comparing two structures

[junky_fellow]

Guys,

Is it a good way to compare two structures for equality by using
"memcmp" ?

If not, what could be the problems associated with this ? And what is
the correct
method of doing this ?

thanks for any help.

[Richard Heathfield]

ju**********@ya hoo.co.in said:

Guys,

Is it a good way to compare two structures for equality by using
"memcmp" ?

No.

If not, what could be the problems associated with this ?

Padding bytes, and dynamically allocated memory pointed to by struct
members.

And what is the correct method of doing this ?

int cmp(const struct T *a, const struct T *b);

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Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Joachim Schmitz]

<ju**********@y ahoo.co.inschri eb im Newsbeitrag
news:11******** *************@w 3g2000hsg.googl egroups.com...

Guys,

Is it a good way to compare two structures for equality by using
"memcmp" ?

If not, what could be the problems associated with this ? And what is
the correct
method of doing this ?

thanks for any help.

http://c-faq.com/struct/compare.html

[CBFalconer]

Richard Heathfield wrote:

ju**********@ya hoo.co.in said:

>Is it a good way to compare two structures for equality by using
"memcmp" ?

No.

>If not, what could be the problems associated with this ?

Padding bytes, and dynamically allocated memory pointed to by
struct members.

>And what is the correct method of doing this ?

int cmp(const struct T *a, const struct T *b);

Which should resolve to (very roughly}:

for (c = eachcomponentof a) {
if ((a->c) != (b->c)) return 0; /* unequal */
}
return 1; /* i.e. structures equal */

Which means you have to write a cmp() each structure type. You may
have to resolve embedded pointers.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>
--
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[Richard Heathfield]

CBFalconer said:

Richard Heathfield wrote:

<snip>

>>
int cmp(const struct T *a, const struct T *b);

Which should resolve to (very roughly}:

for (c = eachcomponentof a) {
if ((a->c) != (b->c)) return 0; /* unequal */
}
return 1; /* i.e. structures equal */

Personally, I would find a relational comparison more generally useful.

<snip>

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[John Gordon]

In <11************ *********@w3g20 00hsg.googlegro ups.com"ju***** *****@yahoo.co. in" <ju**********@y ahoo.co.inwrite s:

Is it a good way to compare two structures for equality by using
"memcmp" ?

No.

If not, what could be the problems associated with this?

One such problem is comparison of strings. The contents of a string are
meaningless after the terminating null byte, but memcmp() wouldn't know
that.

--
John Gordon A is for Amy, who fell down the stairs
go****@panix.co m B is for Basil, assaulted by bears
-- Edward Gorey, "The Gashlycrumb Tinies"

[Tor Rustad]

CBFalconer wrote:

[...]

Which should resolve to (very roughly}:

for (c = eachcomponentof a) {
if ((a->c) != (b->c)) return 0; /* unequal */
}
return 1; /* i.e. structures equal */

Which means you have to write a cmp() each structure type. You may
have to resolve embedded pointers.

Very confusing name choice, unless cmp() return 0 if equal.

--
Tor <torust [at] online [dot] no>

[CBFalconer]

Richard Heathfield wrote:

CBFalconer said:

>Richard Heathfield wrote:

<snip>

>>>
int cmp(const struct T *a, const struct T *b);

Which should resolve to (very roughly}:

for (c = eachcomponentof a) {
if ((a->c) != (b->c)) return 0; /* unequal */
}
return 1; /* i.e. structures equal */

Personally, I would find a relational comparison more generally
useful.

Granted, but generally impossible. For:

struct foobar {int foo; char bar);

please define a suitable cmp function that is relational, without
somehow defining the relative importance of foo and bar. :-) I'm
sure you know this, but the comment is for general consumption.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
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[CBFalconer]

Tor Rustad wrote:

CBFalconer wrote:

[...]

>Which should resolve to (very roughly}:

for (c = eachcomponentof a) {
if ((a->c) != (b->c)) return 0; /* unequal */
}
return 1; /* i.e. structures equal */

Which means you have to write a cmp() each structure type. You
may have to resolve embedded pointers.

Very confusing name choice, unless cmp() return 0 if equal.

You have my full permission to interchange 0 and 1. :-)

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

--
Posted via a free Usenet account from http://www.teranews.com

[Richard Heathfield]

Tor Rustad said:

<snip>

Furthermore, if writing a cmp() function, I rather prefer to use the
same interface and return value as qsort() and bsearch() expect:

int cmp_mystruct(co nst void *, const void *)

why compare "struct's", unless doing sorting and searching?

Well said, although I think I'd like to add an extra void * to that, so
that the comparison function can use (or provide) additional
information about the comparison without resorting to global data.
This, of course, requires ISO to change the spec for qsort and bsearch,
or provide sensibly-designed versions thereof.

Jump to it, lads...

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999