Enforcing C90 style declarations in gcc with -pedantic-errors

[fyquah]

I've got asked this frequently, that I think is worth pointing out, as it might be useful for writing test cases etc.

To force gcc to throw and error with non-C89 compliant type declaration, one has to pass std=c89 and -pedantic-errors to the compiler.

$ cat main.c 
int
main()
{
  int a;
  a = 1;
  int b = 2;
  return 0;
}
$ gcc -std=c89 main.c  # succeeds
$ gcc -pedantic -std=c89 -Wall main.c
main.c:6:7: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]
  int b = 2;
      ^
main.c:6:7: warning: unused variable 'b' [-Wunused-variable]
2 warnings generated.
$ gcc -pedantic-errors -std=c89  main.c
main.c:6:7: error: ISO C90 forbids mixing declarations and code [-Werror,-Wdeclaration-after-statement]
  int b = 2;