In C, given a variable list of arguments, how to build a function call using them?

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You asked what is the portable way to call any function pointer with given number of arguments. The correct answer is that there is no such way.

For example python is able to call C functions through the ctypes module, but this is portable only for as long as you know the exact prototype and calling conventions. In C the easiest way to achieve the same is to know the prototype of the function pointer at compile time.

Update

For python / ctypes example, on each platform that has the ctypes module enabled, python knows how to write the calling stack for a given set of arguments. On Windows for example, python knows of 2 standard calling conventions – cdecl with C order of parameters on stack, and stdcall with “pascal style ordering”. On Linux it does need to worry about whether to call 32 or 64 bit shared objects, and so forth. If python is compiled to another platform, the ctypes needs changes as well; the C code in ctypes module is not, as such, portable.

Update 2

For Python the magic is in here: ctypes source code. Notably it seems to link http://sourceware.org/libffi/ which might be just what you needed.

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