PERLCALL(1)

PERLCALL(1) — straight out of the Perl Programmers Reference Guide, and bestie, it's a whole journey 🦋

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✨ NAME ✨

perlcall — Perl calling conventions from C 💅

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🎀 DESCRIPTION 🎀

okay so the whole point of this doc? it's here to show you how to call Perl subroutines directly from C — aka, how to write callbacks. that's the vibe. 🫶

and it's not just dry theory either. besides walking through the C interface Perl gives you for writing callbacks, it uses a whole series of examples to show how that interface actually works out in the real world. plus it covers some techniques for coding callbacks along the way. iconic. ✨

so like, when do you even need a callback? examples include:

• An Error Handler 😭

picture this: you've created an XSUB interface to some application's C API. and a pretty common feature in applications is letting you define a C function that gets called whenever something nasty happens. what we want instead is the ability to specify a Perl subroutine that gets called instead. slay.

• An Event-Driven Program 🦋

the classic callback situation — writing an event driven program, like 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. very main character energy.

ngl, the techniques described here do apply when you're embedding Perl in a C program — but that is not the primary goal of this document, bestie. there are other details you've gotta consider that are specific to embedding Perl. for the full tea on embedding Perl in C, go read perlembed.

and before you launch yourself head first into the rest of this document? it would be a really good idea to have already read these two: perlxs and perlguts. do your homework first, i'm begging 🙈

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👑 THE CALL_ FUNCTIONS 👑

so this stuff is genuinely easier to explain with examples, but first you need to be aware of a few important definitions. no skipping.

Perl has a number of C functions that let you 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 here is call_sv. all the other functions are just fairly simple wrappers that 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. periodt. 💅

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, so no surprises. the settings available in the bit mask get discussed in "FLAG VALUES".

okay now, each of the functions, one by one:

💖 call_sv

call_sv takes two parameters. the first, "sv", is an SV*. this lets you specify the Perl subroutine to be called either as a C string (which has first been converted to an SV) or as a reference to a subroutine. the section "Using call_sv" shows you how to make use of call_sv. 🎀

💖 call_pv

the function call_pv is similar to call_sv, except it expects its first parameter to be a C char* which identifies the Perl subroutine you want to call, e.g., call_pv("fred", 0). and if the subroutine you want to call lives in another package, just include the package name in the string, e.g., pkg::fred. easy. ✨

💖 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 the method belongs to is passed on the Perl stack rather than in the parameter list — yes, really, that's just how it is. 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 info on static and virtual methods, and "Using call_method" for an example of using call_method. 🦋

💖 call_argv

call_argv calls the Perl subroutine specified by the C string stored in the "subname" parameter. it also takes the usual "flags" parameter. the final parameter, "argv", consists of a NULL-terminated list of C strings to be passed as parameters to the Perl subroutine. see "Using call_argv". 🌸

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so, 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. 🫶

and as a general rule? you should ALWAYS check the return value from these functions. even if you're only expecting a particular number of values back from the Perl subroutine, there is literally nothing stopping someone from doing something unexpected — don't say you haven't been warned, bestie 💀

🚩 FLAG VALUES 🚩

okay bestie so the flags parameter in all the call_* functions is one of G_VOID, G_SCALAR, or G_LIST — those tell it the call context — and you OR them together with a bit mask of literally any combo of the other G_* symbols listed below. mix and match, it's giving ✨customization✨

🌸 G_VOID 🌸

Calls the Perl subroutine in a void context. iconic.

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. zero. nada. nothing. 💅

💖 G_SCALAR 💖

Calls the Perl subroutine in a scalar context. This is the default context flag setting for all the call_* functions.

This flag has 2 effects:

1. It indicates to the subroutine being called that it is executing in a scalar context (if it executes wantarray the result will be false).
2. It ensures that only a scalar is actually returned from the subroutine. The subroutine can, of course, ignore the wantarray and return a list anyway. If so, then only the last element of the list will be returned.

The value returned by the call_* function indicates how many items have been returned by the Perl subroutine — in this case it will be either 0 or 1.

If 0, then you have specified the G_DISCARD flag.

