🚀 go-pugleaf

GCC 2.2.2 doesn't warn about the common C coding error of using `=' when you
meant to use `=='.  For example, in the program

	int main(int argc, char **argv) {
		char **a = argv;
		for (;;) {
			printf("%s", *a);
			if (*++a = 0)
				break;
			printf(" ");
		}
		printf("\n");
		return 0;
	}

there's no option to make GCC warn that `if (*++a = 0) ...' does not do what
the programmer probably expected.

The following patch causes GCC to warn about using `=' in a truth value
context if the -Wparentheses option is specified.  If you really mean `='
instead of `==', you can prevent the warning by writing an extra set of
parentheses around the assignment, e.g. `if ((*++a = 0)) ...'.  The extra
parentheses help signal the reader of the unusual case where an assignment
appears where an equality test is normally expected.


Thu Aug 13 23:59:47 1992  Paul Eggert  (eggert@twinsun.com)

	* c-common.c (truthvalue_conversion): Optionally warn if an
	unparenthesized assignment is used as a truth value.
	* c-parse.y (expr_no_commas): Mark assignments for warning.

===================================================================
RCS file: c-common.c,v
retrieving revision 2.2
diff -c -r2.2 c-common.c
*** c-common.c	1992/05/26 19:22:26	2.2
--- c-common.c	1992/08/13 23:59:47
***************
*** 879,884 ****
--- 879,889 ----
  			      fold (build1 (NOP_EXPR,
  					    TREE_TYPE (TREE_OPERAND (expr, 0)),
  					    TREE_OPERAND (expr, 1))), 1);
+ 
+     case MODIFY_EXPR:
+       if (warn_parentheses && C_EXP_ORIGINAL_CODE (expr) == MODIFY_EXPR)
+ 	warning ("suggest parentheses around assignment used as truth value");
+       break;
      }
  
    return build_binary_op (NE_EXPR, expr, integer_zero_node, 1);
===================================================================
RCS file: c-parse.y,v
retrieving revision 2.2.2.2
diff -c -r2.2.2.2 c-parse.y
*** c-parse.y	1992/08/01 03:25:39	2.2.2.2
--- c-parse.y	1992/08/13 23:59:47
***************
*** 428,436 ****
  	| expr_no_commas '?' xexpr ':' expr_no_commas
  		{ $$ = build_conditional_expr ($1, $3, $5); }
  	| expr_no_commas '=' expr_no_commas
! 		{ $$ = build_modify_expr ($1, NOP_EXPR, $3); }
  	| expr_no_commas ASSIGN expr_no_commas
! 		{ $$ = build_modify_expr ($1, $2, $3); }
  	;
  
  primary:
--- 428,438 ----
  	| expr_no_commas '?' xexpr ':' expr_no_commas
  		{ $$ = build_conditional_expr ($1, $3, $5); }
  	| expr_no_commas '=' expr_no_commas
! 		{ $$ = build_modify_expr ($1, NOP_EXPR, $3);
! 		  C_SET_EXP_ORIGINAL_CODE ($$, MODIFY_EXPR); }
  	| expr_no_commas ASSIGN expr_no_commas
! 		{ $$ = build_modify_expr ($1, $2, $3);
! 		  C_SET_EXP_ORIGINAL_CODE ($$, MODIFY_EXPR); }
  	;
  
  primary: