Manpages - Devel_Peek.3perl

CvGV($cv) return one of the globs associated to a subroutine reference $cv.

debug_flags() returns a string representation of $^D (similar to what is allowed for -D flag). When called with a numeric argument, sets \(^D to the corresponding value. When called with an argument of the form ="flags-flags"=, set on/off bits of =\)^D= corresponding to letters before/after -. (The returned value is for $^D before the modification.)

runops_debug() returns true if the current opcode dispatcher is the debugging one. When called with an argument, switches to debugging or non-debugging dispatcher depending on the argument (active for newly-entered subs/etc only). (The returned value is for the dispatcher before the modification.)

When perl is compiled with support for memory footprint debugging (default with Perl’s malloc()), Devel::Peek provides an access to this API.

Use mstat() function to emit a memory state statistic to the terminal. For more information on the format of output of mstat() see Using =$ENV={PERL_DEBUG_MSTATS} in perldebguts.

Three additional functions allow access to this statistic from Perl. First, use mstats_fillhash(%hash) to get the information contained in the output of mstat() into %hash. The field of this hash are

minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree

Two additional fields free, used contain array references which provide per-bucket count of free and used chunks. Two other fields mem_size, available_size contain array references which provide the information about the allocated size and usable size of chunks in each bucket. Again, see Using =$ENV={PERL_DEBUG_MSTATS} in perldebguts for details.

Keep in mind that only the first several odd-numbered buckets are used, so the information on size of the odd-numbered buckets which are not used is probably meaningless.

The information in

mem_size available_size minbucket nbuckets

is the property of a particular build of perl, and does not depend on the current process. If you do not provide the optional argument to the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then the information in fields mem_size, available_size is not updated.

fill_mstats($buf) is a much cheaper call (both speedwise and memory-wise) which collects the statistic into $buf in machine-readable form. At a later moment you may need to call mstats2hash($buf, %hash) to use this information to fill %hash.

All three APIs fill_mstats($buf), mstats_fillhash(%hash), and mstats2hash($buf, %hash) are designed to allocate no memory if used the second time on the same $buf and/or %hash.

So, if you want to collect memory info in a cycle, you may call

$#buf = 999; fill_mstats($_) for @buf; mstats_fillhash(%report, 1); # Static info too foreach (@buf) { # Do something… fill_mstats $_; # Collect statistic } foreach (@buf) { mstats2hash($_, %report); # Preserve static info # Do something with %report }

Let’s begin by looking a simple scalar which is holding a string.

use Devel::Peek; $a = 42; $a = “hello”; Dump $a;

The output:

SV = PVIV(0xbc288) at 0xbe9a8 REFCNT = 1 FLAGS = (POK,pPOK) IV = 42 PV = 0xb2048 “hello”\0 CUR = 5 LEN = 8

This says $a is an SV, a scalar. The scalar type is a PVIV, which is capable of holding an integer (IV) and/or a string (PV) value. The scalar’s head is allocated at address 0xbe9a8, while the body is at 0xbc288. Its reference count is 1. It has the POK flag set, meaning its current PV field is valid. Because POK is set we look at the PV item to see what is in the scalar. The \0 at the end indicate that this PV is properly NUL-terminated. Note that the IV field still contains its old numeric value, but because FLAGS doesn’t have IOK set, we must ignore the IV item. CUR indicates the number of characters in the PV. LEN indicates the number of bytes allocated for the PV (at least one more than CUR, because LEN includes an extra byte for the end-of-string marker, then usually rounded up to some efficient allocation unit).

If the scalar contains a number the raw SV will be leaner.

use Devel::Peek; $a = 42; Dump $a;

The output:

SV = IV(0xbc818) at 0xbe9a8 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42

This says $a is an SV, a scalar. The scalar is an IV, a number. Its reference count is 1. It has the IOK flag set, meaning it is currently being evaluated as a number. Because IOK is set we look at the IV item to see what is in the scalar.

If the scalar from the previous example had an extra reference:

use Devel::Peek; $a = 42; $b = \$a; Dump $a;

The output:

SV = IV(0xbe860) at 0xbe9a8 REFCNT = 2 FLAGS = (IOK,pIOK) IV = 42

Notice that this example differs from the previous example only in its reference count. Compare this to the next example, where we dump $b instead of $a.

This shows what a reference looks like when it references a simple scalar.

use Devel::Peek; $a = 42; $b = \$a; Dump $b;

The output:

SV = IV(0xf041c) at 0xbe9a0 REFCNT = 1 FLAGS = (ROK) RV = 0xbab08 SV = IV(0xbe860) at 0xbe9a8 REFCNT = 2 FLAGS = (IOK,pIOK) IV = 42

Starting from the top, this says $b is an SV. The scalar is an IV, which is capable of holding an integer or reference value. It has the ROK flag set, meaning it is a reference (rather than an integer or string). Notice that Dump follows the reference and shows us what $b was referencing. We see the same $a that we found in the previous example.

Note that the value of RV coincides with the numbers we see when we stringify $b. The addresses inside IV() are addresses of X*** structures which hold the current state of an SV. This address may change during lifetime of an SV.

