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Full Discussion: help with data type sizes
Top Forums Programming help with data type sizes Post 302488131 by Driver on Saturday 15th of January 2011 09:41:46 AM
Old 01-15-2011
The reason pointer arithmetic is scaled for the base type is simply to support array operations.

Given

int x[2];

... we access the second element of the array by writing x[1] or equivalently *(x + 1). You cannot fully separate arrays and pointers because the value of an array is a pointer to its first element, such that most array operations will involve pointers to their elements. It would be quite inconvenient and error-prone if we had to multiply the element number with the base type size to access the element.

Quote:
for chars if i do a="a" then get the address (&a) i get some large negative number, but i know the size of a char is 1 byte or 8 bits, so shouldn't the value be between 0 and 255 ?

also if converting an address of an int to hex and printing it why is it never negative?
The address of your char has a different type - it's char *, not char - and will probably (but not necessarily) have the same size as any other pointer, and typically amounts to 32 or 64 bits on most systems.

Signedness is a matter of how the data is interpreted. The %x format specifier expects an unsigned argument and will therefore treat all bits as data, whereas if you're using %d, the most significant bit of your value will be treated as sign bit - if it's set, the number is negative, otherwise it isn't.

Pointer encoding is system-specific, but it is generally not signed, so you should not print it as a signed value.

Quote:
I think it's reasonably portable among 32-bit and 64-bit varieties of UNIX, and 32-bit varieties of Windows. Perhaps not 64-bit windows, in which long inexplicably doesn't change size even though the integer width of the processor doubles...
On Windows (and arguably other systems as well, though I tend to use unsigned long) it is best to convert your pointer to size_t, because that will give you an __int64.
 

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LEARN ABOUT SUSE

funcolumnlookup

funcolumnlookup(3)						SAORD Documentation						funcolumnlookup(3)

NAME

       FunColumnLookup - lookup a Funtools column

SYNOPSIS

	 #include <funtools.h>

	 int FunColumnLookup(Fun fun, char *s, int which,
			     char **name, int *type, int *mode,
			     int *offset, int *n, int *width)

DESCRIPTION

       The FunColumnLookup() routine returns information about a named (or indexed) column.  The first argument is the Fun handle associated with
       this set of columns. The second argument is the name of the column to look up.  If the name argument is NULL, the argument that follows is
       the zero-based index into the column array of the column for which information should be returned.  The next argument is a pointer to a
       char *, which will contain the name of the column. The arguments that follow are the addresses of int values into which the following
       information will be returned:

       o   type: data type of column:

	   o   A: ASCII characters

	   o   B: unsigned 8-bit char

	   o   I: signed 16-bit int

	   o   U: unsigned 16-bit int (not standard FITS)

	   o   J: signed 32-bit int

	   o   V: unsigned 32-bit int (not standard FITS)

	   o   E: 32-bit float

	   o   D: 64-bit float

       o   mode: bit flag status of column, including:

	   o   COL_ACTIVE      1 is column activated?

	   o   COL_IBUF        2 is column in the raw input data?

	   o   COL_PTR	       4 is column a pointer to an array?

	   o   COL_READ      010 is read mode selected?

	   o   COL_WRITE     020 is write mode selected?

	   o   COL_REPLACEME 040 is this column being replaced by user data?

       o   offset: byte offset in struct

       o   n: number of elements (i.e. size of vector) in this column

       o   width: size in bytes of this column

       If the named column exists, the routine returns a positive integer, otherwise zero is returned. (The positive integer is the index+1 into
       the column array where this column was located.)

       If NULL is passed as the return address of one (or more) of these values, no data is passed back for that information.  For example:

	 if( !FunColumnLookup(fun, "phas", 0, NULL NULL, NULL, NULL, &npha, NULL) )
	   gerror(stderr, "can't find phas column
");

       only returns information about the size of the phas vector.

SEE ALSO

       See funtools(7) for a list of Funtools help pages

version 1.4.2							  January 2, 2008						funcolumnlookup(3)
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