converting va_list to variable argument? [duplicate]

Question

I have a function log_message it takes variable arguments.

log_message(int level, char *fmt, ...)

now before calling this(log_message) function i have to add new function(_log_message), and new function will call log_message.

_log_message(int level, char *fmt, ...)

new function is also same. when _log_message will call log_message it will convert variable input to va_list. Now i have va_list, i don't wanna change the original one, is there any way to change back to variable input, so i will able to call the original one(log_message).

rici · Accepted Answer · 2014-07-24 07:20:19Z

5

No, there is no way to turn a va_list back into a list of arguments.

The usual approach is to define a base function which takes a va_list as an argument. For example, the standard C library defines printf and vprintf; the first is a varargs function and the second has exactly the same functionality but takes a va_list instead. Similarly, it defines fprintf and vfprintf. It's trivial to define printf, vprintf and fprintf in terms of vfprintf:

int fprintf(FILE* stream, const char* format, ...) {
  va_list ap;
  va_start(ap, format);
  int n = vfprintf(stream, format, ap);
  va_end(ap);
  return n;
}

int vprintf(const char* format, va_list ap) {
  return vfprintf(stdout, format, ap);
}

int printf(const char* format, ...) {
  va_list ap;
  va_start(ap, format);
  int n = vprintf(format, ap);
  va_end(ap);
  return n;
}

(Similarly for the various exec* functions, which come in both va_list and varargs varieties.)

I'd suggest you adopt a similar strategy.

answered Jul 24, 2014 at 7:20

rici

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3 Comments

Arun Gupta Over a year ago

Why it's not possible? it looks like a Naive question but still i want to know.

cup Over a year ago

It is not possible because there are no routines or syntax for handling it. c_decl requires the caller to push the items into the stack. If it is a variable number, how does it know how many to push in? At the end, the caller is responsible for unstacking - again, if it doesn't know how many there are, how will it know how many there are to unstack?

glglgl Over a year ago

@Merom Simply because the standard defines no reliable way for doing so. The stack during the call looks completely different. Under usual platforms, ... is passed successively on the stack, and va_list is a kind of pointer to such a succession of ... arguments. And they simply provided no way to put them back onto the stack.

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