MISRA.ETYPE.COMP.CAST.EXPL.DIFFERENT.2012

MISRA C 2012 Rule 10.8: The value of a composite expression shall not be cast to a different essential type category or a wider essential type

Category: Required

Analysis: Decidable, Single Translation Unit

Applies to: C90, C99

Rationale

The rationale is described in the introduction on composite operators and expressions (see Section 8.10.3).

( uint32_t ) ( u16a + u16b );

On a 16-bit machine the addition will be performed in 16 bits with the result wrapping modulo-2 before it is cast to 32 bits. However, on a 32-bit machine the addition will take place in 32 bits and would preserve high-order bits that would have been lost on a 16-bit machine.

Casting to a narrower type with the same essential type category is acceptable as the explicit truncation of the result always leads to the same loss of information.

Example

( uint16_t ) ( u32a + u32b )   /* Compliant                            */
( uint16_t ) ( s32a + s32b )   /* Non-compliant - different essential
                                * type category                        */
( uint16_t ) s32a              /* Compliant - s32a is not composite    */
( uint32_t ) ( u16a + u16b )   /* Non-compliant - cast to wider
                                * essential type                       */