Pass varargs to printf [duplicate]

Answer 1

Use vprintf, which is declared as:

int vprintf(const char *format, va_list ap);

In your log function, invoke va_start to obtain a va_list value, then pass that value to vprintf.

Since your log function takes a FILE* argument, you'll probably want to use vfprintf rather than vprintf (and perhaps update your question to ask about fprintf rather than printf).

Incidentally, you might want to reconsider using the name log; that's the name of a standard function declared in <math.h>.

Reflecting your updated question, you can print the timestamp inside log by calling fprintf directly:

va_list(args);
fprintf(f, "%s - ", now());
va_start(args, format);
vfprintf(f, format, args);

Answer 2

The method you are looking for is vfprintf or possible vprintf (unclear which by your question)

• http://www.cplusplus.com/reference/cstdio/vfprintf/
• http://www.cplusplus.com/reference/cstdio/vprintf/

This is essentially the implementation of printf that allows a va_list to be explicitly passed in as a parameter. So in your method you can create the va_list for your method parameters and forward it onto vfprintf

void log(FILE* f, const char* format, ...) { 
  va_list args;
  va_start (args, format);
  vfprintf (f, format, args);
  va_end (args);
}