The presence of unused values may indicate significant logic errors. To prevent such errors, unused values should be identified and removed from code.

This recommendation is a specific case of MSC12-C. Detect and remove code that has no effect or is never executed.

Noncompliant Code Example

In this example, p2 is assigned the value returned by bar(), but that value is never used. Note this example assumes that foo() and bar() return valid pointers (see DCL30-C. Declare objects with appropriate storage durations).

int *p1;
int *p2;
p1 = foo();
p2 = bar();

if (baz()) {
  return p1;
}
else {
  p2 = p1;
}
return p2;

Compliant Solution

This example can be corrected in many different ways, depending on the intent of the programmer. In this compliant solution, p2 is found to be extraneous. The calls to bar() and baz() can be removed if they do not produce any side effects.

int *p1 = foo();

/* Removable if bar() does not produce any side effects */
(void)bar();

/* Removable if baz() does not produce any side effects */
(void)baz();
return p1;

Exceptions

MSC13-EX1: Initializing a variable with a default value, such as 0, which gets subsequently overwritten may be inefficient, but is less of a problem than reading an uninitialized value, as per EXP33-C. Do not read uninitialized memory.

Risk Assessment

Unused values may indicate significant logic errors.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported, but no explicit checker
CodeSonar		LANG.STRUCT.UUVAL	Unused value
Coverity		UNUSED_VALUE	Finds variables that are assigned pointer values returned from a function call but never used
Helix QAC		C1500, C1502, C3203, C3205, C3206, C3207, C3229 DF2980, DF2981, DF2982, DF2983, DF2984, DF2985, DF2986
Klocwork		LV_UNUSED.GEN VA_UNUSED.GEN VA_UNUSED.INIT
LDRA tool suite		1 D, 8 D, 105 D, 94 D, 15 D	Fully implemented
Parasoft C/C++test		CERT_C-MSC13-a	Avoid unnecessary local variables
PC-lint Plus		438, 505, 529, 715, 838	Partially supported
Polyspace Bug Finder		CERT C: Rec. MSC13-C	Checks for: Unused parameter Write without a further read Rec. partially covered.
PVS-Studio		V519, V596, V603, V714, V744, V751, V763, V1001, V5003
SonarQube C/C++ Plugin		S1854

Related Vulnerabilities

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

Related Guidelines

Bibliography