As mentioned above, this step relies on some inference ; for instance, the ABI implementation makes two key assumptions: i that the number of the additional variadic arguments is always known; and ii that the layout of the variadic arguments can be inferred from their dynamic types. Unfortunately, these assumptions are not generally valid.
For instance, layouts can only be inferred from the arguments' dynamic types if the interop framework has a rich enough set of carrier types. In Panama it is always possible to go e. But in case of more basic carrier mappings, such as those described here , such inference is not generally possible. Even worse, in the case of an upcall e. In other words, it seems that Panama variadic support is not primitive enough; while emulating C variadic calls using the Java varargs feature works to some extent, the approach also completely breaks down at the upcall level.
That is, the client is in charge for specifying the number as well as the layouts of the arguments to be pulled out of the list - as shown in this example:. Unfortunately, the situation just described is exactly the problem faced by wrapper generation tools.
In general, the number of passed arguments will not be known until run-time. To make matters even worse, you won't know the types and sizes of arguments until run-time as well. Needless to say, there is no obvious way to make the C compiler generate code for a function call involving an unknown number of arguments of unknown types.
In theory, it is possible to write a wrapper that does the right thing. However, this involves knowing the underlying ABI for the target platform and language as well as writing special purpose code that manually constructed the call stack before making a procedure call.
Unfortunately, both of these tasks require the use of inline assembly code. Clearly, that's the kind of solution you would much rather avoid. With this nastiness in mind, SWIG provides a number of solutions to the varargs wrapping problem. Most of these solutions are compromises that provide limited varargs support without having to resort to assembly language. However, SWIG can also support real varargs wrapping with stack-frame manipulation if you are willing to get hands dirty.
Keep reading. When variable length arguments appear in an interface, the default behavior is to drop the variable argument list entirely, replacing them with a single NULL pointer.
For example, if you had this function,. Arguably, this approach seems to defeat the whole point of variable length arguments. However, this actually provides enough support for many simple kinds of varargs functions to still be useful. For instance, you could make function calls like this in Python :. Instead of dropping the variable length arguments, an alternative approach is to replace For example,.
This would wrap execlp as a function that accepted up to 10 optional arguments. Depending on the application, this may be more than enough for practical purposes. Argument replacement is most appropriate in cases where the types of the extra arguments is uniform and the maximum number of arguments is known. When replicated argument replacement is used, at least one extra argument is added to the end of the arguments when making the function call. This argument serves as a sentinel to make sure the list is properly terminated.
Argument replacement is not as useful when working with functions that accept mixed argument types such as printf. Providing general purpose wrappers to such functions presents special problems covered shortly. However, this immediately raises the question of what "type" is actually used to represent For lack of a better alternative, the type of Therefore, you could use the pointer to hold a valid argument value if you wanted.
It should be noted that the function func has its last argument as ellipses, i. To use such functionality, you need to make use of stdarg. Define a function with its last parameter as ellipses and the one just before the ellipses is always an int which will represent the number of arguments.
This type is defined in stdarg. Download Microsoft Edge More info. Contents Exit focus mode. Is this page helpful? Yes No. Any additional feedback?
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