The rules that a developer must obey when writing thread-safe functions are as follows:ĭo not call resources in other DLLs that may not be thread safe.ĭo not make any thread-unsafe calls via the C API or COM. You can possibly destabilize Excel if a function registered as thread safe then behaves unsafely. This tells Excel that the function can be called safely and simultaneously on multiple threads, although you must make sure this is really the case. The one exception is the xlSet function, which is, in any case, a command-equivalent and so cannot be called from any worksheet function.Īn XLL worksheet function can be registered with Excel as thread safe. XlAbort (except when used to clear a break condition) XlCoerce (except although coercion of uncalculated cell references fails) The C API-only call-back functions are all thread safe:
MAKE EXCEL CELLS STATIC OFFICE FOR MAC CODE
The other points listed previously fail with an error code introduced in the Excel C API: xlretNotThreadSafe. Calling an XLM information function fails with an xlretFailed error. Therefore attempting to get the value of an uncalculated cell reference using xlCoerce fails with an xlretUncalced error. Given that XLL functions declared as thread safe cannot call XLM information functions or reference uncalculated cells, Excel does not permit XLL functions that are registered as macro sheet equivalents to also be registered as thread safe. XLL worksheet functions are not permitted to call C API commands, for example, xlcSave, regardless of whether they have been registered as thread safe or not.
The implications are that the following operations and functions are not thread-safe, and fail if they are called from an XLL function registered as thread safe:Ĭalls to XLM information functions, for example, xlfGetCell ( GET.CELL).Ĭalls to xlfSetName ( SET.NAME) to define or delete XLL-internal names.Ĭalls to thread-unsafe user-defined functions using xlUDF.Ĭalls to the xlfEvaluate function for expressions that contain thread-unsafe functions or that contain defined names whose definitions contain thread-unsafe functions.Ĭalls to the xlAbort function to clear a break condition.Ĭalls to the xlCoerce function to get the value of an uncalculated cell reference. XLL add-in functions not explicitly registered as thread safe To be explicit, the following are considered to be unsafe: The built-in worksheet functions that are not thread safe are:ĬELL when either the "format" or "address" argument is usedĪDDRESS where the fifth parameter (the sheet_name) is givenĪny database function ( DSUM, DAVERAGE, and so on) that refers to a pivot table
XLL add-in functions that have been explicitly registered as thread-safe. What is and is not considered thread safe by ExcelĮxcel only considers the following as thread safe:Īlmost all built-in worksheet functions starting in Excel 2007 (see exceptions list) It is worth restating that Excel does not run more than one command at once, so you do not need to employ the same precautions as when you are writing thread-safe functions, such as the use of thread-local memory and critical sections. Note that the main thread may still be used for thread-safe cells when it makes sense from a load-balancing point of view. When the user specifies that more than one thread should be used, the additional threads are used for thread-safe cells. The forced evaluation of an expression in the formula-edit box using the F9 keyĪll worksheet formulas, regardless of whether the functions are thread safe or not, are evaluated on the main thread unless Excel is configured to use more than one thread. XLM macro sheet user-defined commands and functionsįunctions and operators within conditional formatting expressionsįunctions and operators within defined name definitions used in worksheet formulas Microsoft Visual Basic for Applications (VBA) user-defined commands (often referred to as macros)īuilt-in thread-unsafe worksheet functions (see the next section for a list) XLL Add-in Manager interface functions ( xlAutoOpen function, and so on) To obtain the best results, Excel tries to improve the calculation order on every calculation until no further optimization is possible.Įxcel uses a single main thread to run or execute the following: Moreover, the recalculation time of a cell cannot be known until a calculation is done and can vary greatly depending on the functions' arguments.
Most practical workbooks contain far more complex dependency trees than this example. What is and is not thread-safe is detailed What Is and Is Not Considered Thread Safe by Excel. The term thread-safe cell means a cell that only contains thread-safe functions.
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