into the code.
5. Because we specified G_DISCARD, it is not necessary
to check the value returned from call_pv. It will
always be 0.
Passing Parameters
Now let's make a slightly more complex example. This time
we want to call a Perl subroutine, "LeftString", which
will take 2 parameters--a string ($s) and an integer ($n).
The subroutine will simply print the first $n characters
of the string.
So the Perl subroutine would look like this
sub LeftString
{
my($s, $n) = @_ ;
print substr($s, 0, $n), "\n" ;
}
The C function required to call LeftString would look like
this.
static void
call_LeftString(a, b)
char * a ;
int b ;
{
dSP ;
ENTER ;
SAVETMPS ;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSVpv(a, 0)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
call_pv("LeftString", G_DISCARD);
FREETMPS ;
LEAVE ;
}
Here are a few notes on the C function call_LeftString.
1. Parameters are passed to the Perl subroutine using
the Perl stack. This is the purpose of the code
beginning with the line "dSP" and ending with the
line "PUTBACK". The "dSP" declares a local copy of
the stack pointer. This local copy should always be
accessed as "SP".
2. If you are going to put something onto the Perl
stack, you need to know where to put it. This is the
purpose of the macro "dSP"--it declares and initial
izes a local copy of the Perl stack pointer.
All the other macros which will be used in this exam
ple require you to have used this macro.
The exception to this rule is if you are calling a
Perl subroutine directly from an XSUB function. In
this case it is not necessary to use the "dSP" macro
explicitly--it will be declared for you automati
cally.
3. Any parameters to be pushed onto the stack should be
bracketed by the "PUSHMARK" and "PUTBACK" macros.
The purpose of these two macros, in this context, is
to count the number of parameters you are pushing
automatically. Then whenever Perl is creating the
"@_" array for the subroutine, it knows how big to
make it.
The "PUSHMARK" macro tells Perl to make a mental note
of the current stack pointer. Even if you aren't
passing any parameters (like the example shown in the
section No Parameters, Nothing returned) you must
still call the "PUSHMARK" macro before you can call
any of the call_* functions--Perl still needs to know
that there are no parameters.
The "PUTBACK" macro sets the global copy of the stack
pointer to be the same as our local copy. If we
didn't do this call_pv wouldn't know where the two
parameters we pushed were--remember that up to now
all the stack pointer manipulation we have done is
with our local copy, not the global copy.
4. Next, we come to XPUSHs. This is where the parameters
actually get pushed onto the stack. In this case we
are pushing a string and an integer.
See the XSUBs and the Argument Stack entry in the
perlguts manpage for details on how the XPUSH macros
work.
5. Because we created temporary values (by means of
sv_2mortal() calls) we will have to tidy up the Perl
stack and dispose of mortal SVs.
This is the purpose of
ENTER ;
SAVETMPS ;
at the start of the function, and
FREETMPS ;
LEAVE ;
at the end. The "ENTER"/"SAVETMPS" pair creates a
boundary for any temporaries we create. This means
that the temporaries we get rid of will be limited to
those which were created after these calls.
The "FREETMPS"/"LEAVE" pair will get rid of any val
ues returned by the Perl subroutine (see next exam
ple), plus it will also dump the mortal SVs we have
created. Having "ENTER"/"SAVETMPS" at the beginning
of the code makes sure that no other mortals are
destroyed.
Think of these macros as working a bit like using "{"
and "}" in Perl to limit the scope of local
variables.
See the section Using Perl to dispose of temporaries
for details of an alternative to using these macros.
6. Finally, LeftString can now be called via the call_pv
function. The only flag specified this time is
G_DISCARD. Because we are passing 2 parameters to the
Perl subroutine this time, we have not specified
G_NOARGS.
Returning a Scalar
Now for an example of dealing with the items returned from
a Perl subroutine.
Here is a Perl subroutine, Adder, that takes 2 integer
parameters and simply returns their sum.
sub Adder
{
my($a, $b) = @_ ;
$a + $b ;
}
Because we are now concerned with the return value from
Adder, the C function required to call it is now a bit
more complex.
static void
call_Adder(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = call_pv("Adder", G_SCALAR);
SPAGAIN ;
if (count != 1)
croak("Big trouble\n") ;
printf ("The sum of %d and %d is %d\n", a, b, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
Points to note this time are
1. The only flag specified this time was G_SCALAR. That
means the "@_" array will be created and that the
value returned by Adder will still exist after the
call to call_pv.
