A string literal is a sequence of zero or more multibyte characters enclosed in double quotes ("xyz", for example). A wide string literal is the same, except prefixed by the letter L (L"xyz", for example).
At compile time, string literals are used to create an array of static storage duration and sufficient length to contain the character sequence and a null-termination character. It is unspecified whether these arrays are distinct. The behavior is undefined if a program attempts to modify string literals but frequently results in an access violation, as string literals are typically stored in read-only memory.
Do not attempt to modify a string literal. Use a named array of characters to obtain a modifiable string.
Non-Compliant Code Example
In this non-compliant code example, the char pointer p is initialized to the address of a string literal. Attempting to modify the string literal results in undefined behavior.
char *p = "string literal"; p[0] = 'S';
Compliant Solution
As an array initializer, a string literal specifies the initial values of characters in an array (as well as the size of the array). This code creates a copy of the string literal in the space allocated to the character array a. The string stored in a can be safely modified.
char a[] = "string literal"; a[0] = 'S';
Non-Compliant Code Example
In this non-compliant example, the mktemp() function modifies its string argument.
mktemp("/tmp/edXXXXXX");
Compliant Solution
Instead of passing a string literal, use a named array:
static char fname[] = "/tmp/edXXXXXX"; mktemp(fname);
Risk Assessment
Modifying string literals can lead to abnormal program termination and possibly denial-of-service attacks.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
STR30-C |
1 (low) |
3 (likely) |
3 (low) |
P9 |
L2 |
Automated Detection
The LDRA tool suite V 7.6.0 is able to detect violations of this rule.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[ISO/IEC 9899-1999]] Section 6.4.5, "String literals"
[[Summit 95]] comp.lang.c FAQ list - Question 1.32
[[Plum 91]] Topic 1.26, "strings - string literals"
STR06-A. Do not assume that strtok() leaves the parse string unchanged 07. Characters and Strings (STR)