PERLCALL(1) Perl Programmers Reference Guide PERLCALL(1) #
PERLCALL(1) Perl Programmers Reference Guide PERLCALL(1)
NNAAMMEE #
perlcall - Perl calling conventions from C
DDEESSCCRRIIPPTTIIOONN #
The purpose of this document is to show you how to call Perl subroutines
directly from C, i.e., how to write _c_a_l_l_b_a_c_k_s.
Apart from discussing the C interface provided by Perl for writing
callbacks the document uses a series of examples to show how the
interface actually works in practice. In addition some techniques for
coding callbacks are covered.
Examples where callbacks are necessary include
• An Error Handler
You have created an XSUB interface to an 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, 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.
TTHHEE CCAALLLL__ FFUUNNCCTTIIOONNSS #
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 _c_a_l_l___s_v. 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 _c_a_l_l___s_v to invoke the Perl
subroutine.
All the _c_a_l_l___* 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
_c_a_l_l___s_v 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 _c_a_l_l___s_v.
call_pv
The function, _c_a_l_l___p_v, is similar to _c_a_l_l___s_v 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 _c_a_l_l___m_e_t_h_o_d 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 _c_a_l_l___m_e_t_h_o_d.
call_argv
_c_a_l_l___a_r_g_v 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 _a_l_w_a_y_s 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.
FFLLAAGG VVAALLUUEESS #
The "flags" parameter in all the _c_a_l_l___* functions is one of "G_VOID",
"G_SCALAR", or "G_LIST", which indicate the call context, OR'ed together
with a bit mask of any combination of the other G_* symbols defined
below.
GG__VVOOIIDD #
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 _w_a_n_t_a_r_r_a_y the result will be the
undefined value).
2. It ensures that nothing is actually returned from the subroutine.
The value returned by the _c_a_l_l___* function indicates how many items have
been returned by the Perl subroutine--in this case it will be 0.
GG__SSCCAALLAARR #
Calls the Perl subroutine in a scalar context. This is the default
context flag setting for all the _c_a_l_l___* 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 _w_a_n_t_a_r_r_a_y 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 _w_a_n_t_a_r_r_a_y
and return a list anyway. If so, then only the last element of the
list will be returned.
The value returned by the _c_a_l_l___* 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 _c_a_l_l___* function. The
section "Returning a List in Scalar Context" shows an example of this
behavior.
GG__LLIISSTT #
Calls the Perl subroutine in a list context. Prior to Perl version 5.35.1
this was called "G_ARRAY".
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 _w_a_n_t_a_r_r_a_y the result will be true).
2. It ensures that all items returned from the subroutine will be
accessible when control returns from the _c_a_l_l___* function.
The value returned by the _c_a_l_l___* 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_LIST flag
and the mechanics of accessing the returned items from the Perl stack.
GG__DDIISSCCAARRDD #
By default, the _c_a_l_l___* 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_LIST.
If you do not set this flag then it is _v_e_r_y 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.
GG__NNOOAARRGGSS #
Whenever a Perl subroutine is called using one of the _c_a_l_l___* 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 _c_a_l_l___* 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".
GG__EEVVAALL #
It is possible for the Perl subroutine you are calling to terminate
abnormally, e.g., by calling _d_i_e 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 _e_v_a_l _{ _} around the subroutine call.
Whenever control returns from the _c_a_l_l___* function you need to check the
$@ variable as you would in a normal Perl script.
The value returned from the _c_a_l_l___* 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 _c_a_l_l___* 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_LIST is specified _a_n_d an error has occurred, the return value
will always be 0.
• If G_SCALAR is specified _a_n_d an error has occurred, the return value
will be 1 and the value on the top of the stack will be _u_n_d_e_f. 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 _u_n_d_e_f
from the stack.
See "Using G_EVAL" for details on using G_EVAL.
GG__KKEEEEPPEERRRR #
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
_c_a_l_l___* 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.
DDeetteerrmmiinniinngg tthhee CCoonntteexxtt As mentioned above, you can determine the context of the currently executing subroutine in Perl with _w_a_n_t_a_r_r_a_y. The equivalent test can be made in C by using the “GIMME_V” macro, which returns “G_LIST” 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”.
EEXXAAMMPPLLEESS #
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 _c_a_l_l___p_v function. This has been done to keep the
code simpler and ease you into the topic. Wherever possible, if the
choice is between using _c_a_l_l___p_v and _c_a_l_l___s_v, you should always try to use
_c_a_l_l___s_v. See "Using call_sv" for details.
NNoo PPaarraammeetteerrss,, NNootthhiinngg RReettuurrnneedd This first trivial example will call a Perl subroutine, _P_r_i_n_t_U_I_D, to print out the UID of the process.
sub PrintUID
{
print "UID is $<\n";
}
and here is a C function to call it
static void
call_PrintUID()
{
dSP;
PUSHMARK(SP); #
call_pv("PrintUID", G_DISCARD|G_NOARGS);
}
Simple, eh?