If 1, then the item actually returned by the Perl subroutine will be stored on the Perl stack — the section "Returning a Scalar" shows how to access this value on the stack. Remember that regardless of how many items the Perl subroutine returns, only the last one will be accessible from the stack — think of the case where only one value is returned as being a list with only one element. Any other items that were returned will not exist by the time control returns from the call_* function. The section "Returning a List in Scalar Context" shows an example of this behavior. the last one ate and the rest got left on read 😭

🌈 G_LIST 🌈

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 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_LIST 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_LIST.

If you do not set this flag then it is very important that you make sure that any temporaries (i.e., parameters passed to the Perl subroutine and values returned from the subroutine) are disposed of yourself. bestie please don't leave temporaries just vibing on the stack, that's how you get leaks 😭 The section "Returning a Scalar" gives details of how to dispose of these temporaries explicitly and the section "Using Perl to Dispose of Temporaries" discusses the specific circumstances where you can ignore the problem and let Perl deal with it for you.

🙅 G_NOARGS 🙅

Whenever a Perl subroutine is called using one of the call_* functions, it is assumed by default that parameters are to be passed to the subroutine. If you are not passing any parameters to the Perl subroutine, you can save a bit of time by setting this flag. It has the effect of not creating the @_ array for the Perl subroutine.

Although the functionality provided by this flag may seem straightforward, it should be used only if there is a good reason to do so. The reason for being cautious is that, even if you have specified the G_NOARGS flag, it is still possible for the Perl subroutine that has been called to think that you have passed it parameters. plot twist!!

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. not fred stealing joe's whole @_ array 💀

🪤 G_EVAL 🪤

It is possible for the Perl subroutine you are calling to terminate abnormally, e.g., by calling die explicitly or by not actually existing. By default, when either of these events occurs, the process will terminate immediately. If you want to trap this type of event, specify the G_EVAL flag. It will put an eval { } around the subroutine call.

Whenever control returns from the call_* function you need to check the $@ variable as you would in a normal Perl script.

The value returned from the call_* function is dependent on what other flags have been specified and whether an error has occurred. Here are all the different cases that can occur:

• 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_LIST 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. don't forget to pop it, it'll just sit there being undef and weird ✨

See "Using G_EVAL" for details on using G_EVAL.

🛡️ G_KEEPERR 🛡️

Using the G_EVAL flag described above will always set $@: clearing it if there was no error, and setting it to describe the error if there was an error in the called code. This is what you want if your intention is to handle possible errors, but sometimes you just want to trap errors and stop them interfering with the rest of the program.

This scenario will mostly be applicable to code that is meant to be called from within destructors, asynchronous callbacks, and signal handlers. In such situations, where the code being called has little relation to the surrounding dynamic context, the main program needs to be insulated from errors in the called code, even if they can't be handled intelligently. It may also be useful to do this with code for __DIE__ or __WARN__ hooks, and tie functions.

The G_KEEPERR flag is meant to be used in conjunction with G_EVAL in call_* functions that are used to implement such code, or with eval_sv. This flag has no effect on the call_* functions when G_EVAL is not used.

When G_KEEPERR is used, any error in the called code will terminate the call as usual, and the error will not propagate beyond the call (as usual for G_EVAL), but it will not go into $@. Instead the error will be converted into a warning, prefixed with the string "\t(in cleanup)". This can be disabled using no warnings 'misc'. If there is no error, $@ will not be cleared.

Note that the G_KEEPERR flag does not propagate into inner evals; these may still set $@.

The G_KEEPERR flag was introduced in Perl version 5.002. a classic 🫶

See "Using G_KEEPERR" for an example of a situation that warrants the use of this flag.

🧭 Determining the Context 🧭

As mentioned above, you can determine the context of the currently executing subroutine in Perl with wantarray. The equivalent test can be made in C by using the GIMME_V macro, which returns G_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). A deprecated 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". deprecated = lowkey still there but don't @ me 💅

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💖 EXAMPLES 💖

Enough of the definition talk! Let's have a few examples. finally, the fun part bestie ✨

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. call_sv is the main character here, no cap 👑

🌸 No Parameters, Nothing Returned 🌸

This first trivial example will call a Perl subroutine, PrintUID, to print out the UID of the process.

    sub PrintUID
    {
        print "UID is $<\n";
    }

and here is a C function to call it

    static void
    call_PrintUID()
    {
        dSP;

