[previous] MISRA-C:2004  Rule  12.1:  (Advisory) [next] Limited dependence should be placed on C's operator precedence rules in expressions.
In addition to the use of parentheses to override default operator precedence, parentheses should also be used to emphasise it. It is easy to make a mistake with the rather complicated precedence rules of C, and this approach helps to avoid such errors, and helps to make the code easier to read. However, do not add too many parentheses so as to clutter the code and make it unreadable.

The following guidelines are suggested in deciding when parentheses are required:

* no parentheses are required for the right-hand operand of an assignment operator unless the right-hand side itself contains an assignment expression: 

x = a + b;              /* acceptable      */
x = ( a + b );          /* () not required */

* no parentheses are required for the operand of a unary operator: 

x = a * -1;             /* acceptable      */
x = a * ( -1 );         /* () not required */

* otherwise, the operands of binary and ternary operators shall be cast-expressions (see section 6.3.4 of ISO/IEC 9899:1990 [2]) unless all the operators in the expression are the same. 

x = a + b + c;                   /* acceptable, but care needed */
x = f( a + b, c );               /* no () required for a + b    */
x = ( a == b ) ? a : ( a - b );
if ( a && b && c )               /* acceptable                  */
x = ( a + b ) - ( c + d );
x = ( a * 3 ) + c + d;
x = ( uint16_t ) a + b;          /* no need for ( ( uint16_t ) a )  */

* even if all operators are the same, parentheses may be used to control the order of operation. Some operators (e.g. addition and multiplication) that are associative in algebra are not necessarily associative in C. Similarly, integer operations involving mixed types (prohibited by several rules) may produce different results because of the integral promotions. The following example written for a 16-bit implementation demonstrates that addition is not associative and that it is important to be clear about the structure of an expression: 

uint16_t a = 10U;
uint16_t b = 65535U;
uint32_t c = 0U;
uint32_t d;

d = ( a + b ) + c;   /* d is 9; a + b wraps modulo 65536 */
d = a + ( b + c );   /* d is 65545                       */
/* this example also deviates from several other rules */

Note that Rule is a special case of this rule applicable solely to the logical operators, && and ||.

QAC messages that encompass this guideline:

3389 Extra parentheses recommended to clarify the ordering of a % operator and another arithmetic operator (* / % + -).
3391 Extra parentheses recommended. A conditional operation is the operand of another conditional operator.
3392 Extra parentheses recommended. A shift, relational or equality operation is the operand of a second identical operator.
3393 Extra parentheses recommended. An arithmetic operation (* / + -) is the operand of a different operator with the same precedence.
3394 Extra parentheses recommended. A shift, relational or equality operation is the operand of a different operator with the same precedence.
3395 Extra parentheses recommended. A * or / operation is the operand of a + or - operator.
3396 Extra parentheses recommended. A binary operation is the operand of a conditional operator.
3397 Extra parentheses recommended. A binary operation is the operand of a binary operator with different precedence.


(c) The Motor Industry Research Association, 2004
QA C Source Code Analyser 8.1.2
MISRA-C:2004 Compliance Module 3.2
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