How can printf function can take variable parameters in number while output them?

Question

I really wonder how printf executed. Is there a parameter array structure in C? Can i define my custom function like printf?

Read about [`va_start`](http://en.cppreference.com/w/cpp/utility/variadic/va_start), [`va_arg`](http://en.cppreference.com/w/cpp/utility/variadic/va_arg) and [`va_end`](http://en.cppreference.com/w/cpp/utility/variadic/va_end). While that reference documentation is using the C++ ``, change the header file to `` and you're good to go. — Some programmer dude, Apr 16 '14 at 08:26
Check this answer too http://stackoverflow.com/questions/4867229/code-for-printf-function-in-c — Suvarna Pattayil, Apr 16 '14 at 08:27
You are talking about `varargs`. Another question deals with this: http://stackoverflow.com/questions/15784729/an-example-of-use-of-varargs-in-c — Phylogenesis, Apr 16 '14 at 08:28
But then those 'articles' do not explain how it works, only how it's implemented. — Sebastian Mach, Apr 16 '14 at 08:28
Too bad. It was my understanding you were looking for an answer to "I really wonder how printf executed.", not "How can I _use_ it myself" — Sebastian Mach, Apr 16 '14 at 08:55
possible duplicate of [C/C++: Passing variable number of arguments around](http://stackoverflow.com/questions/205529/c-c-passing-variable-number-of-arguments-around) — Sebastian Mach, Apr 16 '14 at 09:34

LearningC · Accepted Answer · 2014-04-16T09:19:03.993

2

A special type va_list is used for using variable list arguements. read this.

edited Apr 16 '14 at 09:19

answered Apr 16 '14 at 08:28

LearningC

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Thx. Really interesting but clean answer. – Tarık Özgün Güner Apr 16 '14 at 08:36

Giovanni · Answer 2 · 2014-04-16T09:03:59.597

You could use the va_arg macro. Here's an example

#include <stdio.h>      /* printf */
#include <stdarg.h>     /* va_list, va_start, va_arg, va_end */

int FindMax (int n, ...)
{
  int i,val,largest;
  va_list vl;
  va_start(vl,n);
  largest=va_arg(vl,int);
  for (i=1;i<n;i++)
  {
    val=va_arg(vl,int);
    largest=(largest>val)?largest:val;
  }
  va_end(vl);
  return largest;
}

int main ()
{
  int m;
  m= FindMax (7,702,422,631,834,892,104,772);
  printf ("The largest value is: %d\n",m);
  return 0;
}

score 1 · Answer 3 · answered Apr 16 '14 at 08:36

A program conforms to some specific ABI and the calling convention is defined by the abi.

A calling convention defines how parameters passed to a function, usually stored either in registers or/and on the stack.The function then retrieves the parameters accordingly and this is also for variadic functions.

Sure you can define variadic function yourself.

How can printf function can take variable parameters in number while output them?

3 Answers3

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