sizeof Post: 86324

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Top Forums Programming sizeof Post 86324 by Perderabo on Thursday 13th of October 2005 07:22:32 AM

10-13-2005

Administrator Emeritus

c[] is not legal. Reasonable compilers refuse to produce an executable from your source code. That is the behavior I would prefer to see and I think it's unfortunate that g++ compiles it. But the standard does not require g++ to reject illegal code. So g++ is free to do anything it wants with your code. The resulting program can output 1 or 0. Or 8,732. Or it could whistle dixie. Or it could dump core. No behavior is specified for a non-c++ program by the c++ standard.

As for what you call "empty" structures, "Objects with an incomplete structure or union type may be mentioned in contexts where their size is not needed". And the sizeof operator needs the size. That too is illegal. gcc allows sizeof to operate on a incomplete structure and for some odd reason decided it should return 1.

Microsoft allows a single unsized array as the last member of a structure. gcc's behavior with unsized arrays is a superset of Microsoft's extention. The allows gcc to compile some Microsoft code. Maybe that is why gcc behaves as it does with unsized arrays. Microsoft says sizeof return the size of the structure minus the size of the array. The array has a non-zero size but the size is not known at compile time. Apparently, you obtain the size of the array somehow and add it in later. Miicrosoft unsized arrays in structures Note that you are abusing Microsoft's screwy extention. You are not supposed to allocate it as you have done.

Incomplete structures are in the same boat. The members are unknown, not absent. gcc is out of its mind in allowing you allocate such a thing. Each object is supposed to have a unique address, so if you allocate two of them, I guess that they consume a byte.

So nonstandard extentions to the language by two different organizations have clashed with one another. Righteous karma, if you ask me. Try writing in C or C++ for a change. Then none of this will affect you.

Perderabo

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LEARN ABOUT LINUX

offsetof

OFFSETOF(3)						     Linux Programmer's Manual						       OFFSETOF(3)

NAME

       offsetof - offset of a structure member

SYNOPSIS

       #include <stddef.h>

       size_t offsetof(type, member);

DESCRIPTION

       The macro offsetof() returns the offset of the field member from the start of the structure type.

       This  macro  is	useful	because the sizes of the fields that compose a structure can vary across implementations, and compilers may insert
       different numbers of padding bytes between fields.  Consequently, an element's offset is not necessarily given by the sum of the  sizes	of
       the previous elements.

       A compiler error will result if member is not aligned to a byte boundary (i.e., it is a bit field).

RETURN VALUE

       offsetof() returns the offset of the given member within the given type, in units of bytes.

CONFORMING TO

       C89, C99, POSIX.1-2001.

EXAMPLE

       On a Linux/i386 system, when compiled using the default gcc(1) options, the program below produces the following output:

	   $ ./a.out
	   offsets: i=0; c=4; d=8 a=16
	   sizeof(struct s)=16

   Program source

       #include <stddef.h>
       #include <stdio.h>
       #include <stdlib.h>

       int
       main(void)
       {
	   struct s {
	       int i;
	       char c;
	       double d;
	       char a[];
	   };

	   /* Output is compiler dependent */

	   printf("offsets: i=%ld; c=%ld; d=%ld a=%ld
",
		   (long) offsetof(struct s, i),
		   (long) offsetof(struct s, c),
		   (long) offsetof(struct s, d),
		   (long) offsetof(struct s, a));
	   printf("sizeof(struct s)=%ld
", (long) sizeof(struct s));

	   exit(EXIT_SUCCESS);
       }

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2008-07-12							       OFFSETOF(3)