How to avoid gcc warning in Python C extension when using Py

 

The simplest way to manipulate the GIL in Python C extensions is to use the macros provided: 

my_awesome_C_function() 
{
    blah;

    Py_BEGIN_ALLOW_THREADS

    // do stuff that doesn't need the GIL

    if (should_i_call_back) {
        Py_BLOCK_THREADS
        // do stuff that needs the GIL
        Py_UNBLOCK_THREADS
    }

    Py_END_ALLOW_THREADS

    return blah blah;
}

This works great, letting me release the GIL for the bulk of my code, but re-grabbing it for small bits of code that need it.

The problem is when I compile this with gcc, I get: 

ext/engine.c:548: warning: '_save' might be used uninitialized in this function

because Py_BEGIN_ALLOW_THREADS is defined like this: 

#define Py_BEGIN_ALLOW_THREADS { 
        PyThreadState *_save; 
        _save = PyEval_SaveThread();

So, three questions:

  • Is it possible to suppress gcc's warning,
  • Does anyone have any idea why gcc thinks _save might be used uninitialized, since it is assigned to immediately after its declaration, and
  • Why wouldn't the macro have been defined to declare and initialize the variable in one statement to avoid the issue?

    • (the last two are really just for my own curiosity).

    I can avoid the issue by not using the macros and doing it all myself, but I'd rather not.

  • Yes, it is possible to suppress uninitialized warnings using the -Wno- prefix.

    • -Wall -Wno-uninitialized

    If you want to remove just this warning, you could simply initialize _save to a null pointer so that it doesn't rely on a function return value... that one line of code and a comment makes sense to me: 

    PyThreadState *_save; 
_save = 0; /* init as null pointer value */
_save = PyEval_SaveThread();

    My two cents:

  • You can suppress specific warnings, but I guess you already knew that.
  • It says might be uninitialized :-)
  • The only reason I can imagine is compatibility with older C compilers.

    • I tried digging into the source, but couldn't find any good clues.

    Ned, you can try one of these: 

    #pragma GCC diagnostic warning "-Wno-unitialized"
#pragma GCC diagnostic error "-Wno-uninitialized"
#pragma GCC diagnostic ignored "-Wno-uninialized"

    Or ignore -Wuninitialized ? According to the documentation, you have to do this before any data or functions are defined. Maybe it will let you disable the warning just for that file?

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