Writing to const qualified object

Object declared with a const qualifier is modified

Description

This defect occurs when you do one of the following:

• Use a const-qualified object as the destination of an assignment.

• Pass a const-qualified object to a function that modifies the argument.

For instance, the defect can occur in the following situations:

• You pass a const-qualified object as first argument of one of the following functions:

  • mkstemp

  • mkostemp

  • mkostemps

  • mkdtemp

• You pass a const-qualified object as the destination argument of one of the following functions:

  • strcpy

  • strncpy

  • strcat

  • memset

• You perform a write operation on a const-qualified object.

Risk

The risk depends upon the modifications made to the const-qualified object.

Situation	Risk
Passing to mkstemp, mkostemp, mkostemps, mkdtemp, and so on.	These functions replace the last six characters of their first argument with a string. Therefore, they expect a modifiable char array as their first argument.
Passing to strcpy, strncpy, strcat, memset and so on.	These functions modify their destination argument. Therefore, they expect a modifiable char array as their destination argument.
Writing to the object	The const qualifier implies an agreement that the value of the object will not be modified. By writing to a const-qualified object, you break the agreement. The result of the operation is undefined.

Fix

The fix depends on the modification made to the const-qualified object.

Situation	Fix
Passing to mkstemp, mkostemp, mkostemps, mkdtemp, and so on.	Pass a non-const object as first argument of the function.
Passing to strcpy, strncpy, strcat, memset and so on.	Pass a non-const object as destination argument of the function.
Writing to the object	Perform the write operation on a non-const object.

See examples of fixes below.

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

• Address Results in Polyspace User Interface Through Bug Fixes or Justifications if you review results in the Polyspace user interface.

• Address Results in Polyspace Access Through Bug Fixes or Justifications (Polyspace Access) if you review results in a web browser.

• Annotate Code and Hide Known or Acceptable Results if you review results in an IDE.

Examples

Writing to const-Qualified Object

#include <string.h>

const char* buffer = "abcdeXXXXXXX";

void func(char* string) {
    char *ptr = (char*)strchr(buffer,'X');
    if(ptr)
        strcpy(ptr,string);
}

In this example, because the pointer buffer is const-qualified, strchr(buffer,'X') returns a const-qualified char* pointer. When this char* pointer is used as the destination argument of strcpy, a Writing to const qualified object error appears.

Correction — Copy const-Qualified Object to Non-const Object

One possible correction is to assign the constant string to a non-const object and use the non-const object as destination argument of strchr.

#include <string.h>

char buffer[] = "abcdeXXXXXXX";

void func(char* string) { 
    char *ptr = (char*)strchr(buffer,'X');
    if(ptr)
        strcpy(ptr,string);
}

Result Information

Group: Programming

Language: C | C++

Default: On for handwritten code, off for generated code

Command-Line Syntax: CONSTANT_OBJECT_WRITE

Impact: High

Version History

Introduced in R2015b