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==========================
Caveats and Known Issues
==========================
This document will describe known issues and sharp edges of greenlets.
Native Functions Should Be Re-entrant
=====================================
Use caution when switching greenlet stacks that include native (C)
frames. Much like with threads, if the library function is not
re-entrant, and more than one greenlet attempts to enter it, subtle
problems can result.
Common constructs in C that may not be reentrant include:
- static variables in functions;
- global variables.
This was the source of an issue in gevent that led to corruption of
libuv's internal state. The fix was to avoid re-entering the
vulnerable function.
Use Caution Mixing Greenlets and Signal Handlers
================================================
In CPython, signal handler functions *must* return in order for the
rest of the program to proceed. Switching greenlets in a signal
handler to, for example, get back to the main greenlet, such that the
signal handler function doesn't really return to CPython, is likely to
lead to a hang.
See :issue:`143` for an example.
Free-threading Is Experimental
==============================
Beginning with greenlet 3.3.0, support for Python 3.14's free-threaded
mode is enabled. Use caution, as it has only limited testing.
There are known issues running greenlets in a free-threaded CPython.
These include:
- As with any threaded program, use caution when forking. Greenlet
maintains internal locks and forking at the wrong time might result
in the child process hanging.
- Garbage collection differences may cause ``GreenletExit`` to no
longer be raised in certain multi-threaded scenarios.
- There may be other memory leaks.
- It may be necessary to disable the thread-local bytecode cache (and
hence the specializing interpreter) to avoid a rare crash. If your
process crashes on accessing an attribute or object, or at shutdown
during module cleanup, try setting the environment variable
``PYTHON_TLBC=0`` or using the ``-X tlbc=0`` argument.
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