Code deactivated by constant false condition (DEACTIVATED_CODE)

R2026b

Code segment deactivated by #if 0 preprocessor directive

Description

This defect occurs when a preprocessor conditional (#if or #elif) uses a compile-time constant expression that always evaluates to zero. The code within the disabled block is never compiled.

Risk

Code deactivated by #if 0 is never compiled. Risks include:

• The compiler cannot test or verify the disabled code.

• The disabled code may silently become stale as surrounding code evolves.

• Maintenance confusion arises about whether the code is intentionally disabled or forgotten.

• Latent defects may surface if the directive is later changed.

Fix

To fix this defect:

• Remove the #if 0 block entirely if the code is no longer needed.

• Replace #if 0 with a named macro (for example, #if FEATURE_ENABLED) so the intent is explicit and the deactivation is conditional rather than absolute.

• If the code serves as a reference or placeholder, move it to a comment or version control history.

If you do not want to fix the issue, add comments to your result or code to avoid another review.

Examples

Code Deactivated by #if 0 Directive

In this example, a block of code is wrapped in #if 0 / #endif. The code within this block is never compiled regardless of any runtime conditions.

#include <stdio.h>

void initialize_system(void) {
    printf("System starting\n");

#if 0 // Noncompliant
    printf("Debug: detailed trace enabled\n");
    run_diagnostics();
#endif

    printf("System ready\n");
}

Polyspace^® Bug Finder™ reports a DEACTIVATED_CODE defect on the #if 0 directive. The constant expression always evaluates to zero, so the enclosed code is never compiled.

Correction — Use a Named Macro

Replace the literal #if 0 with a named feature macro. This makes the intent clear and allows conditional compilation.

#include <stdio.h>

#define ENABLE_DEBUG_TRACE 0

void initialize_system(void) {
    printf("System starting\n");

#if ENABLE_DEBUG_TRACE  // Compliant
    printf("Debug: detailed trace enabled\n");
    run_diagnostics();
#endif

    printf("System ready\n");
}

Result Information

Group: Data flow

Language: C | C++

Default: off

Command-Line Syntax: DEACTIVATED_CODE

Impact: Low

PQL Name: std.defects.DEACTIVATED_CODE

Version History

Introduced in R2013b

R2026b: Checker scope narrowed to preprocessor directives

The checker now reports a defect only for #if 0 and #elif 0 preprocessor directives. Previously, the checker also flagged if(0), while(0), if(false), macros evaluating to false, and template conditions. These cases now produce the DEAD_CODE defect instead.

 #if 0 // Flagged as DEACTIVATED_CODE code()
 #endif 
if (0) { 
  // No longer flagged as DEACTIVATED_CODE code(); 
}