        PUSHMARK(SP);
        call_pv("PrintUID", G_DISCARD|G_NOARGS);
    }

Simple, eh? literally so simple bestie

A few points to note about this example:

1. Ignore dSP and PUSHMARK(SP) for now. They will be discussed in the next example.
2. We aren't passing any parameters to PrintUID so G_NOARGS can be specified.
3. We aren't interested in anything returned from PrintUID, so G_DISCARD is specified. Even if PrintUID was changed to return some value(s), having specified G_DISCARD will mean that they will be wiped by the time control returns from call_pv.
4. As call_pv is being used, the Perl subroutine is specified as a C string. In this case the subroutine name has been 'hard-wired' into the code.
5. Because we specified G_DISCARD, it is not necessary to check the value returned from call_pv. It will always be 0.

🎀 Passing Parameters 🎀

Now let's make a slightly more complex example. This time we want to call a Perl subroutine, LeftString, which will take 2 parameters — a string ($s) and an integer ($n). The subroutine will simply print the first $n characters of the string.

So the Perl subroutine would look like this:

    sub LeftString
    {
        my($s, $n) = @_;
        print substr($s, 0, $n), "\n";
    }

The C function you'd need to call LeftString looks 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;
}

okay bestie, here are the receipts on this call_LeftString C function 💅

1. Parameters get handed to the Perl subroutine via the Perl stack. That's literally the whole job of the chunk starting at the line dSP and ending at the line PUTBACK. The dSP declares a local copy of the stack pointer. That local copy? always access it as SP. no cap.
2. If you're gonna put something onto the Perl stack, you gotta know where to put it. That's the entire reason the macro dSP exists — it declares and initializes a local copy of the Perl stack pointer.

Every other macro in this example needs you to have used that macro first. fr fr.

The one exception, the little plot twist 🎀: if you're calling a Perl subroutine directly from an XSUB function, you don't need to explicitly use the dSP macro — it gets declared for you automatically. iconic.

3. Any parameters you push onto the stack should be bracketed by the PUSHMARK and PUTBACK macros. Why, you ask? In this context, those two macros count how many parameters you're pushing automatically. Then when Perl is creating the @_ array for the subroutine, it knows how big to make it. it's giving organized.

The PUSHMARK macro tells Perl to make a mental note of the current stack pointer. And even if you aren't passing any parameters — like the example in the section "No Parameters, Nothing Returned" — you must still call PUSHMARK before you can call any of the call_* functions. Perl still needs to know there are no parameters. periodt.

The PUTBACK macro sets the global copy of the stack pointer to be the same as your local copy. If we didn't do that, call_pv would have zero clue where the two parameters we pushed were — remember, up until now all the stack pointer manipulation has been on our local copy, not the global one. 🙈

4. Next up: EXTEND and PUSHs. This is where the parameters actually get pushed onto the stack. Here we're pushing a string and an integer.

Alternatively you can use the XPUSHs() macro, which combines EXTEND(SP, 1) and PUSHs(). Heads up though — that's less efficient if you're pushing multiple values. just so you know.

See "XSUBs and the Argument Stack" in perlguts for the details on how the PUSH macros work.

5. Because we created temporary values (via sv_2mortal() calls) we have to tidy up the Perl stack and dispose of mortal SVs. self care ✨

That's the job 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 — meaning the temporaries we get rid of are limited to the ones created after these calls. Obsessed with a clean boundary.

The FREETMPS/LEAVE pair gets rid of any values returned by the Perl subroutine (see next example), plus it dumps the mortal SVs we've created. Having ENTER/SAVETMPS at the beginning of the code makes sure no other mortals get destroyed. 🫶

Think of these macros as working a bit like { and } in Perl, limiting 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 be called via the call_pv function. The only flag specified this time is G_DISCARD. And because we're passing 2 parameters to the Perl subroutine this time, we have not specified G_NOARGS.

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💖 Returning a Scalar 💖

Now for an example of dealing with the items returned from a Perl subroutine. the way that return values need handling… 😭

Here's a Perl subroutine, Adder, that takes 2 integer parameters and simply returns their sum.

sub Adder
{
    my($x, $y) = @_;
    $x + $y;
}

Because we now care about the return value from Adder, the C function required to call it is 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 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's possible the memory allocated to the Perl stack has been reallocated during the call_pv call. lowkey scary but okay

If you're 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. Even though only a single value was expected back from Adder, it's still good practice to check the return code from call_pv anyway.