A few points to note about this example:
1. Ignore "dSP" and "PUSHMARK(SP)" for now. They will be discussed in
the next example.
2. We aren't passing any parameters to _P_r_i_n_t_U_I_D so G_NOARGS can be
specified.
3. We aren't interested in anything returned from _P_r_i_n_t_U_I_D, so
G_DISCARD is specified. Even if _P_r_i_n_t_U_I_D was changed to return some
value(s), having specified G_DISCARD will mean that they will be
wiped by the time control returns from _c_a_l_l___p_v.
4. As _c_a_l_l___p_v is being used, the Perl subroutine is specified as a C
string. In this case the subroutine name has been 'hard-wired' into
the code.
5. Because we specified G_DISCARD, it is not necessary to check the
value returned from _c_a_l_l___p_v. It will always be 0.
PPaassssiinngg PPaarraammeetteerrss Now let’s make a slightly more complex example. This time we want to call a Perl subroutine, “LeftString”, which will take 2 parameters–a string ($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 _L_e_f_t_S_t_r_i_n_g 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 _c_a_l_l___L_e_f_t_S_t_r_i_n_g.
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 aallwwaayyss 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 _l_o_c_a_l 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
_c_a_l_l___* 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, _c_a_l_l___p_v 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, _n_o_t 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 XXPPUUSSHHss(()) 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 ssvv__22mmoorrttaall(()) 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, _L_e_f_t_S_t_r_i_n_g can now be called via the _c_a_l_l___p_v 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. #
RReettuurrnniinngg aa SSccaallaarr Now for an example of dealing with the items returned from a Perl subroutine.
Here is a Perl subroutine, _A_d_d_e_r, 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 _A_d_d_e_r, 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 _A_d_d_e_r will
still exist after the call to _c_a_l_l___p_v.
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
_c_a_l_l___p_v 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 _c_a_l_l___* functions or any other Perl internal function.
3. Although only a single value was expected to be returned from _A_d_d_e_r,
it is still good practice to check the return code from _c_a_l_l___p_v
anyway.
Expecting a single value is not quite the same as knowing that there
will be one. If someone modified _A_d_d_e_r 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 _r_e_a_l_l_y 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.
RReettuurrnniinngg aa LLiisstt ooff VVaalluueess 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_LIST);
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 _c_a_l_l___A_d_d_S_u_b_t_r_a_c_t 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_LIST 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 _r_e_v_e_r_s_e
order.
RReettuurrnniinngg aa LLiisstt iinn SSccaallaarr CCoonntteexxtt 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 _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r will print the
number of items returned from the Perl subroutine and their value (for
simplicity it assumes that they are integer). So if _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r 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. _A_d_d_S_u_b_t_r_a_c_t actually made it back
to _c_a_l_l___A_d_d_S_u_b_S_c_a_l_a_r.
RReettuurrnniinngg DDaattaa ffrroomm PPeerrll vviiaa tthhee PPaarraammeetteerr LLiisstt 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, _I_n_c, below takes 2 parameters and increments each
directly.
sub Inc
{
++ $_[0];
++ $_[1];
}
and here is a C function to call it.
static void
call_Inc(a, b)
int a;
int b;
{
dSP;
int count;
SV * sva;
SV * svb;
ENTER; #
SAVETMPS; #
sva = sv_2mortal(newSViv(a));
svb = sv_2mortal(newSViv(b));
PUSHMARK(SP); #
EXTEND(SP, 2); #
PUSHs(sva);
PUSHs(svb);
PUTBACK; #
count = call_pv("Inc", G_DISCARD);
if (count != 0)
croak ("call_Inc: expected 0 values from 'Inc', got %d\n",
count);
printf ("%d + 1 = %d\n", a, SvIV(sva));
printf ("%d + 1 = %d\n", b, SvIV(svb));
FREETMPS; #
LEAVE; #
}
To be able to access the two parameters that were pushed onto the stack
after they return from _c_a_l_l___p_v it is necessary to make a note of their
addresses--thus the two variables "sva" and "svb".
The reason this is necessary is that the area of the Perl stack which
held them will very likely have been overwritten by something else by the
time control returns from _c_a_l_l___p_v.
UUssiinngg GG__EEVVAALL Now an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If this would result in a negative result, the subroutine calls _d_i_e.
sub Subtract
{
my ($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 _c_a_l_l___S_u_b_t_r_a_c_t 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 _d_i_e so we have used the G_EVAL flag.
Not specifying this flag would mean that the program would terminate
immediately at the _d_i_e statement in the subroutine _S_u_b_t_r_a_c_t.
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
_c_a_l_l___* function invoked with G_EVAL|G_SCALAR returns an error, the
top of the stack holds the value _u_n_d_e_f. Because we want the program
to continue after detecting this error, it is essential that the
stack be tidied up by removing the _u_n_d_e_f.
