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Many functions return useful values whether or not the function has side effects. In most cases, this value is used to signify whether the function successfully completed its task or if some error occurred (see ERR02-C. Avoid in-band error indicators). Other times, the value is the result of some computation and is an integral part of the function's API.

Subclause 6.8.3 of the C Standard [ISO/IEC 9899:2011] states:

The expression in an expression statement is evaluated as a void expression for its side effects.

All expression statements, such as function calls with an ignored value, are implicitly cast to void. Because a return value often contains important information about possible errors, it should always be checked; otherwise, the cast should be made explicit to signify programmer intent. If a function returns no meaningful value, it should be declared with return type void.

This recommendation encompasses ERR33-C. Detect and handle standard library errors. Unlike this recommendation, that rule is restricted to functions from the Standard C library.

Compliance with this recommendation is required in order to comply with ERR00-C. Adopt and implement a consistent and comprehensive error-handling policy

Noncompliant Code Example

This noncompliant code example calls fputs() and fails to check whether a write error occurs:

FILE* f = /* output file stream */
/* ... */
fputs("foo", f);

However, fputs() can fail and return EOF.

Compliant Solution

This compliant solution checks to make sure no output error occurred (see ERR33-C. Detect and handle standard library errors).

FILE* f = /* output file stream */
/* ... */
if (fputs("foo", f) == EOF) {
  /* Handle error */
}

Exceptions

EXP12-C-EX1: If the return value is inconsequential or if any errors can be safely ignored, such as for functions called because of their side effects, the function should be explicitly cast to void to signify programmer intent. For an example of this exception, see "Compliant Solution (Remove Existing Destination File)" under the section "Portable Behavior" in FIO10-C. Take care when using the rename() function, or Exception ERR33-C-EX1 in ERR33-C. Detect and handle standard library errors.

EXP12-C-EX2: If a function cannot fail or if the return value cannot signify an error condition, the return value may be ignored. Such functions should be added to a whitelist when automatic checkers are used.

strcpy(dst, src);

Risk Assessment

Failure to handle error codes or other values returned by functions can lead to incorrect program flow and violations of data integrity.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

EXP12-C

Medium

Unlikely

Medium

P4

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04

error-information-unused
error-information-unused-computed

Fully checked
Axivion Bauhaus Suite

6.9.0

CertC-EXP12Fully implemented
CodeSonar
5.2p0
LANG.FUNCS.IRVIgnored return value
Compass/ROSE




Coverity

2017.07

CHECKED_RETURN

Finds inconsistencies in how function call return values are handled. Coverity Prevent cannot discover all violations of this recommendation, so further verification is necessary

Cppcheck
 1.66
leakReturnValNotUsed, ignoredReturnValue

Return value of memory allocation function is not used.

Ignored return value from function when configuration says it must be used. See the chapter "Library configuration" in the cppcheck manual

ECLAIR

1.2

CC2.EXP12

Fully implemented

Klocwork
2018

MISRA.FUNC.UNUSEDRET.2012
SV.RVT.RETVAL_NOTTESTED


LDRA tool suite
9.7.1

382 S

Fully implemented

Parasoft C/C++test
10.4.2

CERT_C-EXP12-a
CERT_C-EXP12-b

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

Polyspace Bug Finder

R2019b

CERT C: Rec. EXP12-CChecks for situations where returned value of a sensitive function is not checked (rec. fully covered)
PRQA QA-C
9.7
3200Fully implemented
PVS-Studio

6.23

V530, V698, V757, V797
RuleChecker
19.04

error-information-unused

Partially checked
Splint
3.1.1



Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

[ISO/IEC 9899:2011]Subclause 6.8.3, "Expression and Null Statements"



7 Comments

  1. Yes, OK. How many of our compliant solutions do you suppose fail to comply with this recommendation?

    Also, if you write a checker for this will you get tons of true positives, but these are very low value true positives and will contribute quite alot to an adverse signal to noise ratio.

    I wonder if we could replace with this recommendation with more recommendations to check return values from functions? If something is not on the list (like strcpy() it is not necessary to check the return status or cast to void.

    1. splint says:
      10 violations in 61 CCEs
      7 violations in 77 NCCEs

      I'd say this kind of recommendation is not very useful on legacy code because of the signal to noise issue you mention. But if you write your code and remember to cast to void when you are not checking your return value, a violation is almost certainly a bug.

      Would it make you happy if we mentioned the possibility of a white/black list for automatic checkers?

      1. Sort of. We have an example where we cast a call to remove() to void, which makes perfect sense to me, because normally you would want to check the return status. There are a number of functions, such as string functions that only return a copy of the string so you can nest the function calls, where it is not unusual or bad practice IMHO to ignore the return value.

        I guess I would be happier with this rule if we provided our own white list of functions that are excepted. This would be one class of exception. Maybe we could use the remove() example for EXP12-EX1 as an eample of a return value that is inconsequential.

        1. done, added a new exception for white lists

  2. In the first paragraph, what do you mean by "necessary output"?, necessary for what?

    1. I'd guess the output is necessary in order to fulfill the API. For instance, strlen returns a size_t which is critical to its API. Rewrote the paragraph; hopefully its clearer now.

  3. I am getting a coverity error named CHECKED_RETURN although
    I have checked the return value, I am not sure why coverity tool
    throw this error.

    Both of functions are given below.

    The false statement used for func1 is defined as
    #ifndef true
    #define true 1
    #endif
    #ifndef false
    #define false 0

    bool func1(int*pointer)
    {
       bool r = false;
       r = func2()
       if(r == false)

    Unknown macro: {         return r;        r = func3();    return r; }

    The FALSE & TRUE value has been considered from given enum
    typedef enum {
        FALSE = 0,
        TRUE = 1
    } bool_t;

    bool_t func2()
    {
            if(fun4()){
                     return FALSE;

     if(func5())

    Unknown macro: {                  return TRUE; }

    Coverity ERROR:
    unchecked_value: Return value of func2() is not checked
    can any body suggest why coverity error is observed