2. The purpose of the macro "SPAGAIN" is to refresh the
local copy of the stack pointer. This is necessary
because it is possible that the memory allocated to
the Perl stack has been reallocated whilst in the
call_pv call.
If you are making use of the Perl stack pointer in
your code you must always refresh the local copy
using SPAGAIN whenever you make use of the call_*
functions or any other Perl internal function.
3. Although only a single value was expected to be
returned from Adder, it is still good practice to
check the return code from call_pv anyway.
Expecting a single value is not quite the same as
knowing that there will be one. If someone modified
Adder to return a list and we didn't check for that
possibility and take appropriate action the Perl
stack would end up in an inconsistent state. That is
something you really don't want to happen ever.
4. The "POPi" macro is used here to pop the return value
from the stack. In this case we wanted an integer,
so "POPi" was used.
Here is the complete list of POP macros available,
along with the types they return.
POPs SV
POPp pointer
POPn double
POPi integer
POPl long
5. The final "PUTBACK" is used to leave the Perl stack
in a consistent state before exiting the function.
This is necessary because when we popped the return
value from the stack with "POPi" it updated only our
local copy of the stack pointer. Remember, "PUTBACK"
sets the global stack pointer to be the same as our
local copy.
Returning a list of values
Now, let's extend the previous example to return both the
sum of the parameters and the difference.
Here is the Perl subroutine
sub AddSubtract
{
my($a, $b) = @_ ;
($a+$b, $a-$b) ;
}
and this is the C function
static void
call_AddSubtract(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = call_pv("AddSubtract", G_ARRAY);
SPAGAIN ;
if (count != 2)
croak("Big trouble\n") ;
printf ("%d - %d = %d\n", a, b, POPi) ;
printf ("%d + %d = %d\n", a, b, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
If call_AddSubtract is called like this
call_AddSubtract(7, 4) ;
then here is the output
7 - 4 = 3
7 + 4 = 11
Notes
1. We wanted list context, so G_ARRAY was used.
2. Not surprisingly "POPi" is used twice this time
because we were retrieving 2 values from the stack.
The important thing to note is that when using the
"POP*" macros they come off the stack in reverse
order.
Returning a list in a scalar context
Say the Perl subroutine in the previous section was called
in a scalar context, like this
static void
call_AddSubScalar(a, b)
int a ;
int b ;
{
dSP ;
int count ;
int i ;
ENTER ;
SAVETMPS;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = call_pv("AddSubtract", G_SCALAR);
SPAGAIN ;
printf ("Items Returned = %d\n", count) ;
for (i = 1 ; i <= count ; ++i)
printf ("Value %d = %d\n", i, POPi) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
The other modification made is that call_AddSubScalar will
print the number of items returned from the Perl subrou
tine and their value (for simplicity it assumes that they
are integer). So if call_AddSubScalar is called
call_AddSubScalar(7, 4) ;
then the output will be
Items Returned = 1
Value 1 = 3
In this case the main point to note is that only the last
item in the list is returned from the subroutine, AddSub_
tract actually made it back to call_AddSubScalar.
Returning Data from Perl via the parameter list
It is also possible to return values directly via the
parameter list - whether it is actually desirable to do it
is another matter entirely.
The Perl subroutine, Inc, below takes 2 parameters and
increments each directly.
sub Inc
{
++ $_[0] ;
++ $_[1] ;
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a ;
int b ;
{
dSP ;
int count ;
SV * sva ;
SV * svb ;
ENTER ;
SAVETMPS;
sva = sv_2mortal(newSViv(a)) ;
svb = sv_2mortal(newSViv(b)) ;
PUSHMARK(SP) ;
XPUSHs(sva);
XPUSHs(svb);
PUTBACK ;
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count) ;
printf ("%d + 1 = %d\n", a, SvIV(sva)) ;
printf ("%d + 1 = %d\n", b, SvIV(svb)) ;
FREETMPS ;
LEAVE ;
}
To be able to access the two parameters that were pushed
onto the stack after they return from call_pv it is neces
sary to make a note of their addresses--thus the two vari
ables "sva" and "svb".
