Absolute symbols

Macros
#define	ABS_SYMBOL(name)   name[]
	Declare an absolute symbol (e.g. More...
#define	ABS_VALUE_INIT(name)   ( ( typeof ( name[0] ) ) name )
	Get value of an absolute symbol for use in a static initializer. More...
#define	ABS_VALUE(name)   ( ( typeof ( name[0] ) ) ( intptr_t ) name )
	Get value of an absolute symbol. More...

Detailed Description

Macro Definition Documentation

ABS_SYMBOL

#define ABS_SYMBOL

(

name

)

name[]

Declare an absolute symbol (e.g.

a value defined by a linker script)

Use as e.g.:

extern int ABS_SYMBOL ( _my_symbol );

Definition at line 655 of file compiler.h.

ABS_VALUE_INIT

#define ABS_VALUE_INIT

(

name

)

( ( typeof ( name[0] ) ) name )

Get value of an absolute symbol for use in a static initializer.

Use as e.g.:

extern int ABS_SYMBOL ( _my_symbol ); static int my_symbol = ABS_VALUE_INIT ( _my_symbol );

Note that the declared type must be at least as large as a pointer type, since the compiler sees the absolute symbol as being an address.

Definition at line 668 of file compiler.h.

ABS_VALUE

#define ABS_VALUE

(

name

)

( ( typeof ( name[0] ) ) ( intptr_t ) name )

Get value of an absolute symbol.

In a position-dependent executable, where all addresses are fixed at link time, we can use the standard technique as documented by GNU ld, e.g.:

extern char _my_symbol[];

printf ( "Absolute symbol value is %x\n", ( ( int ) _my_symbol ) );

This technique may not work in a position-independent executable. When dynamic relocations are applied, the runtime addresses will no longer be equal to the link-time addresses. If the code to obtain the address of _my_symbol uses PC-relative addressing, then it will calculate the runtime "address" of the absolute symbol, which will no longer be equal the the link-time "address" (i.e. the correct value) of the absolute symbol.

We can work around this by instead declaring a static variable to contain the absolute value, and returning the contents of this static variable:

extern char _my_symbol[]; static void * volatile my_symbol = _my_symbol;

printf ( "Absolute symbol value is %x\n", ( ( int ) my_symbol ) );

The value of the static variable cannot possibly use PC-relative addressing (since there is no applicable program counter for non-code), and so will instead be filled in with the correct absolute value at link time. (No dynamic relocation will be generated that might change its value, since the symbol providing the value is an absolute symbol.)

This second technique will work for both position-dependent and position-independent code, but incurs the unnecssary overhead of an additional static variable in position-dependent code. The ABS_VALUE() macro abstracts away these differences, using the most efficient available technique. Use as e.g.:

extern int ABS_SYMBOL ( _my_symbol ); #define my_symbol ABS_VALUE ( _my_symbol )

printf ( "Absolute symbol value is %x\n", my_symbol );

The ABS_VALUE() macro uses the (otherwise redundant) type declared on the ABS_SYMBOL() array to automatically determine the correct type for the ABS_VALUE() expression.

Unlike ABS_VALUE_INIT(), there is no restriction that the type must be at least as large as a pointer type.

Definition at line 723 of file compiler.h.