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According to the C Standard, 6.4.5, paragraph 3 [ISO/IEC 9899:20112024]:

A A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes, as in "xyz". A UTF−8 UTF-8 string literal is the same, except prefixed by u8. A wide wchar_t string literal is the same, except prefixed by the letter L, u, or U.L. A UTF-16 string literal is the same, except prefixed by u. A UTF-32 string literal is the same, except prefixed by U. Collectively, wchar_t, UTF-16, and UTF-32 string literals are called wide string literals.

At compile time, string literals are used to create an array of static storage duration of sufficient length to contain the character sequence and a terminating null character. String literals are usually referred to by a pointer to (or array of) characters. Ideally, they should be assigned only to pointers to (or arrays of) const char or const wchar_t. It is unspecified whether these arrays of string literals are distinct from each other. The behavior is undefined if a program attempts to modify any portion of a string literal. Modifying a string literal frequently results in an access violation because string literals are typically stored in read-only memory. (See undefined behavior 33.)

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In this noncompliant code example, the char pointer p is str is initialized to the address of a string literal. Attempting to modify the string literal is undefined behavior:

char *pstr  = "string literal";
pstr[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. (See STR11-C. Do not specify the bound of a character array initialized with a string literal.) This code creates a copy of the string literal in the space allocated to the character array a str. The string stored in a can str can be modified safely.

char astr[] = "string literal";
astr[0] = 'S';

Noncompliant Code Example (POSIX)

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Modifying string literals can lead to abnormal program termination and possibly denial-of-service attacks.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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