The reason this is necessary is that the area of the Perl
stack which held them will very likely have been overwrit
ten by something else by the time control returns from
call_pv.
Using G_EVAL
Now an example using G_EVAL. Below is a Perl subroutine
which computes the difference of its 2 parameters. If this
would result in a negative result, the subroutine calls
die.
sub Subtract
{
my ($a, $b) = @_ ;
die "death can be fatal\n" if $a < $b ;
$a - $b ;
}
and some C to call it
static void
call_Subtract(a, b)
int a ;
int b ;
{
dSP ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = call_pv("Subtract", G_EVAL|G_SCALAR);
SPAGAIN ;
/* Check the eval first */
if (SvTRUE(ERRSV))
{
STRLEN n_a;
printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
POPs ;
}
else
{
if (count != 1)
croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
count) ;
printf ("%d - %d = %d\n", a, b, POPi) ;
}
PUTBACK ;
FREETMPS ;
LEAVE ;
}
If call_Subtract is called thus
call_Subtract(4, 5)
the following will be printed
Uh oh - death can be fatal
Notes
1. We want to be able to catch the die so we have used
the G_EVAL flag. Not specifying this flag would mean
that the program would terminate immediately at the
die statement in the subroutine Subtract.
2. The code
if (SvTRUE(ERRSV))
{
STRLEN n_a;
printf ("Uh oh - %s\n", SvPV(ERRSV, n_a)) ;
POPs ;
}
is the direct equivalent of this bit of Perl
print "Uh oh - $@\n" if $@ ;
"PL_errgv" is a perl global of type "GV *" that
points to the symbol table entry containing the
error. "ERRSV" therefore refers to the C equivalent
of "$@".
3. Note that the stack is popped using "POPs" in the
block where "SvTRUE(ERRSV)" is true. This is neces
sary because whenever a call_* function invoked with
G_EVAL|G_SCALAR returns an error, the top of the
stack holds the value undef. Because we want the pro
gram to continue after detecting this error, it is
essential that the stack is tidied up by removing the
undef.
Using G_KEEPERR
Consider this rather facetious example, where we have used
an XS version of the call_Subtract example above inside a
destructor:
package Foo;
sub new { bless {}, $_[0] }
sub Subtract {
my($a,$b) = @_;
die "death can be fatal" if $a < $b ;
$a - $b;
}
sub DESTROY { call_Subtract(5, 4); }
sub foo { die "foo dies"; }
package main;
eval { Foo->new->foo };
print "Saw: $@" if $@; # should be, but isn't
This example will fail to recognize that an error occurred
inside the "eval {}". Here's why: the call_Subtract code
got executed while perl was cleaning up temporaries when
exiting the eval block, and because call_Subtract is
implemented with call_pv using the G_EVAL flag, it
promptly reset "$@". This results in the failure of the
outermost test for "$@", and thereby the failure of the
error trap.
Appending the G_KEEPERR flag, so that the call_pv call in
call_Subtract reads:
count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
will preserve the error and restore reliable error han
dling.
Using call_sv
In all the previous examples I have 'hard-wired' the name
of the Perl subroutine to be called from C. Most of the
time though, it is more convenient to be able to specify
the name of the Perl subroutine from within the Perl
script.
Consider the Perl code below
sub fred
{
print "Hello there\n" ;
}
CallSubPV("fred") ;
Here is a snippet of XSUB which defines CallSubPV.
void
CallSubPV(name)
char * name
CODE:
PUSHMARK(SP) ;
call_pv(name, G_DISCARD|G_NOARGS) ;
That is fine as far as it goes. The thing is, the Perl
subroutine can be specified as only a string. For Perl 4
this was adequate, but Perl 5 allows references to subrou
tines and anonymous subroutines. This is where call_sv is
useful.