Expecting a single value is not quite the same as knowing there will be one. If someone modified Adder to return a list and we didn't check for that possibility and take appropriate action, the Perl stack would end up in an inconsistent state. And that is something you really don't want to happen. ever. 💀

4. The POPi macro is used here to pop the return value off the stack. In this case we wanted an integer, so POPi was used.

Here's 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 off 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. ✨

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🌸 Returning a List of Values 🌸

Now let's extend the previous example to return both the sum of the parameters and the difference. multi-value era 👑

Here's the Perl subroutine:

sub AddSubtract
{
   my($x, $y) = @_;
   ($x+$y, $x-$y);
}

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 call_AddSubtract is called like this

call_AddSubtract(7, 4);

then here's 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 reverse order.

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🦋 Returning a List in Scalar Context 🦋

Say the Perl subroutine in the previous section was called in a scalar context, like this

static void
call_AddSubScalar(a, b)
int a;
int b;
{
    dSP;
    int count;
    int i;

    ENTER;
    SAVETMPS;

    PUSHMARK(SP);
    EXTEND(SP, 2);
    PUSHs(sv_2mortal(newSViv(a)));
    PUSHs(sv_2mortal(newSViv(b)));
    PUTBACK;

    count = call_pv("AddSubtract", G_SCALAR);

    SPAGAIN;

    printf ("Items Returned = %d\n", count);

    for (i = 1; i <= count; ++i)
        printf ("Value %d = %d\n", i, POPi);

    PUTBACK;
    FREETMPS;
    LEAVE;
}

The other modification made is that call_AddSubScalar will print the number of items returned from the Perl subroutine and their value (for simplicity it assumes they're integers). So if call_AddSubScalar is called

call_AddSubScalar(7, 4);

then the output will be

Items Returned = 1
Value 1 = 3

In this case the main point to note is that only the last item in the list is returned from the subroutine. AddSubtract actually made it back to call_AddSubScalar. it really did the thing 🥹

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💜 Returning Data from Perl via the Parameter List 💜

It's also possible to return values directly via the parameter list — whether it's actually desirable to do it is another matter entirely. iykyk

The Perl subroutine Inc, below, takes 2 parameters and increments each directly.

ok so, bestie, here's the Perl sub in question:

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;
}

So the way you get to actually access those two parameters that were pushed onto the stack, after they come back from call_pv, is to make a note of their addresses — that is literally why the two variables sva and svb exist as their own things. 🫶

And the reason that's non-negotiable? Because 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. 💀 The stack is not your friend after the call, ok? Save the pointers.

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✨ Using G_EVAL ✨

Now an example using G_EVAL. Below is a Perl subroutine which computes the difference of its 2 parameters. If that would result in a negative result, the subroutine calls die.

sub Subtract
{
    my ($x, $y) = @_;

    die "death can be fatal\n" if $x < $y;

    $x - $y;
}

and some C to call it

static void
call_Subtract(a, b)
int a;
int b;
{
    dSP;
    int count;
    SV *err_tmp;

    ENTER;
    SAVETMPS;

    PUSHMARK(SP);
    EXTEND(SP, 2);
    PUSHs(sv_2mortal(newSViv(a)));
    PUSHs(sv_2mortal(newSViv(b)));
    PUTBACK;

    count = call_pv("Subtract", G_EVAL|G_SCALAR);

    SPAGAIN;

    /* Check the eval first */
    err_tmp = ERRSV;
    if (SvTRUE(err_tmp))
    {
        printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
        POPs;
    }
    else
    {
      if (count != 1)
       croak("call_Subtract: wanted 1 value from 'Subtract', got %d\n",
             count);

        printf ("%d - %d = %d\n", a, b, POPi);
    }

    PUTBACK;
    FREETMPS;
    LEAVE;
}

If call_Subtract is called thus

call_Subtract(4, 5)

the following will be printed

Uh oh - death can be fatal

✨ Notes ✨

1. We want to be able to catch the die, so we've used the G_EVAL flag. Not specifying this flag would mean the program would terminate immediately at the die statement in the subroutine Subtract. 😭
2. The code

err_tmp = ERRSV;
if (SvTRUE(err_tmp))
{
    printf ("Uh oh - %s\n", SvPV_nolen(err_tmp));
    POPs;
}

is the direct equivalent of this bit of Perl

print "Uh oh - $@\n" if $@;

