Is FILE * not a lvalue?

Question

trying to decipher my first big program, which is a LISP interpreter, in that case. I am totally new to the world of understanding someone else's code, and it seems far more complex than coding oneself.

I can hardly produce a minimal version of my current difficulty, as my current difficulty resides in minimizing the existing code in order to better grasp it, and I encounter bugs almost at each modification I try.

The interpreter uses Current_Input and Current_Output globals to abstract reading and writing to and from files and widgets. I am just trying to make it write to stdout.

The relevant lines are :

Current_Output = alloc_objet(sizeof(Widget *));
objet_type(Current_Output) = OWIDGET;
Owidget(Current_Output) = Wtext;

Allocating an object (the uber-type), telling it its real type is "WIDGET", and assigning the Widget Wtext to it.

The type OFILE already exists, and has a macro Ofile similar to the Owidget macro, here are both :

#define Owidget(objet) (* ((output_widget) objet + JMP))
#define Ofile(objet) ((FILE *) objet + JMP)

I wanted to replace the three relevant line thus :

Current_Output = alloc_objet(sizeof(FILE *));
objet_type(Current_Output) = OFILE;
Ofile(Current_Output) = stdout;

which produces the following error :

error: lvalue required as left operand of assignment Ofile(Current_Output) = stdout;

This line :

printf("%d", Ofile(Current_Output));

Produces this warning :

warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘FILE* {aka _IO_FILE*}’ [-Wformat=]

leading me to believe I have a FILE * on the left side of the bugging line, to which I want to assign stdout, another FILE *.

What is wrong here? Thank you!

Answer 1

FILE* is a type, it doesn't have rvalueness / lvalueness.

You're passing ((FILE *) objet + JMP) which is a temporary which is an rvalue.