MISRA C:2023 Rule 17.7

The value returned by a function having non-void return type shall be used

Since R2024a

Description

Rule Definition

The value returned by a function having non-void return type shall be used¹ .

Rationale

You can unintentionally call a function with a non-void return type but not use the return value. Because the compiler allows the call, you might not catch the omission. This rule forbids calls to a non-void function where the return value is not used. If you do not intend to use the return value of a function, explicitly cast the return value to void.

Polyspace Implementation

The checker flags functions with non-void return if the return value is not used or not explicitly cast to a void type.

The checker does not flag the functions memcpy, memset, memmove, strcpy, strncpy, strcat, strncat because these functions simply return a pointer to their first arguments.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

expand all

Used and Unused Return Values

unsigned int cutOff(unsigned int val) {
    if (val > 10 && val < 100) {
        return val;
    }
    else {
        return 0;
    }
}

unsigned int getVal(void);

void func2(void) {
    unsigned int val = getVal(), res;
    cutOff(val);           /* Non-compliant */
    res = cutOff(val);     /* Compliant */
    (void)cutOff(val);     /* Compliant */
}

In this example, the rule is violated when the return value of cutOff is not used subsequently.

The rule is not violated when the return value is:

Assigned to another variable.
Explicitly cast to void.

Check Information

Group: Function

Category: Required

AGC Category: Readability

Version History

Introduced in R2024a