PL_errgv is a perl global of type GV * that points to the symbol table entry containing the error. ERRSV therefore refers to the C equivalent of $@. We use a local temporary, err_tmp, since ERRSV is a macro that calls a function, and SvTRUE(ERRSV) would end up calling that function multiple times. 😮‍💨

3. Note that the stack is popped using POPs in the block where SvTRUE(err_tmp) is true. This is necessary because whenever a call_* function invoked with G_EVAL|G_SCALAR returns an error, the top of the stack holds the value undef. Because we want the program to continue after detecting this error, it is essential that the stack be tidied up by removing the undef. 🧹

---

✨ Using G_KEEPERR ✨

Consider this rather facetious example, where we have used an XS version of the call_Subtract example above inside a destructor:

package Foo;
sub new { bless {}, $_[0] }
sub Subtract {
    my($x,$y) = @_;
    die "death can be fatal" if $x < $y;
    $x - $y;
}
sub DESTROY { call_Subtract(5, 4); }
sub foo { die "foo dies"; }

package main;
{
    my $foo = Foo->new;
    eval { $foo->foo };
}
print "Saw: $@" if $@;             # should be, but isn't

This example will fail to recognize that an error occurred inside the eval {}. Here's why: the call_Subtract code got executed while perl was cleaning up temporaries when exiting the outer braced block, and because call_Subtract is implemented with call_pv using the G_EVAL flag, it promptly reset $@. This results in the failure of the outermost test for $@, and thereby the failure of the error trap. 💀

Appending the G_KEEPERR flag, so that the call_pv call in call_Subtract reads:

count = call_pv("Subtract", G_EVAL|G_SCALAR|G_KEEPERR);

will preserve the error and restore reliable error handling. ✨

---

✨ Using call_sv ✨

In all the previous examples I have 'hard-wired' the name of the Perl subroutine to be called from C. Most of the time though, it is more convenient to be able to specify the name of the Perl subroutine from within the Perl script, and you'll want to use call_sv. 🦋

Consider the Perl code below

sub fred
{
    print "Hello there\n";
}

CallSubPV("fred");

Here is a snippet of XSUB which defines CallSubPV.

void
CallSubPV(name)
    char *  name
    CODE:
    PUSHMARK(SP);
    call_pv(name, G_DISCARD|G_NOARGS);

That is fine as far as it goes. The thing is, the Perl subroutine can be specified as only a string, however, Perl allows references to subroutines and anonymous subroutines. This is where call_sv is useful. 👑

The code below for CallSubSV is identical to CallSubPV except that the name parameter is now defined as an SV* and we use call_sv instead of call_pv.

void
CallSubSV(name)
    SV *    name
    CODE:
    PUSHMARK(SP);
    call_sv(name, G_DISCARD|G_NOARGS);

Because we are using an SV to call fred the following can all be used:

CallSubSV("fred");
CallSubSV(\&fred);
$ref = \&fred;
CallSubSV($ref);
CallSubSV( sub { print "Hello there\n" } );

As you can see, call_sv gives you much greater flexibility in how you can specify the Perl subroutine. 🌈

You should note that, if it is necessary to store the SV (name in the example above) which corresponds to the Perl subroutine so that it can be used later in the program, it is not enough just to store a copy of the pointer to the SV. Say the code above had been like this:

static SV * rememberSub;

void
SaveSub1(name)
    SV *    name
    CODE:
    rememberSub = name;

void
CallSavedSub1()
    CODE:
    PUSHMARK(SP);
    call_sv(rememberSub, G_DISCARD|G_NOARGS);

The reason this is wrong is that, by the time you come to use the pointer rememberSub in CallSavedSub1, it may or may not still refer to the Perl subroutine that was recorded in SaveSub1. This is particularly true for these cases:

SaveSub1(\&fred);
CallSavedSub1();

SaveSub1( sub { print "Hello there\n" } );
CallSavedSub1();

By the time each of the SaveSub1 statements above has been executed, the SV*s which corresponded to the parameters will no longer exist. Expect an error message from Perl of the form

Can't use an undefined value as a subroutine reference at ...

for each of the CallSavedSub1 lines. 🙈

Similarly, with this code

$ref = \&fred;
SaveSub1($ref);
$ref = 47;
CallSavedSub1();

you can expect one of these messages (which you actually get is dependent on the version of Perl you are using)

Not a CODE reference at ...
Undefined subroutine &main::47 called ...

