Technical Note 	: SLE0007

Author	: Scott Evans

Created/Modified	: 4/9/98

Description	: GNU C extensions

This technical note describes extensions to the C language supported by
GNU compilers.

Pointer arithmetic

With GNU C you can perform arithmetic on void pointers and functions.
The sizeof() can also be used on void pointers. It assumes void pointers
are the same as byte pointers. So sizeof() will return 1.

Example

void *vp;

vp=(void *)0x80090000;

printf(“Size of void pointer = %d\n”,sizeof(void *));

printf(“Address = %x\n”,vp);

vp++;

printf(“Address = %x\n”,vp);

Case ranges

In case statements you can specify a range of values with ‘...’.

Example

switch(character)

{

	case ‘A’ ... ‘Z’:

		puts(“Upper case”);

	break;

	case ‘a’ ... ‘z’:

		puts(“Lower case”);

	break;

	case ‘0’ ... ‘9’:

		puts(“Digit”);

	break;

}

Array element ranges

It is possible to assign a value to a range of array elements. The
‘...’ is used.

Example 

int a[30]={[0 ... 9]=23,[10 ... 12]=0,[20 ... 29]=100};

This will fill the first 10 elements of the array with 23. Elements
10-12 will be set to 0 and 20-29 will be 100.

You can also do the following.

int a[10]={[6]=23,[8]=250,[0]=21};

This is the same as the following bit of code.

int a[10]={21,0,0,0,0,0,23,0,250,0};

Alignment of variables

The __alignof__ keyword can be used to find out how a type should be
aligned. For example the following will return 4 since a long on R3000
needs to be 4 byte aligned.

printf(“Alignemnt of long %d\n”,__alignof__(long)); 

You can set the alignment of a variable using the __attribute__ keyword.


Example

long a __attribute__ ((aligned(4)))=21;

This will make sure the variable a is aligned on a 4 byte boundary. It
also sets the value of a to 21. 

Address of labels

The address of a label can be found using the ‘&&’ operator. It is
returns type void *.

Example

void main(void)

{

	static void *jump_table[]={&&here,&&there,&&everywhere};

	goto *jump_table[0];

	puts(“Never getes here...”);

here:

	goto *jump_table[1];

	puts(“or here...”);

there:

	goto *jump_table[2];

	puts(“or here...”);

everywhere:

}