Re: [Tinycc-devel] A TCC C parser question about the grammar

[Fred Weigel]

Wei

Not looking at the standard right now, but the following is valid:

x = 1, 2, 3; // x gets 3

A test for the second case would be

x = 3;
y = 3 * x = 2;

This should be an error, because 3 * x is not an lvalue.

Does this compile with tcc? (don't know, the machine I am on doesn't
have the compiler).

On Sat, 2009-09-05 at 01:05 +0800, Wei wrote:
>     Hi, TCC stackholders,
>     
>     I am tracing TCC's source codes, and find something I don't understand.
>     What I don't understand is about the grammar.
>     
>     From the C89 or C99 standard:
>     
>     ======================================================================
>     assignment_expression
>     : lvalue assignment_operator assignment_expression
>     | conditional_expression
>     ;
>     ======================================================================
>     
>     However, in TCC,
>     
>     ======================================================================
>     static void gexpr(void)
>     {
>     while (1) {
>     expr_eq();
>     if (tok != ',')
>     break;
>     vpop();
>     next();
>     }
>     }
>     ======================================================================
>     
>     and expr_eq( ) seems to parse C conditional _expression_ (ie, xxx ? xxx : xxx).
>     It seems that it doesn't parse codes like "a = b = c = d = 4;".
>     
>     And in the C standard,
>     
>     ======================================================================
>     multiplicative_expression
>     : (cast_expression) ('*' cast_expression | '/' cast_expression | '%' cast_expression)*
>     ;
>     ======================================================================
>     
>     However, in TCC, the function to parse "multiplicative_expression" is expr_prod( ):
>     
>     ======================================================================
>     static void expr_prod(void)
>     {
>     int t;
>     
>     uneq();
>     while (tok == '*' || tok == '/' || tok == '%') {
>     t = tok;
>     next();
>     uneq();
>     gen_op(t);
>     }
>     }
>     ======================================================================
>     
>     and uneq( ) seems to parse the so-call "assignment_expression.", not the standard-defined "cast_expression"
>     
>     Why TCC uses such a grammar? it seems not compatible with the C standard.
>     
>     Wei.
>     
>     
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