The code below for CallSubSV is identical to CallSubPV
except that the "name" parameter is now defined as an SV*
and we use call_sv instead of call_pv.
void
CallSubSV(name)
SV * name
CODE:
PUSHMARK(SP) ;
call_sv(name, G_DISCARD|G_NOARGS) ;
Because we are using an SV to call fred the following can
all be used
CallSubSV("fred") ;
CallSubSV(\&fred) ;
$ref = \&fred ;
CallSubSV($ref) ;
CallSubSV( sub { print "Hello there\n" } ) ;
As you can see, call_sv gives you much greater flexibility
in how you can specify the Perl subroutine.
You should note that if it is necessary to store the SV
("name" in the example above) which corresponds to the
Perl subroutine so that it can be used later in the pro
gram, it not enough just to store a copy of the pointer to
the SV. Say the code above had been like this
static SV * rememberSub ;
void
SaveSub1(name)
SV * name
CODE:
rememberSub = name ;
void
CallSavedSub1()
CODE:
PUSHMARK(SP) ;
call_sv(rememberSub, G_DISCARD|G_NOARGS) ;
The reason this is wrong is that by the time you come to
use the pointer "rememberSub" in "CallSavedSub1", it may
or may not still refer to the Perl subroutine that was
recorded in "SaveSub1". This is particularly true for
these cases
SaveSub1(\&fred) ;
CallSavedSub1() ;
SaveSub1( sub { print "Hello there\n" } ) ;
CallSavedSub1() ;
By the time each of the "SaveSub1" statements above have
been executed, the SV*s which corresponded to the parame
ters will no longer exist. Expect an error message from
Perl of the form
Can't use an undefined value as a subroutine reference at ...
for each of the "CallSavedSub1" lines.
Similarly, with this code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = 47 ;
CallSavedSub1() ;
you can expect one of these messages (which you actually
get is dependent on the version of Perl you are using)
Not a CODE reference at ...
Undefined subroutine &main::47 called ...
The variable $ref may have referred to the subroutine
"fred" whenever the call to "SaveSub1" was made but by the
time "CallSavedSub1" gets called it now holds the number
"47". Because we saved only a pointer to the original SV
in "SaveSub1", any changes to $ref will be tracked by the
pointer "rememberSub". This means that whenever "Call
SavedSub1" gets called, it will attempt to execute the
code which is referenced by the SV* "rememberSub". In
this case though, it now refers to the integer "47", so
expect Perl to complain loudly.
A similar but more subtle problem is illustrated with this
code
$ref = \&fred ;
SaveSub1($ref) ;
$ref = \&joe ;
CallSavedSub1() ;
This time whenever "CallSavedSub1" get called it will exe
cute the Perl subroutine "joe" (assuming it exists) rather
than "fred" as was originally requested in the call to
"SaveSub1".
To get around these problems it is necessary to take a
full copy of the SV. The code below shows "SaveSub2" mod
ified to do that
static SV * keepSub = (SV*)NULL ;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name) ;
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name) ;
void
CallSavedSub2()
CODE:
PUSHMARK(SP) ;
call_sv(keepSub, G_DISCARD|G_NOARGS) ;
To avoid creating a new SV every time "SaveSub2" is
called, the function first checks to see if it has been
called before. If not, then space for a new SV is
allocated and the reference to the Perl subroutine, "name"
is copied to the variable "keepSub" in one operation using
"newSVsv". Thereafter, whenever "SaveSub2" is called the
existing SV, "keepSub", is overwritten with the new value
using "SvSetSV".
Using call_argv
Here is a Perl subroutine which prints whatever parameters
are passed to it.
sub PrintList
{
my(@list) = @_ ;
foreach (@list) { print "$_\n" }
}
and here is an example of call_argv which will call Print_
List.
static char * words[] = {"alpha", "beta", "gamma", "delta", NULL} ;
static void
call_PrintList()
{
dSP ;
call_argv("PrintList", G_DISCARD, words) ;
}
Note that it is not necessary to call "PUSHMARK" in this
instance. This is because call_argv will do it for you.
Using call_method
Consider the following Perl code
{
package Mine ;
sub new
{
my($type) = shift ;
bless [@_]
}
sub Display
{
my ($self, $index) = @_ ;
print "$index: $$self[$index]\n" ;
}
sub PrintID
{
my($class) = @_ ;
print "This is Class $class version 1.0\n" ;
}
}
It implements just a very simple class to manage an array.
