C grammar compound statement

Question

I was looking at the C grammer on K&R and i found this:

compound-statement:
{ declaration-list opt statement-list opt }

declaration-list:
declaration
declaration-list declaration

statement-list:
statement
statement-list statement

Which means that we can't have declarations after statements. However i am doing this really often like:

#include <stdio.h>

int main()
{
  printf("Lets use a new block");
  {
    int a=1;
    printf("%d",a);
    int b=3;
    printf("%d",b);
  }
  return 0;
}

This code compiles with no warning and no errors. Am i not understanding the grammar correctly?

This is a duplicate, yes back in days those were the rules, now it is relaxed. — Antti Haapala -- Слава Україні, May 14 '14 at 20:55
Enable more warnings and you'll get what you expect (for `gcc` -- from memory -- `-pedantic` ought to do it). — Jongware, May 14 '14 at 20:57
Also [C89, Mixing Variable Declarations and Code](http://stackoverflow.com/questions/6488503/c89-mixing-variable-declarations-and-code) — Antti Haapala -- Слава Україні, May 14 '14 at 21:02

hyde · Answer 1 · 2014-05-15T05:58:54.997

4

To get the error you want, pass these flags to gcc:

-std=c90 -pedantic-errors

GNU extensions, as well as more recent C standards, allow declarations after other statements in a scope.

edited May 15 '14 at 05:58

answered May 14 '14 at 20:59

hyde

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score 3 · Answer 2 · answered May 14 '14 at 20:55

3

You understand the grammar fine. However, C has advanced since the K&R days and now the grammar accepts interleaved declarations and statements.

answered May 14 '14 at 20:55

rici

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C grammar compound statement

2 Answers2