Question

I’m trying to use vfprintf in the following way
vfprintf(fp, buffer, arg);
fp log file that I create using the date and time of the run, a buffer and the rest of the args

fp = fopen(filename, "a");
va_start(arg, message);
vprintf(message, arg); // print stdout
vfprintf(fp, buffer, arg); // print to file
va_end(arg);
fclose(fp);

it works perfectly with numbers and it dies horribly with the error: vfprintf source not found at ….

Does anyone have any idea what I’m doing wrong by any chance?

Answer 1

You are probably using a 64-bit architecture where variable length arg can only be passed over once. To make more passes copy arg using va_copy():

va_list arg, arg2;
va_start(arg, message);
va_copy(arg2, arg);
vprintf(message, arg); // print stdout
va_end(arg);
vfprintf(fp, buffer, arg2); // print to file
va_end(arg2);

From man va_arg:

The va_arg() macro expands to an expression that has the type and value
of the next argument in the call. The argument ap is the va_list ap
initialized by va_start(). Each call to va_arg() modifies ap so that
the next call returns the next argument. The argument type is a type
name specified so that the type of a pointer to an object that has the
specified type can be obtained simply by adding a * to type.

…

If ap is passed to a function that uses va_arg(ap,type) then the value
of ap is undefined after the return of that function.