STR30-C. Do not attempt to modify string literals

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 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 example, the char pointer p is initialized to the address of the static string. Attempting to modify the string literal result 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);

Priority: P9 Level: L1

Modifying string literals can lead to abnormal program termination and results in undefined behavior that can be used in denial-of-service attacks.

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