PERLCALL(1) Perl Programmers Reference Guide PERLCALL(1)
NAME
perlcall - 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 callbacks.
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
o An Error Handler
You have created an XSUB interface to an application's C API.
A fairly common feature in applications is to allow you to define
a C function that will be called whenever something nasty occurs.
What we would like is to be able to specify a Perl subroutine that
will be called instead.
o An Event-Driven Program
The classic example of where callbacks are used is when writing an
event driven program, such as for an X11 application. In this
case you register functions 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 perlembed.
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--perlxs and perlguts.
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, 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 settings available in the bit mask are
discussed in "FLAG VALUES".
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 subroutine. 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" corresponds 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
perlobj for more information on static and virtual methods and
"Using call_method" 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 parameter, "argv", consists of a NULL-
terminated list of C strings to be passed as parameters to the
Perl subroutine. 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 one of G_VOID,
G_SCALAR, or G_ARRAY, which indicate the call context, OR'ed together
with a bit mask of any combination of the other G_* symbols defined
below.
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 wantarray 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_* functions.
This flag has 2 effects:
1. It indicates to the subroutine being called that it is executing
in a scalar context (if it executes wantarray 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 subroutine 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
accessible 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 section "Returning a List in 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 wantarray the result will be
true).
2. It ensures that all items returned from the subroutine 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 mechanics 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 "Returning a Scalar" gives details of how to
dispose of these temporaries 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 passing 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 executing 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 immediately. If you want to trap this type of event,
specify the G_EVAL flag. It will put an eval { } around the subroutine
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:
o If the call_* function returns normally, then the value returned
is as specified in the previous sections.
o If G_DISCARD is specified, the return value will always be 0.
o If G_ARRAY is specified and an error has occurred, the return
value will always be 0.
o 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
Using the G_EVAL flag described above will always set $@: clearing it
if there was no error, and setting it to describe the error if there
was an error in the called code. This is what you want if your
intention is to handle possible errors, but sometimes you just want to
trap errors and stop them interfering with the rest of the program.
This scenario will mostly be applicable to code that is meant to be
called from within destructors, asynchronous callbacks, and signal
handlers. In such situations, where the code being called has little
relation to the surrounding dynamic context, the main program needs to
be insulated from errors in the called code, even if they can't be
handled intelligently. It may also be useful to do this with code for
"__DIE__" or "__WARN__" hooks, and "tie" functions.
The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in
call_* functions that are used to implement such code, or with
"eval_sv". This flag has no effect on the "call_*" functions when
G_EVAL is not used.
When G_KEEPERR is used, any error in the called code will terminate the
call as usual, and the error will not propagate beyond the call (as
usual for G_EVAL), but it will not go into $@. Instead the error will
be converted into a warning, prefixed with the string "\t(in cleanup)".
This can be disabled using "no warnings 'misc'". If there is no error,
$@ will not be cleared.
Note that the G_KEEPERR flag does not propagate into inner evals; these
may still set $@.
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
context, 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".
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 $<\n";
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP;
PUSHMARK(SP);
call_pv("PrintUID", G_DISCARD|G_NOARGS);
}
Simple, eh?
A few points to note about this example:
1. Ignore "dSP" and "PUSHMARK(SP)" for now. They will be discussed in
the next example.
2. We aren't passing any parameters to PrintUID so G_NOARGS can be
specified.
3. We aren't interested in anything returned from PrintUID, so
G_DISCARD is specified. Even if PrintUID was changed to return
some value(s), having specified G_DISCARD will mean that they will
be wiped by the time control returns from call_pv.
4. As call_pv is being used, the Perl subroutine is specified as a C
string. In this case the subroutine name has been 'hard-wired'
into the code.
5. Because we specified G_DISCARD, it is not necessary to check the
value returned from call_pv. It will always be 0.
Passing Parameters
Now let's make a slightly more complex example. This time we want to
call a Perl subroutine, "LeftString", which will take 2 parameters--a
string ($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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSVpv(a, 0)));
PUSHs(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 initializes a local copy of the Perl stack pointer.
All the other macros which will be used in this example 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
automatically.
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 EXTEND and PUSHs. This is where the parameters
actually get pushed onto the stack. In this case we are pushing a
string and an integer.
Alternatively you can use the XPUSHs() macro, which combines a
"EXTEND(SP, 1)" and "PUSHs()". This is less efficient if you're
pushing multiple values.
See "XSUBs and the Argument Stack" in perlguts for details on how
the PUSH 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 values returned by
the Perl subroutine (see next example), 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 "{" 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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(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 that
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 during
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 (PV)
POPpbytex pointer to bytes (PV)
POPn double (NV)
POPi integer (IV)
POPu unsigned integer (UV)
POPl long
POPul unsigned long
Since these macros have side-effects don't use them as arguments
to macros that may evaluate their argument several times, for
example:
/* Bad idea, don't do this */
STRLEN len;
const char *s = SvPV(POPs, len);
Instead, use a temporary:
STRLEN len;
SV *sv = POPs;
const char *s = SvPV(sv, len);
or a macro that guarantees it will evaluate its arguments only
once:
STRLEN len;
const char *s = SvPVx(POPs, len);
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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(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 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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(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 subroutine 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. AddSubtract 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];
++ $_[1];
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a;
int b;
{
dSP;
int count;
SV * sva;
SV * svb;
ENTER;
SAVETMPS;
sva = sv_2mortal(newSViv(a));
svb = sv_2mortal(newSViv(b));
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sva);
PUSHs(svb);
PUTBACK;
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count);
printf ("%d + 1 = %d\n", a, SvIV(sva));
printf ("%d + 1 = %d\n", b, SvIV(svb));
FREETMPS;
LEAVE;
}
To be able to access the two parameters that were pushed onto the stack
after they return from call_pv it is necessary to make a note of their
addresses--thus the two variables "sva" and "svb".
