ISO/IEC TS 17961 [strmod]

Modifying string literals

Description

Rule Definition

Modifying string literals.¹

Polyspace Implementation

This checker checks for Writing to const qualified object.

Examples

Writing to const qualified object

Issue

Writing to const qualified object 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.

Example - 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 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);
}

Check Information

Decidability: Undecidable

Version History

Introduced in R2019a