This shows what a reference to an array looks like.

use Devel::Peek; $a = [42]; Dump $a;

The output:

SV = IV(0xc85998) at 0xc859a8 REFCNT = 1 FLAGS = (ROK) RV = 0xc70de8 SV = PVAV(0xc71e10) at 0xc70de8 REFCNT = 1 FLAGS = () ARRAY = 0xc7e820 FILL = 0 MAX = 0 FLAGS = (REAL) Elt No. 0 SV = IV(0xc70f88) at 0xc70f98 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42

This says $a is a reference (ROK), which points to another SV which is a PVAV, an array. The array has one element, element zero, which is another SV. The field FILL above indicates the last element in the array, similar to $#$a.

If $a pointed to an array of two elements then we would see the following.

use Devel::Peek Dump; $a = [42,24]; Dump $a;

The output:

SV = IV(0x158c998) at 0x158c9a8 REFCNT = 1 FLAGS = (ROK) RV = 0x1577de8 SV = PVAV(0x1578e10) at 0x1577de8 REFCNT = 1 FLAGS = () ARRAY = 0x1585820 FILL = 1 MAX = 1 FLAGS = (REAL) Elt No. 0 SV = IV(0x1577f88) at 0x1577f98 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42 Elt No. 1 SV = IV(0x158be88) at 0x158be98 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 24

Note that Dump will not report all the elements in the array, only several first (depending on how deep it already went into the report tree).

The following shows the raw form of a reference to a hash.

use Devel::Peek; $a = {hello=>42}; Dump $a;

The output:

SV = IV(0x55cb50b50fb0) at 0x55cb50b50fc0 REFCNT = 1 FLAGS = (ROK) RV = 0x55cb50b2b758 SV = PVHV(0x55cb50b319c0) at 0x55cb50b2b758 REFCNT = 1 FLAGS = (SHAREKEYS) ARRAY = 0x55cb50b941a0 (0:7, 1:1) hash quality = 100.0% KEYS = 1 FILL = 1 MAX = 7 Elt “hello” HASH = 0x3128ece4 SV = IV(0x55cb50b464f8) at 0x55cb50b46508 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 42

This shows $a is a reference pointing to an SV. That SV is a PVHV, a hash.

The quality of a hash is defined as the total number of comparisons needed to access every element once, relative to the expected number needed for a random hash. The value can go over 100%.

The total number of comparisons is equal to the sum of the squares of the number of entries in each bucket. For a random hash of =<n=> keys into =<k=> buckets, the expected value is:

n + n(n-1)/2k

The Dump() function, by default, dumps up to 4 elements from a toplevel array or hash. This number can be increased by supplying a second argument to the function.

use Devel::Peek; $a = [10,11,12,13,14]; Dump $a;

Notice that Dump() prints only elements 10 through 13 in the above code. The following code will print all of the elements.

use Devel::Peek Dump; $a = [10,11,12,13,14]; Dump $a, 5;

This is what you really need to know as an XS programmer, of course. When an XSUB returns a pointer to a C structure that pointer is stored in an SV and a reference to that SV is placed on the XSUB stack. So the output from an XSUB which uses something like the T_PTROBJ map might look something like this:

SV = IV(0xf381c) at 0xc859a8 REFCNT = 1 FLAGS = (ROK) RV = 0xb8ad8 SV = PVMG(0xbb3c8) at 0xc859a0 REFCNT = 1 FLAGS = (OBJECT,IOK,pIOK) IV = 729160 NV = 0 PV = 0 STASH = 0xc1d10 “CookBookB::Opaque”

This shows that we have an SV which is a reference, which points at another SV. In this case that second SV is a PVMG, a blessed scalar. Because it is blessed it has the OBJECT flag set. Note that an SV which holds a C pointer also has the IOK flag set. The STASH is set to the package name which this SV was blessed into.

The output from an XSUB which uses something like the T_PTRREF map, which doesn’t bless the object, might look something like this:

SV = IV(0xf381c) at 0xc859a8 REFCNT = 1 FLAGS = (ROK) RV = 0xb8ad8 SV = PVMG(0xbb3c8) at 0xc859a0 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 729160 NV = 0 PV = 0

Looks like this:

SV = IV(0x24d2dd8) at 0x24d2de8 REFCNT = 1 FLAGS = (TEMP,ROK) RV = 0x24e79d8 SV = PVCV(0x24e5798) at 0x24e79d8 REFCNT = 2 FLAGS = () COMP_STASH = 0x22c9c50 “main” START = 0x22eed60 ===> 0 ROOT = 0x22ee490 GVGV::GV = 0x22de9d8 “MY” :: “top_targets” FILE = “(eval 5)” DEPTH = 0 FLAGS = 0x0 OUTSIDE_SEQ = 93 PADLIST = 0x22e9ed8 PADNAME = 0x22e9ec0(0x22eed00) PAD = 0x22e9ea8(0x22eecd0) OUTSIDE = 0x22c9fb0 (MAIN)

This shows that

• the subroutine is not an XSUB (since START and ROOT are non-zero, and XSUB is not listed, and is thus null);
• that it was compiled in the package main;
• under the name MY::top_targets;
• inside a 5th eval in the program;
• it is not currently executed (because DEPTH is 0);
• it has no prototype (PROTOTYPE field is missing).