safety-critical-patterns

safety-critical-patterns

Knowledge Base

MISRA C compliance and safety-critical firmware patterns.

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Pattern 1: MISRA C Compliant Type Annotations

MISRA mandates explicit types. Use <stdint.h> types throughout.

/* Non-compliant (implicit types): */
int i;
unsigned char flags;

/* MISRA compliant: */
#include <stdint.h>
#include <stdbool.h>

int32_t  i;
uint8_t  flags;
bool     enable;

/* Rule 7.1: Octal constants */
/* VIOLATION: */
uint8_t val = 010;   /* Octal 8, not decimal 10 */
/* COMPLIANT: */
uint8_t val = 0x08U;

/* Rule 7.2: Unsigned constants must have U suffix */
/* VIOLATION: */
uint32_t x = 100;
/* COMPLIANT: */
uint32_t x = 100U;

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Pattern 2: Single-Exit Function Pattern (Rule 15.5)

MISRA 15.5: a function shall have a single point of exit.

/* NON-COMPLIANT: multiple returns */
int32_t process(uint8_t *buf, uint32_t len)
{
    if (buf == NULL) { return -1; }
    if (len == 0U)   { return -2; }
    /* process */
    return 0;
}

/* COMPLIANT: single exit */
int32_t process(uint8_t *buf, uint32_t len)
{
    int32_t result = 0;

    if (buf == NULL) {
        result = -1;
    } else if (len == 0U) {
        result = -2;
    } else {
        /* process */
        result = 0;
    }

    return result;
}

This is advisory in MISRA 2012 (was required in MISRA 2004), but required for DO-178C DAL-A.

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Pattern 3: MISRA Suppression Comments

When a violation is justified, document it with a suppression comment and deviation record.

/* MISRA C:2012 Rule 11.5 Deviation:
   Reason: CMSIS HAL requires void* for peripheral handle cast.
   Risk: Low — type verified by HAL layer.
   Reviewed by: J.Smith, 2024-03-15, ref: DEV-2024-011 */
/*lint -e9087 */  /* PC-lint suppression */
void *handle = (void *)&hspi1;   /* PRQA S 0306 */   /* Polyspace suppression */
/*lint +e9087 */

Deviation records must be maintained in a deviation log linked to each suppression.

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Pattern 4: RAM Test (March-C Algorithm)

March-C tests SRAM integrity at startup (required for IEC 61508 SIL 2+).