I am reading a book namad A Retargetable C Compiler: Design and Implementation. In this book, the C language grammar is specified like this:
expression:
assignment-expression { , assignment-expression }
assignment-expression:
conditional-expression
unary-expression assign-operator assignment-expression
assign-operator:
one of= += -= *= /= %= <<= >>= &= A= I=
conditional-expression:
binary-expression [ ? expression : conditional-expression ]
binary-expression:
unary-expression { binary-operator unary-expression }
binary-operator:
one of || && '|' A & == ! = < > <= >= << >> + - * | %
unary-expression:
postfix-expression
unary-opera tor unary-expression
'(' type-name ')' unary-expression
sizeof unary-expression
sizeof '(' type-name ')'
unary-operator:
one of ++ -- & * + - - !
postfix-expression:
primary-expression { postfix-operator }
postfix-operator:
'[' expression ']'
'(' [ assignment-expression { , assignment-expression } ] ')'
. identifier
-> identifier
++
--
primary-expression:
identifier
constant
string-literal
'(' expression ')'
I have a question about something I observed with:
expression:
assignment-expression
I put unary-expression assign-operator assignment-expression
for the assignment-expression
.
I choose "sizeof '(' type-name ')'"
for the unary-expression.
Then I choose "="
for the assign-operator.
Then I chose "conditional-expression"
for the assignment-expression.
Then I derive like this:
conditional-expression -> binary-expression -> unary-expression ->postfix-expression -> primary-expression -> identifier
As a result of all the above, I can generate an expression like this: "sizeof(int) = 7"
.
But this expression is not possible in C language. Is there a problem with the above grammar listing, or am I producing this expression in the wrong way?
That something is grammatically correct doesn't mean it is logically correct. The expression sizeof(int) = 7
may be grammatically correct, but it doesn't make much sense. So your compiler instead of spilling error: syntax error
will tokenize and interpret the statement properly and tell you error: cannot assign to result of sizeof
.
On the topic you may be interested in the Annex A.1 Lexical grammar in C11 standard draft .
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