Apart from the constructor, "new", it declares methods,
one static and one virtual. The static method, "PrintID",
prints out simply the class name and a version number. The
virtual method, "Display", prints out a single element of
the array. Here is an all Perl example of using it.
$a = new Mine ('red', 'green', 'blue') ;
$a->Display(1) ;
PrintID Mine;
will print
1: green
This is Class Mine version 1.0
Calling a Perl method from C is fairly straightforward.
The following things are required
a reference to the object for a virtual method or the
name of the class for a static method.
the name of the method.
any other parameters specific to the method.
Here is a simple XSUB which illustrates the mechanics of
calling both the "PrintID" and "Display" methods from C.
void
call_Method(ref, method, index)
SV * ref
char * method
int index
CODE:
PUSHMARK(SP);
XPUSHs(ref);
XPUSHs(sv_2mortal(newSViv(index))) ;
PUTBACK;
call_method(method, G_DISCARD) ;
void
call_PrintID(class, method)
char * class
char * method
CODE:
PUSHMARK(SP);
XPUSHs(sv_2mortal(newSVpv(class, 0))) ;
PUTBACK;
call_method(method, G_DISCARD) ;
So the methods "PrintID" and "Display" can be invoked like
this
$a = new Mine ('red', 'green', 'blue') ;
call_Method($a, 'Display', 1) ;
call_PrintID('Mine', 'PrintID') ;
The only thing to note is that in both the static and vir
tual methods, the method name is not passed via the
stack--it is used as the first parameter to call_method.
Using GIMME_V
Here is a trivial XSUB which prints the context in which
it is currently executing.
void
PrintContext()
CODE:
I32 gimme = GIMME_V;
if (gimme == G_VOID)
printf ("Context is Void\n") ;
else if (gimme == G_SCALAR)
printf ("Context is Scalar\n") ;
else
printf ("Context is Array\n") ;
and here is some Perl to test it
PrintContext ;
$a = PrintContext ;
@a = PrintContext ;
The output from that will be
Context is Void
Context is Scalar
Context is Array
Using Perl to dispose of temporaries
In the examples given to date, any temporaries created in
the callback (i.e., parameters passed on the stack to the
call_* function or values returned via the stack) have
been freed by one of these methods
specifying the G_DISCARD flag with call_*.
explicitly disposed of using the "ENTER"/"SAVETMPS" -
"FREETMPS"/"LEAVE" pairing.
There is another method which can be used, namely letting
Perl do it for you automatically whenever it regains con
trol after the callback has terminated. This is done by
simply not using the
ENTER ;
SAVETMPS ;
...
FREETMPS ;
LEAVE ;
sequence in the callback (and not, of course, specifying
the G_DISCARD flag).
If you are going to use this method you have to be aware
of a possible memory leak which can arise under very spe
cific circumstances. To explain these circumstances you
need to know a bit about the flow of control between Perl
and the callback routine.
The examples given at the start of the document (an error
handler and an event driven program) are typical of the
two main sorts of flow control that you are likely to
encounter with callbacks. There is a very important dis
tinction between them, so pay attention.
In the first example, an error handler, the flow of con
trol could be as follows. You have created an interface
to an external library. Control can reach the external
library like this
perl --> XSUB --> external library
Whilst control is in the library, an error condition
occurs. You have previously set up a Perl callback to han
dle this situation, so it will get executed. Once the
callback has finished, control will drop back to Perl
again. Here is what the flow of control will be like in
that situation
perl --> XSUB --> external library
...
error occurs
...
external library --> call_* --> perl
|
perl <-- XSUB <-- external library <-- call_* <----+
After processing of the error using call_* is completed,
control reverts back to Perl more or less immediately.
In the diagram, the further right you go the more deeply
nested the scope is. It is only when control is back with
perl on the extreme left of the diagram that you will have
dropped back to the enclosing scope and any temporaries
you have left hanging around will be freed.
