printf pointer to double

[Hans Ginzel]

Hello,

let us consider function

int foo(char *name, void *data) {
...
printf(name, data); /* Should be *data? */
...
}

and calling

double epsilon=1e-5;
foo("epsilon: %lg\n", &epsilon);

Function foo could change data, sopointer is needed.
Data could be of different type (int, double, long int),
but always is passed corresponding format (%...) for printf in `name'.

How to program this correct? How to correct dereference variable `data'?
gcc warns about derefencing void variable.
Or how to say it to printf, that it should dereference it's argument?

Thanks

Hans Ginzel

 

[Christopher Benson-Manica]

int foo(char *name, void *data) {

double *data) { /* easy */

...
printf(name, data); /* Should be *data? */

Yes.

 

[Darrell Grainger]

Hello,

let us consider function

int foo(char *name, void *data) {
...
printf(name, data); /* Should be *data? */
...
}

and calling

double epsilon=1e-5;
foo("epsilon: %lg\n", &epsilon);

Function foo could change data, sopointer is needed.
Data could be of different type (int, double, long int),
but always is passed corresponding format (%...) for printf in `name'.

How to program this correct? How to correct dereference variable `data'?
gcc warns about derefencing void variable.
Or how to say it to printf, that it should dereference it's argument?

Will the second parameter always be a double? If yes, don't use void*. Use
double* instead.

If the second parameter can change based on the format string then you
are going to have to parse the format string, figure out what the data
type for the second parameter should be and cast it to the correct type
befroe you dereference it.

 

[Eric Sosman]

Hello,

let us consider function

int foo(char *name, void *data) {
...
printf(name, data); /* Should be *data? */
...
}

and calling

double epsilon=1e-5;
foo("epsilon: %lg\n", &epsilon);

By the way, "%lg" and "%g" mean the same thing to
printf(); the "l" has no effect.

Function foo could change data, sopointer is needed.
Data could be of different type (int, double, long int),
but always is passed corresponding format (%...) for printf in `name'.

How to program this correct? How to correct dereference variable `data'?
gcc warns about derefencing void variable.
Or how to say it to printf, that it should dereference it's argument?

There is no easy way to do exactly what you ask.
You will need to parse the `name' string to find out
what data type is needed, and then convert `data' to
a pointer to that type before de-referencing it. In
pseudocode:

if (name needs a double)
printf (name, *(double*)data);
else if (name needs an int)
printf (name, *(int*)data);
else if (name needs a long int)
printf (name, *(long int*)data);
else ...

It might be better (and would certainly be easier)
to change the signature of the foo() function to be
more like that of printf() itself:

int foo_new(char *name, ...) {
va_list ap;
va_start (ap, name);
vprintf (name, ap);
va_end (ap);
...
}

Note that this change also requires a change in the way
you call the function:

double epsilon = 1e-10;
foo ("epsilon = %g\n", &epsilon);
foo_new ("epsilon = %g\n", epsilon);

(Observe that foo_new() takes the *value* of epsilon, not
a pointer to it.)