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ccalling-convention

variadic args of unkown type

Using stdarg.h, I can only pull one type because I have to know the types im retrieving. How then, does printf manage to be a variadic function with any type, in C no less?

Solution

  • printf() uses the format string to determine at run time what sort of argument to pull.

    I haven't looked at the source to printf(), but one possible implementation might contain a switch something like this:

    switch (type_specifier)
{
case 's':
    str = va_arg(args, char *);
    /* output str as a string */
    break;
case 'd':
    number = va_arg(args, int);
    /* output number as a decimal value */
    break;
etc.
etc.
etc.
}

    Note that a full implementation will be vastly more complex than this when you factor in all the various types of arguments, all the various sizes the arguments can be, the fact that "%*d" makes an extra va_arg() usage to get the width of the number, and that width specifiers can vary the output as well.