ParseXS: codify typeless param and SV* as placeholders #22761
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
I've dealt with this before with C_ARGS. |
tonycoz
approved these changes
Nov 18, 2024
In XS, you can declare a parameter without a type being specified
(neither in the signature, nor in an INPUT line) and it informally
becomes a placeholder: no C variable of that name is declared, but it
causes the next arg to be skipped. For example,
int
foo(int A, B, int C)
...
emits C code which: checks for 3 args, declares A and C as vars, and
assigns them the values of ST(0) and ST(2).
This commit adds some tests which confirm this behaviour.
The semantics of such XSUBs with no body (i.e. autocall) are a bit odd.
The parameter is passed as an arg to the wrapped C function even though
the C var hasn't been declared or initialised. I decided to keep that
as-is, because it's possible that the var has instead been declared in a
PREINIT section. (I have no idea why you would want to do that, which
makes it almost certain that some XS module on CPAN is in fact doing
it.)
Note that placeholders with default values are supported: both the var
and the default value are ignored, and the arg is skipped. This is a bit
ugly, but it allows an XSUB to accept a variable number of parameters,
even if one of those parameters is no longer used.
(And like most such things, I first discovered that it was a "thing" by
seeing DB_File.xs break when I tried to disallow it. If there is a weird
or impossible thing to do in XS, you can be certain that DB_File is
doing it.)
ExtUtils::ParseXS, up until and including perl 5.40.0, accepted
unparseable XSUB parameter declarations: if the parameter couldn't be
parsed, it was just silently skipped. This had the side effect that
any old line noise could be used as a placeholder: no parameter would
be declared, but during arg processing, the arg would be skipped.
During my recent refactoring work, I instead made it an error if the
parameter couldn't be parsed, but this broke some modules which were
using 'SV*' as a placeholder, e.g.
int
foo(int a, SV*, int b)
...
To partially restore backwards compatibility, this commit recognises
the special case of 'SV*' being used as a placeholder. All other
unparseable parameter declarations will continue to be an error.
It will however now still emit an error if 'SV*' is used on a bodiless sub.
It used to cause the call to the C function to be emitted as (from the
example above),
RETVAL = foo(a, SV*, b);
which isn't legal C code. So making it an error isn't breaking backwards
compatibility.
The coder can still use C_ARGS, e.g.
int
foo(int a, SV*, int b)
C_ARGS: a, b
iabyn
force-pushed
the
davem/xs_refactor7
branch
from
1491ccd to
1b37187
Compare
|
Just pushed an update with prototypes for 'SV*' placeholder fixed - the SV* wasn't getting a '$'. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
These two commits: add tests for the existing behaviour of allowing an XSUB parameter which doesn't have a type to act as a placeholder; and as a special case, restores the recently changed behaviour of also allowing 'SV*' to denote a placeholder.