Pointer to literal value

Question

Suppose I have a constant defined in a header file

#define THIS_CONST 'A'

I want to write this constant to a stream. I do something like:

char c = THIS_CONST;
write(fd, &c, sizeof(c))

However, what I would like to do for brevity and clarity is:

write(fd, &THIS_CONST, sizeof(char)); // error
                                      // lvalue required as unary ‘&’ operand

Does anyone know of any macro/other trick for obtaining a pointer to a literal? I would like something which can be used like this:

write(fd, PTR_TO(THIS_CONST), sizeof(char))

---

Note: I realise I could declare my constants as static const variables, but then I can't use them in switch/case statements. i.e.

static const char THIS_CONST = 'A'
...
switch(c) {
  case THIS_CONST: // error - case label does not reduce to an integer constant
    ...
}

Unless there is a way to use a const variable in a case label?

Answer 1

There's no reason the compiler has to put the literal into any memory location, so your question doesn't make sense. For example a statement like

int a;
a = 10;

would probably just be directly translated into "put a value ten into a register". In the assembly language output of the compiler, the value ten itself never even exists as something in memory which could be pointed at, except as part of the actual program text.

You can't take a pointer to it.

If you really want a macro to get a pointer,

#include <stdio.h>
static char getapointer[1];

#define GETAPOINTER(x)  &getapointer, getapointer[0] = x

int main ()
{
    printf ("%d\n",GETAPOINTER('A'));
}

Answer 2

Since calling write() to write a single character to a file descriptor is almost certainly a performance killer, you probably want to just do fputc( THIS_CONST, stream ).

Answer 3

The only way you can obtain a pointer is by putting the literal into a variable (your first code example). You can then use the variable with write() and the literal in switch.

Answer 4

#define THIS_CONST 'a'

Is just a macro. The compiler basically just inserts 'a' everywhere you use THIS_CONST. You could try:

const char THIS_CONST = 'a';

But I suspect that will not work wither (don't have a c-compiler handy to try it out on, and it has been quite a few years since I've written c code).

Answer 5

C simply does not allow the address of character literals like 'A'. For what it's worth, the type of character literals in C is int (char in C++ but this question is tagged C). 'A' would have an implementation defined value (such as 65 on ASCII systems). Taking the address of a value doesn't make any sense and is not possible.

Now, of course you may take the address of other kinds of literals such as string literals, for example the following is okay:

write(fd, "potato", sizeof "potato");

This is because the string literal "potato" is an array, and its value is a pointer to the 'p' at the start.

To elaborate/clarify, you may only take the address of objects. ie, the & (address-of) operator requires an object, not a value.

And to answer the other question that I missed, C doesn't allow non-constant case labels, and this includes variables declared const.

Answer 6

Ok, I've come up with a bit of a hack, for chars only - but I'll put it here to see if it inspires any better solutions from anyone else

static const char *charptr(char c) {
    static char val[256];
    val[(unsigned char)c] = c;
    return &val[(unsigned char)c];
}

...

write(fd, charptr(THIS_CONST), sizeof(char));

Answer 7

I can see what you're trying to do here, but you're trying to use two fundamentally different things here. The crux of the matter is that case statements need to use values which are present at compile time, but pointers to data in memory are available only at run time.

When you do this:

#define THIS_CONST 'A'
char c = THIS_CONST;
write(fd, &c, sizeof(c))

you are doing two things. You are making the macro THIS_CONST available to the rest of the code at compile time, and you are creating a new char at runtime which is initialised to this value. At the point at which the line write(fd, &c, sizeof(c)) is executed, the concept of THIS_CONST no longer exists, so you have correctly identified that you can create a pointer to c, but not a pointer to THIS_CONST.

Now, when you do this: static const char THIS_CONST = 'A'; switch(c) { case THIS_CONST: // error - case label does not reduce to an integer constant ... }

you are writing code where the value of the case statement needs to be evaluated at compile time. However, in this case, you have specified THIS_CONST in a way where it is a variable, and therefore its value is available only at runtime despite you "knowing" that it is going to have a particular value. Certainly, other languages allow different things to happen with case statements, but those are the rules with C.

Here's what I'd suggest:

1) Don't call a variable THIS_CONST. There's no technical reason not to, but convention suggests that this is a compile-time macro, and you don't want to confuse your readers.

2) If you want the same values to be available at compile time and runtime, find a suitable way of mapping compile-time macros into run-time variables. This may well be as simple as:

#define CONST_STAR '*'
#define CONST_NEWLINE '\n'

static const char c_star CONST_STAR;
static const char c_newline CONST_NEWLINE;

Then you can do:

switch(c) {
  case CONST_STAR:
    ...
    write(fd, &c_star, sizeof(c_star))
    ...
}

(Note also that sizeof(char) is always one, by definition. You may know that already, but it's not as widely appreciated as perhaps it should be.)

Answer 8

For chars, you may use extra global static variables. Maybe something like:

#define THIS_CONST 'a'
static char tmp;
#define PTR_TO(X) ((tmp = X),&tmp)

write(fd,PTR_TO(THIS_CONST),sizeof(char));

Answer 9

There is no way to do this directly in C89. You would have to use a set of macros to create such an expression.

In C99, it is allowed to declare struct-or-union literals, and initializers to scalars can be written using a similar syntax. Therefore, there is one way to achieve the desired effect:

#include <stdio.h>

void f(const int *i) {
    printf("%i\n", *i);
}

int main(void) {
    f(&(int){1});
    return 0;
}