The variable $ref may have referred to the subroutine fred whenever the call to SaveSub1 was made but by the time CallSavedSub1 gets called it now holds the number 47. Because we saved only a pointer to the original SV in SaveSub1, any changes to $ref will be tracked by the pointer rememberSub. This means that whenever CallSavedSub1 gets called, it will attempt to execute the code which is referenced by the SV* rememberSub. In this case though, it now refers to the integer 47, so expect Perl to complain loudly. 📢

A similar but more subtle problem is illustrated with this code:

$ref = \&fred;
SaveSub1($ref);
$ref = \&joe;
CallSavedSub1();

This time whenever CallSavedSub1 gets called it will execute the Perl subroutine joe (assuming it exists) rather than fred as was originally requested in the call to SaveSub1. 💀

To get around these problems it is necessary to take a full copy of the SV. The code below shows SaveSub2 modified to do that.

/* this isn't thread-safe */
static SV * keepSub = (SV*)NULL;

void
SaveSub2(name)
    SV *    name
    CODE:
    /* Take a copy of the callback */
    if (keepSub == (SV*)NULL)
        /* First time, so create a new SV */
        keepSub = newSVsv(name);
    else
        /* Been here before, so overwrite */
        SvSetSV(keepSub, name);

void
CallSavedSub2()
    CODE:
    PUSHMARK(SP);
    call_sv(keepSub, G_DISCARD|G_NOARGS);

To avoid creating a new SV every time SaveSub2 is called, the function first checks to see if it has been called before. If not, then space for a new SV is allocated and the reference to the Perl subroutine name is copied to the variable keepSub in one operation using newSVsv. Thereafter, whenever SaveSub2 is called, the existing SV, keepSub, is overwritten with the new value using SvSetSV. 💅

Note: using a static or global variable to store the SV isn't thread-safe. You can either use the MY_CXT mechanism documented in "Safely Storing Static Data in XS" in perlxs which is fast, or store the values in perl global variables, using get_sv(), which is much slower.

---

✨ Using call_argv ✨

Here is a Perl subroutine which prints whatever parameters are passed to it.

sub PrintList
{
    my(@list) = @_;

    foreach (@list) { print "$_\n" }
}

And here is an example of call_argv which will call PrintList.

static char * words[] = {"alpha", "beta", "gamma", "delta", NULL};

static void
call_PrintList()
{
    call_argv("PrintList", G_DISCARD, words);
}

and notice how there's no PUSHMARK in here at all?? that's not me forgetting bestie, that's the whole point — call_argv will do it for you. it's giving ✨effortless✨ 💅

---

🎀 Using call_method 🎀

okay so 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.

$x = Mine->new('red', 'green', 'blue');
$x->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:

$x = Mine->new('red', 'green', 'blue');
call_Method($x, 'Display', 1);
call_PrintID('Mine', 'PrintID');

The only thing to note is that, in both the static and virtual methods, the method name is not passed via the stack--it is used as the first parameter to call_method. yes literally the name itself is an argument, not a stack thing, don't @ me 🫶

---

✨ Using GIMME_V ✨

Here is a trivial XSUB which prints the context in which it is currently executing.

void
PrintContext()
    CODE:
    U8 gimme = GIMME_V;
    if (gimme == G_VOID)
        printf ("Context is Void\n");
    else if (gimme == G_SCALAR)
        printf ("Context is Scalar\n");
    else
        printf ("Context is Array\n");

And here is some Perl to test it.

PrintContext;
$x = PrintContext;
@a = PrintContext;

The output from that will be

Context is Void
Context is Scalar
Context is Array

it just knows what context it's in, no cap, main character energy fr fr 👑

---

💅 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 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 bestie 👀

In the first example, an error handler, the flow of control could be as follows. You have created an interface to an external library. Control can reach the external library like this

perl --> XSUB --> external library

Whilst control is in the library, an error condition occurs. You have previously set up a Perl callback to handle this situation, so it will get executed. Once the callback has finished, control will drop back to Perl again. Here is what the flow of control will be like in that situation

perl --> XSUB --> external library
                  ...
                  error occurs
                  ...
                  external library --> call_* --> perl
                                                      |
perl <-- XSUB <-- external library <-- call_* <----+

After processing of the error using call_* is completed, control reverts back to Perl more or less immediately.