In the second example, an event driven program, the flow
of control will be more like this
perl --> XSUB --> event handler
...
event handler --> call_* --> perl
|
event handler <-- call_* <----+
...
event handler --> call_* --> perl
|
event handler <-- call_* <----+
...
event handler --> call_* --> perl
|
event handler <-- call_* <----+
In this case the flow of control can consist of only the
repeated sequence
event handler --> call_* --> perl
for practically the complete duration of the program.
This means that control may never drop back to the sur
rounding scope in Perl at the extreme left.
So what is the big problem? Well, if you are expecting
Perl to tidy up those temporaries for you, you might be in
for a long wait. For Perl to dispose of your temporaries,
control must drop back to the enclosing scope at some
stage. In the event driven scenario that may never hap
pen. This means that as time goes on, your program will
create more and more temporaries, none of which will ever
be freed. As each of these temporaries consumes some mem
ory your program will eventually consume all the available
memory in your system--kapow!
So here is the bottom line--if you are sure that control
will revert back to the enclosing Perl scope fairly
quickly after the end of your callback, then it isn't
absolutely necessary to dispose explicitly of any tempo
raries you may have created. Mind you, if you are at all
uncertain about what to do, it doesn't do any harm to tidy
up anyway.
Strategies for storing Callback Context Information
Potentially one of the trickiest problems to overcome when
designing a callback interface can be figuring out how to
store the mapping between the C callback function and the
Perl equivalent.
To help understand why this can be a real problem first
consider how a callback is set up in an all C environment.
Typically a C API will provide a function to register a
callback. This will expect a pointer to a function as one
of its parameters. Below is a call to a hypothetical
function "register_fatal" which registers the C function
to get called when a fatal error occurs.
register_fatal(cb1) ;
The single parameter "cb1" is a pointer to a function, so
you must have defined "cb1" in your code, say something
like this
static void
cb1()
{
printf ("Fatal Error\n") ;
exit(1) ;
}
Now change that to call a Perl subroutine instead
static SV * callback = (SV*)NULL;
static void
cb1()
{
dSP ;
PUSHMARK(SP) ;
/* Call the Perl sub to process the callback */
call_sv(callback, G_DISCARD) ;
}
void
register_fatal(fn)
SV * fn
CODE:
/* Remember the Perl sub */
if (callback == (SV*)NULL)
callback = newSVsv(fn) ;
else
SvSetSV(callback, fn) ;
/* register the callback with the external library */
register_fatal(cb1) ;
where the Perl equivalent of "register_fatal" and the
callback it registers, "pcb1", might look like this
# Register the sub pcb1
register_fatal(\&pcb1) ;
sub pcb1
{
die "I'm dying...\n" ;
}
The mapping between the C callback and the Perl equivalent
is stored in the global variable "callback".
This will be adequate if you ever need to have only one
callback registered at any time. An example could be an
error handler like the code sketched out above. Remember
though, repeated calls to "register_fatal" will replace
the previously registered callback function with the new
one.
Say for example you want to interface to a library which
allows asynchronous file i/o. In this case you may be
able to register a callback whenever a read operation has
completed. To be of any use we want to be able to call
separate Perl subroutines for each file that is opened.
As it stands, the error handler example above would not be
adequate as it allows only a single callback to be defined
at any time. What we require is a means of storing the
mapping between the opened file and the Perl subroutine we
want to be called for that file.
Say the i/o library has a function "asynch_read" which
associates a C function "ProcessRead" with a file handle
"fh"--this assumes that it has also provided some routine
to open the file and so obtain the file handle.
asynch_read(fh, ProcessRead)
This may expect the C ProcessRead function of this form
void
ProcessRead(fh, buffer)
int fh ;
char * buffer ;
{
...
