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 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 addi-
tion some techniques for coding callbacks are covered.
Examples where callbacks are necessary include
* 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.
* 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 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 con-
sidered 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
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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 "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 iden-
tifies 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
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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 par-
ticular number of values to be returned from the Perl sub-
routine, there is nothing to stop someone from doing some-
thing 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 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.
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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 subrou-
tine 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 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 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.
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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 subrou-
tine) 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 tem-
poraries 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 creat-
ing 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 possi-
ble 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
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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:
+ If the call_* function returns normally, then the value
returned is as specified in the previous sections.
+ 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 prob-
lematic 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
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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 $@. an error will not be appended
if that same error string is already at the end of $@.
In addition, a warning is generated using the appended
string. This can be disabled using "no warnings 'misc'".
The G_KEEPERR flag was introduced in Perl version 5.002.
See Using G_KEEPERR for an example of a situation that war-
rants 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.
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sub PrintUID
{
print "UID is $<\n";
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP;
PUSHMARK(SP);
call_pv("PrintUID", G_DISCARD|G_NOARGS);
}
Simple, eh.
A few points to note about this example.
1. Ignore "dSP" and "PUSHMARK(SP)" for now. They will be
discussed in the next example.
2. We aren't passing any parameters to PrintUID so
G_NOARGS can be specified.
3. We aren't interested in anything returned from Prin-
tUID, so G_DISCARD is specified. Even if PrintUID was
changed to return some value(s), having specified
G_DISCARD will mean that they will be wiped by the time
control returns from call_pv.
4. As call_pv is being used, the Perl subroutine is speci-
fied as a C string. In this case the subroutine name
has been 'hard-wired' into the code.
5. Because we specified G_DISCARD, it is not necessary to
check the value returned from call_pv. It will always
be 0.
Passing Parameters
Now let's make a slightly more complex example. This time we
want to call a Perl subroutine, "LeftString", which will
take 2 parameters--a string ($s) and an integer ($n). The
subroutine will simply print the first $n characters of the
string.
So the Perl subroutine would look like this
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sub LeftString
{
my($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 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
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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 automat-
ically. 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 pass-
ing any parameters (like the example shown in the sec-
tion 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 "XSUBs and the Argument Stack" in perlguts 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 boun-
dary 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.
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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");
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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 know-
ing that there will be one. If someone modified Adder
to return a list and we didn't check for that possibil-
ity 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
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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;
}
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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 impor-
tant 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);
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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, 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 param-
eter 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];
++ $_[1];
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a;
int b;
{
dSP;
int count;
SV * sva;
SV * svb;
ENTER;
SAVETMPS;
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sva = sv_2mortal(newSViv(a));
svb = sv_2mortal(newSViv(b));
PUSHMARK(SP);
XPUSHs(sva);
XPUSHs(svb);
PUTBACK;
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count);
printf ("%d + 1 = %d\n", a, SvIV(sva));
printf ("%d + 1 = %d\n", b, SvIV(svb));
FREETMPS;
LEAVE;
}
To be able to access the two parameters that were pushed
onto the stack after they return from call_pv it is neces-
sary to make a note of their addresses--thus the two vari-
ables "sva" and "svb".
The reason this is necessary is that the area of the Perl
stack which held them will very likely have been overwritten
by something else by the time control returns from call_pv.
Using G_EVAL
Now an example using G_EVAL. Below is a Perl subroutine
which computes the difference of its 2 parameters. If this
would result in a negative result, the subroutine calls die.
sub Subtract
{
my ($a, $b) = @_;
die "death can be fatal\n" if $a < $b;
$a - $b;
}
and some C to call it
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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
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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 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 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
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exiting the eval block, and because call_Subtract is imple-
mented 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.
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 sub-
routine can be specified as only a string. For Perl 4 this
was adequate, but Perl 5 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);
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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 have
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.
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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);
$ref = \&joe;
CallSavedSub1();
This time whenever "CallSavedSub1" get called it will exe-
cute the Perl subroutine "joe" (assuming it exists) rather
than "fred" as was originally requested in the call to
"SaveSub1".
To get around these problems it is necessary to take a full
copy of the SV. The code below shows "SaveSub2" modified to
do that
static SV * keepSub = (SV*)NULL;
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void
SaveSub2(name)
SV * name
CODE:
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name);
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name);
void
CallSavedSub2()
CODE:
PUSHMARK(SP);
call_sv(keepSub, G_DISCARD|G_NOARGS);
To avoid creating a new SV every time "SaveSub2" is called,
the function first checks to see if it has been called
before. If not, then space for a new SV is allocated and
the reference to the Perl subroutine, "name" is copied to
the variable "keepSub" in one operation using "newSVsv".
Thereafter, whenever "SaveSub2" is called the existing SV,
"keepSub", is overwritten with the new value using
"SvSetSV".
Using call_argv
Here is a Perl subroutine which prints whatever parameters
are passed to it.
sub PrintList
{
my(@list) = @_;
foreach (@list) { print "$_\n" }
}
and here is an example of call_argv which will call
PrintList.
static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};
static void
call_PrintList()
{
dSP;
call_argv("PrintList", G_DISCARD, words);
}
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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.
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+ 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.
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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 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
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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
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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 con-
sider 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
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of its parameters. Below is a call to a hypothetical func-
tion "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 call-
back it registers, "pcb1", might look like this
# Register the sub pcb1
register_fatal(\&pcb1);
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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 com-
pleted. 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 ade-
quate 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 asso-
ciates 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 subrou-
tine 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;
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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);
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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 dis-
tinct advantage that it allows an unlimited number of call-
backs 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" parame-
ter 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 prede-
fine 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();
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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;
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/* 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.
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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 suit-
able 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*".
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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 necessary
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
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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.
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
Version 1.3, 14th Apr 1997
perl v5.8.8 2006-06-30 36
