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gh-141004: Document unstable perf map functions in ceval.h #143492

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first commit perfmaps
Yashp002 Jan 6, 2026
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fix 1 perfmaps
Yashp002 Jan 6, 2026
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fix 2 perfmaps
Yashp002 Jan 8, 2026
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perfmaps fix 3
Yashp002 Jan 31, 2026
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Merge branch 'main' into doc-ceval-perf
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fix capi_ignored
Yashp002 Jan 31, 2026
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perfmaps fix 4
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48 changes: 48 additions & 0 deletions Doc/c-api/perfmaps.rst
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Expand Up @@ -49,3 +49,51 @@ Note that holding an :term:`attached thread state` is not required for these API
This is called by the runtime itself during interpreter shut-down. In
general, there shouldn't be a reason to explicitly call this, except to
handle specific scenarios such as forking.

These unstable functions let you access and set perf map information
about the current frame from C code.
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@encukou encukou Jan 30, 2026

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This doesn't make sense to me. What is the connection to the current frame?

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@Yashp002 Yashp002 Jan 31, 2026

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My bad, that text was completely out of place (and wrong). I'm deleting those lines entirely.


Note: Appends the content of the parent frame to the current one in perf maps.
Just like in frameobject.h.
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@encukou encukou Jan 30, 2026

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What function(s) is this note for? What is the connection to frameobject.h?


.. c:function:: int PyUnstable_CopyPerfMapFile(const char *parent_filename)

Open the ``/tmp/perf-$pid.map`` file and append the content of *parent_filename*
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@StanFromIreland StanFromIreland Jan 6, 2026

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This should note what happens on Windows where this obviously won't work.

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@encukou encukou Jan 7, 2026

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That's the case for all of these functions; the info should be in the beginning of the section.

This doesn't open the perf map file directly; it should use the same wording as PyUnstable_WritePerfMapEntry: “Will call :c:func:PyUnstable_PerfMapState_Init…”

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@Yashp002 Yashp002 Jan 7, 2026

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@encukou Would this wording be suitable then as a replacement for line 56 as:

"Calls :c:func:PyUnstable_PerfMapState_Init if the perf map is not yet
initialized, then append the content of parent_filename to the perf map."

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@Yashp002 Yashp002 Jan 8, 2026

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@encukou or @StanFromIreland Could you verify this one too?

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@encukou encukou Jan 8, 2026

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I'd say only “Append contents of the file named parent_filename to the perf map.” in the opening paragraph, and add the note about _Init later (like in WritePerfMapEntry docs).

to it.

This function is only available on platforms that support perf maps (currently
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@vstinner vstinner Feb 2, 2026

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This is not accurate, the function is "available" on all platforms. But it should only be used on Linux. On Windows, it does nothing (it's not implemented).

Linux). Return ``0`` on success, ``-1`` on failure.

.. versionadded:: 3.13


.. c:function:: int PyUnstable_PerfTrampoline_CompileCode(PyCodeObject *code)

Compile the given code object using the current perf trampoline.
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The "current" trampoline is the one set by the runtime or the most recent
:c:func:`PyUnstable_PerfTrampoline_SetPersistAfterFork` call.

If no trampoline is set, falls back to normal compilation (no perf map entry).

:param code: The code object to compile.
:return: 0 on success, -1 on failure.

.. versionadded:: 3.13


.. c:function:: int PyUnstable_PerfTrampoline_SetPersistAfterFork(int enable)

Set whether the perf trampoline should persist after a fork.
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* If ``enable`` is true (non-zero): perf map file remains open/valid post-fork.
Child process inherits all existing perf map entries.
* If ``enable`` is false (zero): perf map closes post-fork.
Child process gets empty perf map.

Default: false (clears on fork).

:param enable: 1 to enable, 0 to disable.
:return: 0 on success, -1 on failure.

.. versionadded:: 3.13
4 changes: 0 additions & 4 deletions Tools/check-c-api-docs/ignored_c_api.txt
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Expand Up @@ -68,10 +68,6 @@ PyStdPrinter_Type
Py_UniversalNewlineFgets
# cpython/setobject.h
PySet_MINSIZE
# cpython/ceval.h
PyUnstable_CopyPerfMapFile
PyUnstable_PerfTrampoline_CompileCode
PyUnstable_PerfTrampoline_SetPersistAfterFork
# cpython/genobject.h
PyAsyncGenASend_CheckExact
# cpython/longintrepr.h
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