UUssiinngg GG__KKEEEEPPEERRRR 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 _c_a_l_l___p_v 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 _c_a_l_l___p_v call in call_Subtract
reads:
count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);
will preserve the error and restore reliable error handling.
UUssiinngg ccaallll__ssvv 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 _C_a_l_l_S_u_b_P_V.
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 _c_a_l_l___s_v is useful.
The code below for _C_a_l_l_S_u_b_S_V is identical to _C_a_l_l_S_u_b_P_V except that the
"name" parameter is now defined as an SV* and we use _c_a_l_l___s_v instead of
_c_a_l_l___p_v.
void
CallSubSV(name)
SV * name
CODE: #
PUSHMARK(SP); #
call_sv(name, G_DISCARD|G_NOARGS);
Because we are using an SV to call _f_r_e_d the following can all be used:
CallSubSV("fred");
CallSubSV(\&fred);
$ref = \&fred;
CallSubSV($ref);
CallSubSV( sub { print "Hello there\n" } );
As you can see, _c_a_l_l___s_v 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);
$ref = \&joe;
CallSavedSub1();
This time whenever "CallSavedSub1" gets called it will execute the Perl
subroutine "joe" (assuming it exists) rather than "fred" as was
originally requested in the call to "SaveSub1".
To get around these problems it is necessary to take a full copy of the
SV. The code below shows "SaveSub2" modified to do that.
/* this isn't thread-safe */
static SV * keepSub = (SV*)NULL;
void
SaveSub2(name)
SV * name
CODE: #
/* Take a copy of the callback */
if (keepSub == (SV*)NULL)
/* First time, so create a new SV */
keepSub = newSVsv(name);
else
/* Been here before, so overwrite */
SvSetSV(keepSub, name);
void
CallSavedSub2()
CODE: #
PUSHMARK(SP); #
call_sv(keepSub, G_DISCARD|G_NOARGS);
To avoid creating a new SV every time "SaveSub2" is called, the function
first checks to see if it has been called before. If not, then space for
a new SV is allocated and the reference to the Perl subroutine "name" is
copied to the variable "keepSub" in one operation using "newSVsv".
Thereafter, whenever "SaveSub2" is called, the existing SV, "keepSub", is
overwritten with the new value using "SvSetSV".
Note: using a static or global variable to store the SV isn't thread-
safe. You can either use the "MY_CXT" mechanism documented in "Safely
Storing Static Data in XS" in perlxs which is fast, or store the values
in perl global variables, using ggeett__ssvv(()), which is much slower.
UUssiinngg ccaallll__aarrggvv Here is a Perl subroutine which prints whatever parameters are passed to it.
sub PrintList
{
my(@list) = @_;
foreach (@list) { print "$_\n" }
}
And here is an example of _c_a_l_l___a_r_g_v which will call _P_r_i_n_t_L_i_s_t.
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 _c_a_l_l___a_r_g_v will do it for you.
UUssiinngg ccaallll__mmeetthhoodd 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:
• A reference to the object for a virtual method or the name of the
class for a static method
• The name of the method
• Any other parameters specific to the method
Here is a simple XSUB which illustrates the mechanics of calling both the
"PrintID" and "Display" methods from C.
void
call_Method(ref, method, index)
SV * ref
char * method
int index
CODE: #
PUSHMARK(SP); #
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 _c_a_l_l___m_e_t_h_o_d.
UUssiinngg GGIIMMMMEE__VV 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
UUssiinngg PPeerrll ttoo DDiissppoossee ooff TTeemmppoorraarriieess In the examples given to date, any temporaries created in the callback (i.e., parameters passed on the stack to the _c_a_l_l___* function or values returned via the stack) have been freed by one of these methods:
• Specifying the G_DISCARD flag with _c_a_l_l___*
• 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 _c_a_l_l___* 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 _n_e_v_e_r 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.
SSttrraatteeggiieess ffoorr SSttoorriinngg CCaallllbbaacckk CCoonntteexxtt IInnffoorrmmaattiioonn 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 _P_r_o_c_e_s_s_R_e_a_d 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.
AAlltteerrnnaattee SSttaacckk MMaanniippuullaattiioonn 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_LIST);
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.
CCrreeaattiinngg aanndd CCaalllliinngg aann AAnnoonnyymmoouuss SSuubbrroouuttiinnee iinn CC 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.
LLIIGGHHTTWWEEIIGGHHTT CCAALLLLBBAACCKKSS #
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 ssoorrtt(()). You can pass a comparison function to ssoorrtt(()),
which will then be invoked for every pair of values that needs to be
compared. The ffiirrsstt(()) and rreedduuccee(()) 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_LIST, 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 ffiirrsstt(()) and
rreedduuccee(()) functions of List::Util 1.18. There you will also find a header
file that emulates the multicall API on older versions of perl.
SSEEEE AALLSSOO #
perlxs, perlguts, perlembed
AAUUTTHHOORR #
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.
DDAATTEE #
Last updated for perl 5.23.1.
perl v5.36.3 2023-02-15 PERLCALL(1)