In the diagram, the further right you go the more deeply nested the scope is. It is only when control is back with perl on the extreme left of the diagram that you will have dropped back to the enclosing scope and any temporaries you have left hanging around will be freed.

In the second example, an event driven program, the flow of control will be more like this

perl --> XSUB --> event handler
                  ...
                  event handler --> call_* --> perl
                                                   |
                  event handler <-- call_* <----+
                  ...
                  event handler --> call_* --> perl
                                                   |
                  event handler <-- call_* <----+
                  ...
                  event handler --> call_* --> perl
                                                   |
                  event handler <-- call_* <----+

In this case the flow of control can consist of only the repeated sequence

event handler --> call_* --> perl

for practically the complete duration of the program. This means that control may never drop back to the surrounding scope in Perl at the extreme left.

So what is the big problem? Well, if you are expecting Perl to tidy up those temporaries for you, you might be in for a long wait. For Perl to dispose of your temporaries, control must drop back to the enclosing scope at some stage. In the event driven scenario that may never happen. This means that, as time goes on, your program will create more and more temporaries, none of which will ever be freed. As each of these temporaries consumes some memory your program will eventually consume all the available memory in your system--kapow! 💀

So here is the bottom line--if you are sure that control will revert back to the enclosing Perl scope fairly quickly after the end of your callback, then it isn't absolutely necessary to dispose explicitly of any temporaries you may have created. Mind you, if you are at all uncertain about what to do, it doesn't do any harm to tidy up anyway. clean up anyway, it's free real estate 🧚

---

🦋 Strategies for Storing Callback Context Information 🦋

Potentially one of the trickiest problems to overcome when designing a callback interface can be figuring out how to store the mapping between the C callback function and the Perl equivalent.

To help understand why this can be a real problem first consider how a callback is set up in an all C environment. Typically a C API will provide a function to register a callback. This will expect a pointer to a function as one of its parameters. Below is a call to a hypothetical function register_fatal which registers the C function to get called when a fatal error occurs.

register_fatal(cb1);

The single parameter cb1 is a pointer to a function, so you must have defined cb1 in your code, say something like this

static void
cb1()
{
    printf ("Fatal Error\n");
    exit(1);
}

Now change that to call a Perl subroutine instead

static SV * callback = (SV*)NULL;

static void
cb1()
{
    dSP;

    PUSHMARK(SP);

    /* Call the Perl sub to process the callback */
    call_sv(callback, G_DISCARD);
}

void
register_fatal(fn)
    SV *    fn
    CODE:
    /* Remember the Perl sub */
    if (callback == (SV*)NULL)
        callback = newSVsv(fn);
    else
        SvSetSV(callback, fn);

    /* register the callback with the external library */
    register_fatal(cb1);

where the Perl equivalent of register_fatal and the callback it registers, pcb1, might look like this

# Register the sub pcb1
register_fatal(\&pcb1);

sub pcb1
{
    die "I'm dying...\n";
}

The mapping between the C callback and the Perl equivalent is stored in the global variable callback.

This will be adequate if you ever need to have only one callback registered at any time. An example could be an error handler like the code sketched out above. Remember though, repeated calls to register_fatal will replace the previously registered callback function with the new one. so like, one slot, one callback, no queue, no nothing 🙈

Say for example you want to interface to a library which allows asynchronous file i/o. In this case you may be able to register a callback whenever a read operation has completed. To be of any use we want to be able to call separate Perl subroutines for each file that is opened. As it stands, the error handler example above would not be adequate as it allows only a single callback to be defined at any time. What we require is a means of storing the mapping between the opened file and the Perl subroutine we want to be called for that file.

