Macro expansion in armcc

Discussion in 'C Programming' started by srinu.fsl@gmail.com, Jul 11, 2006.

  1. Guest

    there's a MACRO call :

    MACRO1(cnf)

    and its expansion is :

    #define MACRO1(cnf) (((cnf) != TRUE)? (CLEANUP(FAIL)):(err =
    SUCCESS));

    #define CLEANUP(a)
    \
    {
    \
    if ((err = (a)) != SUCCESS)
    \
    {
    \
    goto cleanup;
    \
    }
    \
    }
    \


    Its giving me error:
    line 392: Error: expected an expression
    MACRO1(cnf)
    ^

    Same code is getting compiled in gcc but armcc gives the above error..
    Can anybody pls comment on this?
    , Jul 11, 2006
    #1
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  2. Richard Bos Guest

    wrote:

    > there's a MACRO call :
    >
    > MACRO1(cnf)
    >
    > and its expansion is :
    >
    > #define MACRO1(cnf) (((cnf) != TRUE)? (CLEANUP(FAIL)):(err =
    > SUCCESS));


    I assume that in your code it's all on one line; if not, it should have
    given an error for this declaration, not for your call.

    > #define CLEANUP(a)
    > {
    > if ((err = (a)) != SUCCESS)
    > {
    > goto cleanup;
    > }
    > }


    Ditto here; you provided line continuation backslashes, but they
    themselves had wrapped to the next line. I've snipped them here for
    brevity.

    > Its giving me error:
    > line 392: Error: expected an expression
    > MACRO1(cnf)
    > ^


    The CLEANUP macro is declared to be equivalent to an entire block
    statement containing an if statement. However, in MACRO1, you use it
    essentially like this:

    value? CLEANUP(FAIL): expr;

    In ISO C, the ternary operator takes three expressions, not one
    expression and three bits of whatever executable code. IOW, this:

    value? { if (x) y; }: expr;

    is invalid, but that is what you are declaring.

    Since your CLEANUP macro involves a goto statement (see the castigations
    for that flying in from a dozen posters, btw), you can't turn CLEANUP
    itself into a ?: expression, which would have solved this problem; so
    you're left with only one solution: change your MACRO1 to use an if
    statement as well, so that you get something like

    if (!cnf) {
    if ((err=FAIL)!=SUCCESS) { goto cleanup; }
    } else err=SUCCESS;

    It's still ugly, and you will probably want to consult the FAQ on how to
    write a statement macro more solidly, but it should work - FSVO.

    > Same code is getting compiled in gcc but armcc gives the above error..


    That's because gcc supports complete statements in ?: (and IIRC in all
    expression contexts). Beats me why; its main use is to create illegible
    code.

    Richard
    Richard Bos, Jul 11, 2006
    #2
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  3. writes:
    > there's a MACRO call :
    >
    > MACRO1(cnf)
    >
    > and its expansion is :
    >
    > #define MACRO1(cnf) (((cnf) != TRUE)? (CLEANUP(FAIL)):(err =
    > SUCCESS));
    >
    > #define CLEANUP(a)
    > \
    > {
    > \
    > if ((err = (a)) != SUCCESS)
    > \
    > {
    > \
    > goto cleanup;
    > \
    > }
    > \
    > }
    > \
    >
    >
    > Its giving me error:
    > line 392: Error: expected an expression
    > MACRO1(cnf)
    > ^
    >
    > Same code is getting compiled in gcc but armcc gives the above error..
    > Can anybody pls comment on this?


    I see a number of problems.

    As Richard Bos has mentioned, some of your lines wrapped when you
    posted this.

    Stylistically, there's way too much vertical space in the definition
    of CLEANUP, and some of the '\'s wrapped to the next line.

    It's easy enough to fix the lin wrapping in MACRO1, either by
    re-joining the line or by splitting it and adding a '\'.

    Here's a re-formatted version of CLEANUP:

    #define CLEANUP(a) \
    { \
    if ((err = (a)) != SUCCESS) \
    { \
    goto cleanup; \
    } \
    }

    I presume TRUE is defined somewhere else. It's almost never a good
    idea to compare a boolean expression for equality with TRUE or FALSE.
    Remember than an expression with the value zero is considered false
    when used as a condition, and an expression with *any* non-zero value
    is considered true. Your test
    ((cnf) != TRUE)
    will fail if cnf is true (non-zero) but doesn't happen to have the
    same value as TRUE (presumably 1). cnf is already a condition; just
    use it directly. ((cnf) != TRUE) can be replaced by (!(cnf)). See
    section 9 of the comp.lang.c FAQ, <http://www.c-faq.com/>.

    A macro usually expands to either an expression or a statement. An
    expression can be used as a statement by adding a ';', but a statement
    cannot be used in an expression context.

    Your MACRO1 definition *almost* expands to an expression, but you've
    added a semicolon, so you can't use MACRO1 in an expression context.
    That's what the error message is telling you.

    I might define MACRO1 something like this:

    #define MACRO1(cnf) ( (cnf) ? err = SUCCESS : CLEANUP(FAIL) )

    (but I'd give it a better name!)

    Your CLEANUP macro expands to a compound statement. This means, of
    course, that you can't use it in an expression context. Presumably
    you haven't made that mistake, but since you only showed us the
    definitions of your macros and not the code that invokes them, we
    can't really tell. But there's still a problem using CLEANUP even in
    a statement context; see question 10.4 in the FAQ.

    --
    Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
    San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
    We must do something. This is something. Therefore, we must do this.
    Keith Thompson, Jul 11, 2006
    #3
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