I think C's automatic conversion of char[] to char* is trying to drive
me insane :-)
In Asterisk's code, we've got a very simple macro for selecting between
two strings if the first one is NULL or empty:
#define S_OR(a, b) (ast_strlen_zero(a) ? (b) : (a))
However, as written, this macro evaluates 'a' twice, so if it is
actually a function call or has side effects, the results are non-optimal.
I tried to rewrite it as follows:
#define S_OR(a, b) ({typeof(a) __x = a; (ast_strlen_zero(__x) ? (b) :
__x);})
When called like this, it compiles just fine:
char *str1, *str2;
....
str2 = S_OR(str1, "");
In this case, typeof(a) is 'char *', so the declaration "char *__x =
str1;' works great.
However, when called like this, it fails to compile:
char str1[20], *str2;
....
str2 = S_OR(str1, "");
This is because the declaration becomes "char[] __x = str1;', which is
invalid. In the simpler version of the macro this works fine, because
assigning the other way (str2 = str1;) is completely valid, as the
compiler 'promotes' the char[] reference into a char* since they are
the same thing anyway when used as an r-value.
So, in an attempt to be even more clever, I rewrote it as this:
#define S_OR(a, b)
__builtin_choose_expr(__builtin_types_compatible_p(typeof(a), char[]), \
({char *__x = (a);
(ast_strlen_zero(__x) ? (b) : __x);}), \
({typeof (a) __x = (a);
(ast_strlen_zero(__x) ? (b) : __x);}))
Unfortunately, this does not accomplish the desired result, because
__builtin_types_compatible_p appears to return 1 (true) for 'char*' and
'char[]'. I suppose in the grand scheme of things this is true, but it's
not helping me accomplish what I want to accomplish :-)
There is one workaround that I've found, which is to call the macro like
this:
char str1[20], *str2;
....
str2 = S_OR(&str1, "");
This makes the 'promotion' to 'char *' explicit, and resolves the issue.
It would require changing about 60 lines of code, which I could deal
with, but if possible I'd really rather come up with a way to make the
macro do the 'right thing'.
Am I just barking up the wrong tree here?
--
Kevin P. Fleming
Director of Software Technologies
Digium, Inc. - "The Genuine Asterisk Experience" (TM)
