Re: question about linker

On 2024-12-09, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:

An unambiguous grammar is something quite essential; how would you
parse code if it were ambiguous?

Here's an ambiguity in the C grammar:

    <statement>:
        ...
        <selection-statement>
        ...

    <selection-statement>:
        ...
        if ( <expression> ) <statement>
        if ( <expression> ) <statement> else <statement>
        ...

The following selection-statement is grammatically ambiguous:

    if (E1) if (E2) S1 else S2

it has two possible parsings:

    if (E1) <statement> else S2

    where <statement> expands to

        if (E2) S1

or

    if (E1) <statement>

    where <statement> expands to

        if (E2) S1 else S2

The grammatical ambiguity is resolved by an additional rule in the
'Semantics' section for selection-statement:

  3 An else is associated with the lexically nearest preceding if that is
    allowed by the syntax.

gcc -Wall will issue a warning for such an ambiguous statement:

    warning: suggest explicit braces to avoid ambiguous 'else' [-Wdangling-else]