CERT C: Rec. STR06-C

Do not assume that strtok() leaves the parse string unchanged

Since R2025a

Description

Rule Definition

Do not assume that strtok() leaves the parse string unchanged¹

Polyspace Implementation

Polyspace^® checks for the issue String passed to strtok() without copying.

Examples

String passed to strtok() without copying

Issue

This issue occurs if these conditions are true:

• A C-string (char*) is not the destination argument of strcpy() or memcpy().

• The same C-string is passed to the function strtok() as the string to be parsed.

That is, this issue occurs if the string that strtok() parses is not a copied string.

Risk

The function strtok() tokenizes the string to be parsed in-place by using the delimiter parameter. For example, in this code, the string path is initialized to "/usr/bin:/bin:/usr/sbin:/sbin". After the call to strtok(), the string path becomes "/usr/bin":

   char path* = "/usr/bin:/bin:/usr/sbin:/sbin";
   strtok(path,":");

This modification to the string path can be unexpected. If you use the string in later code without accounting for the modification, the code behavior can be incorrect and unexpected.

Fix

To fix violations of this rule, copy the strings to be parsed and pass the copied strings to the function strtok().

Example

In this example, the string str is passed to strtok() as the string to be parsed without copying the string beforehand. Polyspace reports a violation.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

void foo()
{
	char *token;
	char *str = "apple,banana,orange,grape";
	token = strtok(str, ",");  //noncompliant
	puts(token);


}

Correction

To correct this violation, copy the string str to preserve the original value and pass the copy to strtok().

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

void foo() {
    char *token;
    const char *str = "apple,banana,orange,grape";

    // Allocate memory for the copy of the string
    char *str_copy = malloc(strlen(str) + 1);
    if (str_copy == NULL) {
        perror("Failed to allocate memory");
        exit(EXIT_FAILURE);
    }

    // Copy the string literal to the allocated memory
    strcpy(str_copy, str);

    // Use strtok on the copied string
    token = strtok(str_copy, ","); //Compliant
    puts(token);

    // Free the allocated memory
    free(str_copy);
}

Check Information

Group: Rec. 07. Characters and Strings (STR)

Version History

Introduced in R2025a