The reason this is necessary is that the area of the Perl stack which
held them will very likely have been overwritten by something else by
the time control returns from call_pv.
Using G_EVAL
Now an example using G_EVAL. Below is a Perl subroutine which computes
the difference of its 2 parameters. If this would result in a negative
result, the subroutine calls die.
sub Subtract
{
my ($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;
SV *err_tmp;
ENTER;
SAVETMPS;
PUSHMARK(SP);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(sv_2mortal(newSViv(b)));
PUTBACK;
count = call_pv("Subtract", G_EVAL|G_SCALAR);
SPAGAIN;
/* Check the eval first */
err_tmp = ERRSV;
if (SvTRUE(err_tmp))
{
printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
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
err_tmp = ERRSV;
if (SvTRUE(err_tmp))
{
printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
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 $@. We use a local temporary, "err_tmp",
since "ERRSV" is a macro that calls a function, and
"SvTRUE(ERRSV)" would end up calling that function multiple times.
3. Note that the stack is popped using "POPs" in the block where
"SvTRUE(err_tmp)" is true. This is necessary 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
program to continue after detecting this error, it is essential
that the stack be 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;
{
my $foo = Foo->new;
eval { $foo->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 outer braced 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 handling.
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, and you'll want to use call_sv.
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, however, Perl allows references to
subroutines 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 program, 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 has been executed,
the SV*s which corresponded to the parameters 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 "CallSavedSub1"
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);
$ref = \&joe;
CallSavedSub1();
This time whenever "CallSavedSub1" gets called it will execute the Perl
subroutine "joe" (assuming it exists) rather than "fred" as was
originally requested in the call to "SaveSub1".
To get around these problems it is necessary to take a full copy of the
SV. The code below shows "SaveSub2" modified to do that.
/* this isn't thread-safe */
static SV * keepSub = (SV*)NULL;
void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name);
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name);
void
CallSavedSub2()
CODE:
PUSHMARK(SP);
call_sv(keepSub, G_DISCARD|G_NOARGS);
To avoid creating a new SV every time "SaveSub2" is called, the
function first checks to see if it has been called before. If not,
then space for a new SV is allocated and the reference to the Perl
subroutine "name" is copied to the variable "keepSub" in one operation
using "newSVsv". Thereafter, whenever "SaveSub2" is called, the
existing SV, "keepSub", is overwritten with the new value using
"SvSetSV".
Note: using a static or global variable to store the SV isn't thread-
safe. You can either use the "MY_CXT" mechanism documented in "Safely
Storing Static Data in XS" in perlxs which is fast, or store the values
in perl global variables, using get_sv(), which is much slower.
Using call_argv
Here is a Perl subroutine which prints whatever parameters are passed
to it.
sub PrintList
{
my(@list) = @_;
foreach (@list) { print "$_\n" }
}
And here is an example of call_argv which will call PrintList.
static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
static void
call_PrintList()
{
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 = Mine->new('red', 'green', 'blue');
$a->Display(1);
Mine->PrintID;
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:
o A reference to the object for a virtual method or the name of the
class for a static method
o The name of the method
o 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);
EXTEND(SP, 2);
PUSHs(ref);
PUSHs(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:
$a = Mine->new('red', 'green', 'blue');
call_Method($a, 'Display', 1);
call_PrintID('Mine', 'PrintID');
The only thing to note is that, in both the static and virtual 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:
U8 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:
o Specifying the G_DISCARD flag with call_*
o Explicitly 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 control 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 specific 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 distinction between them, so pay attention.
In the first example, an error handler, the flow of control 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 handle 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 surrounding 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
happen. 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 memory 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 temporaries 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 subroutine 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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(fh)));
PUSHs(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" parameter 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 predefine 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 perlxs for a full
description 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 "Returning 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);
EXTEND(SP, 2);
PUSHs(sv_2mortal(newSViv(a)));
PUSHs(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 necessary to define "ax" as it is already defined
for us.
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 "eval_pv" in
perlapi). Once this code reference is in hand, it can be mixed in with
all the previous examples we've shown.
LIGHTWEIGHT CALLBACKS
Sometimes you need to invoke the same subroutine repeatedly. This
usually happens with a function that acts on a list of values, such as
Perl's built-in sort(). You can pass a comparison function to sort(),
which will then be invoked for every pair of values that needs to be
compared. The first() and reduce() functions from List::Util follow a
similar pattern.
In this case it is possible to speed up the routine (often quite
substantially) by using the lightweight callback API. The idea is that
the calling context only needs to be created and destroyed once, and
the sub can be called arbitrarily many times in between.
It is usual to pass parameters using global variables (typically $_ for
one parameter, or $a and $b for two parameters) rather than via @_. (It
is possible to use the @_ mechanism if you know what you're doing,
though there is as yet no supported API for it. It's also inherently
slower.)
The pattern of macro calls is like this:
dMULTICALL; /* Declare local variables */
U8 gimme = G_SCALAR; /* context of the call: G_SCALAR,
* G_ARRAY, or G_VOID */
PUSH_MULTICALL(cv); /* Set up the context for calling cv,
and set local vars appropriately */
/* loop */ {
/* set the value(s) af your parameter variables */
MULTICALL; /* Make the actual call */
} /* end of loop */
POP_MULTICALL; /* Tear down the calling context */
For some concrete examples, see the implementation of the first() and
reduce() functions of List::Util 1.18. There you will also find a
header file that emulates the multicall API on older versions of perl.
SEE ALSO
perlxs, perlguts, perlembed
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
Last updated for perl 5.23.1.
perl v5.26.3 2018-03-23 PERLCALL(1)
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