}
To provide a Perl interface to this library we need to be
able to map between the "fh" parameter and the Perl sub
routine we want called. A hash is a convenient mechanism
for storing this mapping. The code below shows a possible
implementation
static HV * Mapping = (HV*)NULL ;
void
asynch_read(fh, callback)
int fh
SV * callback
CODE:
/* If the hash doesn't already exist, create it */
if (Mapping == (HV*)NULL)
Mapping = newHV() ;
/* Save the fh -> callback mapping */
hv_store(Mapping, (char*)&fh, sizeof(fh), newSVsv(callback), 0) ;
/* Register with the C Library */
asynch_read(fh, asynch_read_if) ;
and "asynch_read_if" could look like this
static void
asynch_read_if(fh, buffer)
int fh ;
char * buffer ;
{
dSP ;
SV ** sv ;
/* Get the callback associated with fh */
sv = hv_fetch(Mapping, (char*)&fh , sizeof(fh), FALSE) ;
if (sv == (SV**)NULL)
croak("Internal error...\n") ;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(fh))) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
/* Call the Perl sub */
call_sv(*sv, G_DISCARD) ;
}
For completeness, here is "asynch_close". This shows how
to remove the entry from the hash "Mapping".
void
asynch_close(fh)
int fh
CODE:
/* Remove the entry from the hash */
(void) hv_delete(Mapping, (char*)&fh, sizeof(fh), G_DISCARD) ;
/* Now call the real asynch_close */
asynch_close(fh) ;
So the Perl interface would look like this
sub callback1
{
my($handle, $buffer) = @_ ;
}
# Register the Perl callback
asynch_read($fh, \&callback1) ;
asynch_close($fh) ;
The mapping between the C callback and Perl is stored in
the global hash "Mapping" this time. Using a hash has the
distinct advantage that it allows an unlimited number of
callbacks to be registered.
What if the interface provided by the C callback doesn't
contain a parameter which allows the file handle to Perl
subroutine mapping? Say in the asynchronous i/o package,
the callback function gets passed only the "buffer" param
eter like this
void
ProcessRead(buffer)
char * buffer ;
{
...
}
Without the file handle there is no straightforward way to
map from the C callback to the Perl subroutine.
In this case a possible way around this problem is to pre
define a series of C functions to act as the interface to
Perl, thus
#define MAX_CB 3
#define NULL_HANDLE -1
typedef void (*FnMap)() ;
struct MapStruct {
FnMap Function ;
SV * PerlSub ;
int Handle ;
} ;
static void fn1() ;
static void fn2() ;
static void fn3() ;
static struct MapStruct Map [MAX_CB] =
{
{ fn1, NULL, NULL_HANDLE },
{ fn2, NULL, NULL_HANDLE },
{ fn3, NULL, NULL_HANDLE }
} ;
static void
Pcb(index, buffer)
int index ;
char * buffer ;
{
dSP ;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSVpv(buffer, 0))) ;
PUTBACK ;
/* Call the Perl sub */
call_sv(Map[index].PerlSub, G_DISCARD) ;
}
static void
fn1(buffer)
char * buffer ;
{
Pcb(0, buffer) ;
}
static void
fn2(buffer)
char * buffer ;
{
Pcb(1, buffer) ;
}
static void
fn3(buffer)
char * buffer ;
{
Pcb(2, buffer) ;
}
void
array_asynch_read(fh, callback)
int fh
SV * callback
CODE:
int index ;
int null_index = MAX_CB ;
/* Find the same handle or an empty entry */
for (index = 0 ; index < MAX_CB ; ++index)
{
if (Map[index].Handle == fh)
break ;
if (Map[index].Handle == NULL_HANDLE)
null_index = index ;
}
if (index == MAX_CB && null_index == MAX_CB)
croak ("Too many callback functions registered\n") ;
if (index == MAX_CB)
index = null_index ;
/* Save the file handle */
Map[index].Handle = fh ;
/* Remember the Perl sub */
if (Map[index].PerlSub == (SV*)NULL)
Map[index].PerlSub = newSVsv(callback) ;
else
SvSetSV(Map[index].PerlSub, callback) ;
asynch_read(fh, Map[index].Function) ;
void
array_asynch_close(fh)
int fh
CODE:
int index ;
/* Find the file handle */
for (index = 0; index < MAX_CB ; ++ index)
if (Map[index].Handle == fh)
break ;
if (index == MAX_CB)
croak ("could not close fh %d\n", fh) ;
Map[index].Handle = NULL_HANDLE ;
SvREFCNT_dec(Map[index].PerlSub) ;
Map[index].PerlSub = (SV*)NULL ;
asynch_close(fh) ;
In this case the functions "fn1", "fn2", and "fn3" are
used to remember the Perl subroutine to be called. Each of
the functions holds a separate hard-wired index which is
used in the function "Pcb" to access the "Map" array and
actually call the Perl subroutine.
