Comma operator in a macro-help me understand this plz!!

[s.subbarayan]

Dear all,
What does this following piece of code do?
#define CONVERT_SELF_PTR(type,in,out) \
(((in) != NULL) ? ((out) = (type *)(in), RETURN_OK) : ERROR_PARAM)

especially can someone let me understand why theres a comma in between

(out) = (type *)(in) and RETURN_OK in the expression ((out) = (type
*)(in), RETURN_OK)?

It will be also helpful if you could let me know the sequence in how
this expression in whole will be evaluated in steps.

Expecting all your help and advanced thanks for the same.

Regards,
s.subbarayan

[Richard Bos]

s_**********@rediffmail.com (s.subbarayan) wrote:

#define CONVERT_SELF_PTR(type,in,out) \
(((in) != NULL) ? ((out) = (type *)(in), RETURN_OK) : ERROR_PARAM)

especially can someone let me understand why theres a comma in between

(out) = (type *)(in) and RETURN_OK in the expression ((out) = (type
*)(in), RETURN_OK)?
That's just an ordinary comma operator. It works as any other comma
operator does: it separates successive expressions, returning the value
of the last expression as the value of the whole.
It will be also helpful if you could let me know the sequence in how
this expression in whole will be evaluated in steps.

Just peel it off into parts:

(in)!=NULL?
(
out=(type *)in,
RETURN_OK
)
:
ERROR_PARM

That is, it first checks to see whether in is a null pointer. If not, it
casts null to a pointer of type "type" and assigns it to out; and then
evaluates RETURN_OK (which presumably is a single value), and returns
that as the value of the whole expression. If in _is_ a null pointer,
the entire macro evaluates to ERROR_PARM.
No, I can't think of a good reason for writing a macro like that,
either. The context in which you found it might provide an explanation.

Richard

[Francois Grieu]

s_**********@rediffmail.com (s.subbarayan) ask:

What does thefollowing piece of code do?

#define CONVERT_SELF_PTR(type,in,out) \
(((in) != NULL) ? ((out) = (type *)(in), RETURN_OK) : ERROR_PARAM)

Assuming in particular that RETURN_OK and ERROR_PARAM are distinct
constants, and NULL has its usual meaning, a most natural explanation
is that
- out is expected to be a pointer to type
- in is expected to be a pointer
- the macro checks if in is a non-NULL pointer, and returns a
status accordingly; and in the affirmative only, copies
in to out, with type cast.

The comma is here to execute the assignment and produce RETURN_OK
in the second case; this is done so that the macro behaves
somewhat like a function.
For example

unsigned char *myin;
char *myout;

if (CONVERT_SELF_PTR(char,myin,myout) != RETURN_OK)
printf("error\n");

means

if (myin==NULL)
printf("error\n");
else
myout = (char *)myin;

François Grieu