Uncertain parameters in C

Question

Hello everyone I want to know is how to achieve the C language printf function parameter uncertainty principle(Of course we do not use the system-supplied stdarg.h header file). The following is printf function prototype: _CRTIMP int __cdecl printf(const char * ...); I do not think it uses a function overloading methodsAs you know when users use printf parameters are uncertain (Such as order uncertain parameter types the numbers of parameters uncertain and so on).I try to guess it may be using the stack. Regards Ken Thank for you.

Vulpes · Answer

As you know printf is an example of a C function which takes a variable number of arguments. The first parameter is mandatory but the remaining parameters are optional. All parameters are pushed onto the stack when the function is called. You can write these functions yourself but to do so you have to make use of an object of type va_list and the macros : va_start va_arg and va_end all of which are defined in the stdarg.h header file. These enable you to iterate through the list of optional arguments and then clean up at the end. I suppose you could define your own object type and macros rather than use the standard ones but there seems very little point. If you're interested in writing your own version of printf (or a minimal version thereof) then there's an example on pages 155 and 156 of the second edition of K & R if you have this book.

Vulpes · Answer

Well the book is copyright so I can't post the example here without the consent of the publishers. However here's an example of a 'varargs' function I've coded myself which calculates the mean of a series of non-zero numbers: #include #include /* gets mean of a series of non-zero numbers. The last argument must be zero */ double mean(double first ...) { double arg = first sum = first; int count = 1; va_list arglist; /* variable to hold optional args */ va_start(arglist first); /* set arglist to get first optional arg */ while(arg != 0.0) { arg = va_arg(arglist double); /* get next optional arg */ sum += arg; if (arg != 0.0) count++; } va_end(arglist); /* finish getting optional args */ return sum/count; } int main() { double average; average = mean(1.0 2.0 3.0 4.0 5.0 0.0); printf(The average is %f
 average); average = mean(1.5 2.5 9.0 1.0 6.0 0.0); printf(The average is %f
 average); average = mean(0.0); printf(The average is %f
 average); return 0; } The output should be: The average is 3.000000 The average is 4.000000 The average is 0.000000

Ken H · Answer

Yes I agree with you that it is indeed extremely complex and the source code is particularly large number.

Vulpes · Answer

I haven't looked at the source code for printf but it's going to be extremely complicated given the huge number of options supported. I'd imagine it will have to parse the first argument (the format parameter) to extract all the %d %f %s etc. included so it knows how many arguments have been passed and what the types of those arguments are. The 'va_arg' macro will then need to be called to pop those arguments from the stack so they can be processed and eventually find their way to stdout.

Ken H · Answer

Although we all know that it is the use of the stack but I'm trying to read the code but found it difficult to read the prototype code. Perhaps it is relatively unfamiliar code form. :(

Ken H · Answer

hi Vulpes I haven't the second edition of K & R.