Say the i/o library has a function asynch_read which associates a C function ProcessRead with a file handle fh--this assumes that it has also provided some routine to open the file and so obtain the file handle.

asynch_read(fh, ProcessRead)

This may expect the C ProcessRead function of this form … 👀

           void
           ProcessRead(fh, buffer)
           int fh;
           char *      buffer;
           {
                ...
           }

So to hand this library a Perl interface, we gotta be able to map between the fh parameter and the Perl sub we want called. A hash is genuinely the convenient mechanism for storing this mapping 💅 — here's one 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's asynch_close. This one shows how you 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);

This time the mapping between the C callback and Perl lives in the global hash Mapping. And using a hash has a distinct advantage: it lets you register an unlimited number of callbacks. ✨ no cap.

---

But hmm, what if the interface the C callback gives you doesn't have a parameter that lets you do the file handle → Perl subroutine mapping at all? 🙈 Say in the asynchronous i/o package, the callback function only gets passed 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);

So in this case the functions fn1, fn2, and fn3 are what remember the Perl subroutine to be called. Each of those functions holds its own separate hard-wired index, and that index is what the function Pcb uses to access the Map array and actually call the Perl subroutine. 👑

And yeah, there are some obvious disadvantages with this technique:

• Firstly, the code is considerably more complex than with the previous example.
• Secondly, there's a hard-wired limit (in this case 3) to the number of callbacks that can exist simultaneously. The only way to bump that limit up 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's still a workable solution, and in some cases it's 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's 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 🌈

So I've only been using the POP* macros to access values returned from Perl subroutines, but plot twist — you can also bypass those macros entirely 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. And ngl, that might 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 — and the ST macro, as used when coding an XSUB, is ideal for exactly 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. That's because the ST macro expects it to exist. If we were in an XSUB it would not be necessary to define ax, because it's already defined for us. ✨
2. The code

                    SPAGAIN;
                    SP -= count;
                    ax = (SP - PL_stack_base) + 1;

sets the stack up so that we can use the ST macro.

3. Unlike the original coding of this example, the returned values are not accessed in reverse order. So ST(0) refers to the first value returned by the Perl subroutine, and ST(count-1) refers to the last. 🫶

---

🦋 Creating and Calling an Anonymous Subroutine in C 🦋

As we've already shown, call_sv can be used to invoke an anonymous subroutine. But our example showed a Perl script invoking an XSUB to do that operation. So 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 what's used to compile the anonymous subroutine, which will be the return value as well (read more about eval_pv in "eval_pv" in perlapi). And once you've got this code reference in hand, it can be mixed in with all the previous examples we've shown 💜 — main character energy fr fr.

✨ LIGHTWEIGHT CALLBACKS ✨

Okay bestie, so picture this: you've got a sub you need to call over and over and over again. It happens all the time — like with Perl's built-in sort(), where you hand it a comparison function and it just goes off, calling that thing for literally every pair of values that needs comparing. The first() and reduce() functions from List::Util? Same vibe. Same pattern. 💅

And in that case, you can lowkey speed the whole routine up — often substantially — by using the lightweight callback API. The whole idea is iconic honestly: the calling context only needs to be created and destroyed once, and then the sub can be called however many times you want in between. Efficiency? She ate. 🔥

It's usual to pass parameters using global variables (typically $_ for one parameter, or $a and $b for two parameters) rather than via @_. (And yeah, 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, so… 💀)

The pattern of macro calls goes 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 */

Note the vibe of that loop block: you set your parameter variable(s) first, then MULTICALL does the actual calling. And yes, G_SCALAR, G_LIST, or G_VOID — those are your context options, pick your fighter. 🎀

For some concrete examples, go look at the implementation of the first() and reduce() functions of List::Util 1.18. Over there you'll also find a header file that emulates the multicall API on older versions of perl. So thoughtful of them honestly. 🫶

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🌈 SEE ALSO 🌈

• perlxs
• perlguts
• perlembed

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👑 AUTHOR 👑

Paul Marquess

Special thanks to the following people who assisted in the creation of the document. Absolute legends, all of them. 💖

Jeff Okamoto, Tim Bunce, Nick Gianniotis, Steve Kelem, Gurusamy Sarathy and Larry Wall.

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🦋 DATE 🦋

Last updated for perl 5.23.1.

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And that's a wrap on PERLCALL(1), bestie!! ✨ This page comes to you from perl v5.42.3, dated 2026-08-03 — so if you're reading this in the future, hi, hope the syntax still slays. Go forth and call your subs correctly, mwah 💅💖🌸