There are some obvious disadvantages with this technique.
Firstly, the code is considerably more complex than with
the previous example.
Secondly, there is a hard-wired limit (in this case 3) to
the number of callbacks that can exist simultaneously. The
only way to increase the limit is by modifying the code to
add more functions and then recompiling. None the less,
as long as the number of functions is chosen with some
care, it is still a workable solution and in some cases is
the only one available.
To summarize, here are a number of possible methods for
you to consider for storing the mapping between C and the
Perl callback
1. Ignore the problem - Allow only 1 callback
For a lot of situations, like interfacing to an error
handler, this may be a perfectly adequate solution.
2. Create a sequence of callbacks - hard wired limit
If it is impossible to tell from the parameters
passed back from the C callback what the context is,
then you may need to create a sequence of C callback
interface functions, and store pointers to each in an
array.
3. Use a parameter to map to the Perl callback
A hash is an ideal mechanism to store the mapping
between C and Perl.
Alternate Stack Manipulation
Although I have made use of only the "POP*" macros to
access values returned from Perl subroutines, it is also
possible to bypass these macros and read the stack using
the "ST" macro (See the perlxs manpage for a full descrip
tion of the "ST" macro).
Most of the time the "POP*" macros should be adequate, the
main problem with them is that they force you to process
the returned values in sequence. This may not be the most
suitable way to process the values in some cases. What we
want is to be able to access the stack in a random order.
The "ST" macro as used when coding an XSUB is ideal for
this purpose.
The code below is the example given in the section Return_
ing a list of values recoded to use "ST" instead of
"POP*".
static void
call_AddSubtract2(a, b)
int a ;
int b ;
{
dSP ;
I32 ax ;
int count ;
ENTER ;
SAVETMPS;
PUSHMARK(SP) ;
XPUSHs(sv_2mortal(newSViv(a)));
XPUSHs(sv_2mortal(newSViv(b)));
PUTBACK ;
count = call_pv("AddSubtract", G_ARRAY);
SPAGAIN ;
SP -= count ;
ax = (SP - PL_stack_base) + 1 ;
if (count != 2)
croak("Big trouble\n") ;
printf ("%d + %d = %d\n", a, b, SvIV(ST(0))) ;
printf ("%d - %d = %d\n", a, b, SvIV(ST(1))) ;
PUTBACK ;
FREETMPS ;
LEAVE ;
}
Notes
1. Notice that it was necessary to define the variable
"ax". This is because the "ST" macro expects it to
exist. If we were in an XSUB it would not be neces
sary to define "ax" as it is already defined for you.
2. The code
SPAGAIN ;
SP -= count ;
ax = (SP - PL_stack_base) + 1 ;
sets the stack up so that we can use the "ST" macro.
3. Unlike the original coding of this example, the
returned values are not accessed in reverse order.
So "ST(0)" refers to the first value returned by the
Perl subroutine and "ST(count-1)" refers to the last.
Creating and calling an anonymous subroutine in C
As we've already shown, "call_sv" can be used to invoke an
anonymous subroutine. However, our example showed a Perl
script invoking an XSUB to perform this operation. Let's
see how it can be done inside our C code:
...
SV *cvrv = eval_pv("sub { print 'You will not find me cluttering any namespace!' }", TRUE);
...
call_sv(cvrv, G_VOID|G_NOARGS);
"eval_pv" is used to compile the anonymous subroutine,
which will be the return value as well (read more about
"eval_pv" in the eval_pv entry in the perlapi manpage).
Once this code reference is in hand, it can be mixed in
with all the previous examples we've shown.
SEE ALSO
the perlxs manpage, the perlguts manpage, the perlembed
manpage
AUTHOR
Paul Marquess
Special thanks to the following people who assisted in the
creation of the document.
Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem,
Gurusamy Sarathy and Larry Wall.
DATE
Version 1.3, 14th Apr 1997
2001-04-07 perl v5.6.1 PERLCALL(1)
