help with data type sizes Post: 302488131

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Top Forums Programming help with data type sizes Post 302488131 by Driver on Saturday 15th of January 2011 09:41:46 AM

01-15-2011

Registered User

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.

Driver

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LEARN ABOUT DEBIAN

ba_and_ba

BITARR(3pub)						       C Programmer's Manual						      BITARR(3pub)

NAME

       ba_and_ba, ba_clear, ba_clear_all, ba_copy, ba_create, ba_destroy, ba_not, ba_or_ba, ba_or_not_ba, ba_query, ba_resize, ba_set, ba_xor_ba -
       bit array manipulation

SYNOPSIS

       #include <bitarr.h>

       Bitarr *ba_create(void);
       void ba_destroy(Bitarr *ba);
       Bitarr *ba_copy(const Bitarr *ba);
       int ba_resize(Bitarr *ba, size_t max_number);

       int ba_set(Bitarr *ba, unsigned number);
       int ba_clear(Bitarr *ba, unsigned number);
       void ba_clear_all(Bitarr *ba);
       int ba_query(Bitarr *ba, unsigned number);
       void ba_and_ba(Bitarr *ba1, const Bitarr *ba2);
       int ba_or_ba(Bitarr *ba1, const Bitarr *ba2);
       void ba_xor_ba(Bitarr *ba1, const Bitarr *ba2);
       void ba_or_not_ba(Bitarr *ba1, const Bitarr *ba2);
       void ba_not(Bitarr *ba);

DESCRIPTION

       These functions operate on bit arrays.  ba_create creates one and ba_destroy it.  ba_copy makes a copy of one, and  ba_resize  changes  its
       size.   The bit arrays resize themselves automatically, but ba_resize can be used to get rid of extra memory allocated for the array, or to
       make sure there is enough memory allocated for the array (so that further operations are guaranteed to work).

       ba_set sets one element (bit) in the array to 1.  ba_clear clears one element (sets it to 0), ba_clear_all clears all  elements.   ba_query
       returns the current value of an element.

       ba_and_ba, ba_or_ba, ba_xor_ba, and ba_or_ba do logical operations on two arrays.  The result will be stored in the first one.  ba_not will
       invert all elements in the array.

RETURNS

       ba_create and ba_copy return a pointer to the new array, or NULL if they fail.  Those that return an integer, return -1 for error, non-neg-
       ative for success.

SEE ALSO

       publib(3), iset(3)

AUTHOR

       Lars Wirzenius (lars.wirzenius@helsinki.fi)

Publib							       C Programmer's Manual						      BITARR(3pub)

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