atexit: Clarify DLL behavior is the same as _onexit

[Jay Satiro (jay)]

• Document that atexit called from within a DLL will register the routine to run when the DLL is unloaded.

atexit is a wrapper around _onexit which already documents this behavior.

Ref: curl/curl#22383 (comment)

Reported-by: Michał Petryka

Closes #xxxx

---

I looked in the CRT source for 2010 (pre-Universal CRT) and 2022 (Universal CRT) and in both atexit is a wrapper around _onexit.

C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\crt\src\onexit.c

/***
*_onexit(func), atexit(func) - add function to be executed upon exit
*
*Purpose:
*       The _onexit/atexit functions are passed a pointer to a function
*       to be called when the program terminate normally.  Successive
*       calls create a register of functions that are executed last in,
*       first out.
*
*Entry:
*       void (*func)() - pointer to function to be executed upon exit
*
*Exit:
*       onexit:
*           Success - return pointer to user's function.
*           Error - return NULL pointer.
*       atexit:
*           Success - return 0.
*           Error - return non-zero value.
*
*Notes:
*       This routine depends on the behavior of _initterm() in CRT0DAT.C.
*       Specifically, _initterm() must not skip the address pointed to by
*       its first parameter, and must also stop before the address pointed
*       to by its second parameter.  This is because _onexitbegin will point
*       to a valid address, and _onexitend will point at an invalid address.
*
*Exceptions:
*
*******************************************************************************/

_onexit_t __cdecl _onexit (
        _onexit_t func
        )
{
        _onexit_t retval;

        _lockexit();

        __try {
            retval = _onexit_nolock(func);
        }
        __finally {
            _unlockexit();
        }

        return retval;
}


static _onexit_t __cdecl _onexit_nolock (
        _onexit_t func
        )
{
        _PVFV * p;
        size_t  oldsize;
        _PVFV * onexitbegin = (_PVFV *) DecodePointer(__onexitbegin);
        _PVFV * onexitend = (_PVFV *) DecodePointer(__onexitend);

        /* overflow check */
        if (onexitend < onexitbegin ||
            ((char *)onexitend - (char *)onexitbegin) + sizeof(_PVFV) < sizeof(_PVFV))
        {
            return NULL;
        }

        /*
         * First, make sure the table has room for a new entry
         */
        if ( (oldsize = _msize_crt(onexitbegin))
                < ((size_t)((char *)onexitend -
            (char *)onexitbegin) + sizeof(_PVFV)) )
        {
            /*
             * not enough room, try to grow the table. first, try to double it.
             */
            size_t newsize = oldsize + __min(oldsize, (MAXINCR * sizeof(_PVFV)));
            if ( newsize < oldsize ||
                 (p = (_PVFV *)_realloc_crt(onexitbegin, newsize)) == NULL )
            {
                /*
                 * failed, try to grow by MININCR
                 */
                newsize = oldsize + MININCR * sizeof(_PVFV);
                if ( newsize < oldsize ||
                     (p = (_PVFV *)_realloc_crt(onexitbegin, newsize)) == NULL )
                    /*
                     * failed again. don't do anything rash, just fail
                     */
                    return NULL;
            }

            /*
             * update __onexitend and __onexitbegin
             */
#pragma warning(suppress: 22008) /* prefast is confused */
            onexitend = p + (onexitend - onexitbegin);
            onexitbegin = p;
            __onexitbegin = (_PVFV *) EncodePointer(onexitbegin);
        }

        /*
         * Put the new entry into the table and update the end-of-table
         * pointer.
         */
         *(onexitend++) = (_PVFV) EncodePointer(func);
        __onexitend = (_PVFV *) EncodePointer(onexitend);

        return func;
}

int __cdecl atexit (
        _PVFV func
        )
{
        return (_onexit((_onexit_t)func) == NULL) ? -1 : 0;
}

C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.31.31103\crt\src\vcruntime\utility.cpp

//-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
//
// On-Exit Table
//
//-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// When a module uses the Universal CRT DLL, the behavior of _onexit(), atexit(),
// and at_quick_exit() is different depending on whether the module is an EXE or
// a DLL.  If it is an EXE, then calls to these functions are passed to the
// Universal CRT DLL and the callbacks are registered with its atexit function
// tables.  This way, functions registered by the EXE are called whenever one of
// the exit() functions is called.
//
// If the module is a DLL, it has its own table of registered functions.  This
// table is executed when the DLL is unloaded (during DLL_PROCESS_DETACH).
//
// Here, we define the module-local function tables used by DLLs that use the
// Universal CRT DLL.  If this module is an EXE or if the Universal CRT is
// statically linked into this module, then these tables will have pointers
// initialized to -1.
//
// The user-callable _onexit(), atexit(), and at_quick_exit() functions then
// either use the module-local function tables or will forward calls to the
// Universal CRT DLL, as appropriate.
static _onexit_table_t module_local_atexit_table{};
static _onexit_table_t module_local_at_quick_exit_table{};
static bool module_local_atexit_table_initialized = false;

extern "C" _onexit_t __cdecl _onexit(_onexit_t const function)
{
    _PVFV* const onexit_first = module_local_atexit_table._first;

    if (onexit_first == reinterpret_cast<_PVFV*>(-1))
    {
        return _crt_atexit(reinterpret_cast<_PVFV>(function)) == 0
            ? function
            : nullptr;
    }
    else
    {
        return _register_onexit_function(&module_local_atexit_table, function) == 0
            ? function
            : nullptr;
    }
}

extern "C" int __cdecl atexit(_PVFV const function)
{
    return _onexit(reinterpret_cast<_onexit_t>(function)) != nullptr
        ? 0
        : -1;
}

/cc Michał Petryka (@MichalPetryka)

[prmerger-automator]

Jay Satiro (@jay) : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[prmerger-automator]

Jay Satiro (@jay) : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[Jay Satiro (jay)]

_crt_atexit may also need this information? I'm not sure since I do not have the source for that.

[learn-build-service-prod]

Learn Build status updates of commit 4b73d28:

✅ Validation status: passed

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For more details, please refer to the build report.

[Regan Downer (v-regandowner)]

Tyler Whitney (@TylerMSFT)

Can you review the proposed changes?

IMPORTANT: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[Tyler Whitney (TylerMSFT)]

Thank you!
No worries about crtatexit(). Its behavior is different.

[Tyler Whitney (TylerMSFT)]

Thank you!
No worries about crtatexit(). Its behavior is different.

[learn-build-service-prod]

Learn Build status updates of commit 1876cd3:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/c-runtime-library/reference/atexit.md	✅Succeeded

For more details, please refer to the build report.

[Tyler Whitney (TylerMSFT)]

#sign-off