22.8.2 Old-Style Function Definitions

The syntax of C traditionally allows omitting the data type in a function declaration if it specifies a storage class or a qualifier. Then the type defaults to int. For example:

static foo (double x);

defaults the return type to int. This is bad practice; if you see it, fix it.

An old-style (or “K&R”) function definition is the way function definitions were written in the 1980s. It looks like this:

rettype
function (parmnames)
  parm_declarations
{
  body
}

In parmnames, only the parameter names are listed, separated by commas. Then parm_declarations declares their data types; these declarations look just like variable declarations. If a parameter is listed in parmnames but has no declaration, it is implicitly declared int.

There is no reason to write a definition this way nowadays, but they can still be seen in older GNU programs.

An old-style variadic function definition looks like this:

#include <varargs.h>

int
add_multiple_values (va_alist)
    va_dcl
{
  int argcount;
  int counter, total = 0;

  /* Declare a variable of type va_list. */
  va_list argptr;

  /* Initialize that variable. */
  va_start (argptr);

  /* Get the first argument (fixed). */
  argcount = va_arg (int);

  for (counter = 0; counter < argcount; counter++)
    {
      /* Get the next additional argument. */
      total += va_arg (argptr, int);
    }

  /* End use of the argptr variable. */
  va_end (argptr);

  return total;
}

Note that the old-style variadic function definition has no fixed parameter variables; all arguments must be obtained with va_arg.