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According to the C Standard, subclause 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 literalsliteral. Modifying a string literal frequently results in an access violation because string literals are typically stored in read-only memory. (See also undefined behavior 33  of Annex J of the C Standard [ISO/IEC 9899:2011].

String literals are usually referred to via a pointer to, or array of, const characters.

.)

Avoid assigning a string literal to a pointer to non-const or casting a string literal to a pointer to non-const. For the purposes of this rule, When called with a pointer to (or array of) const characters must be treated as a string literal. Similarly, the return returned value from of the following library functions shall must be treated as a pointer to const charactersa string literal if the first argument is a string literal:

• strpbrk(), strchr(), strrchr(), strstr()
• wcspbrk(), wcschr(), wcsrchr(), wcsstr()
• memchr(), wmemchr()

This rule is a specific instance of EXP40-C. Do not attempt to modify a string literal. Instead, use a named array of characters to create a modifiable copy of a string literalmodify constant objects.

Noncompliant Code Example

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 results in 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 STR36 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 modified.

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

Noncompliant Code Example (POSIX)

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The behavior of mkstemp() is described in more detail in FIO43 FIO21-C. Do not create temporary files in shared directories.

Compliant Solution (POSIX)

Instead This compliant solution uses a named array instead of passing a string literal, use a named array:

#include <stdlib.h>
 
void func(void) {
  static char fname[] = "/tmp/edXXXXXX";
  mkstemp(fname);
}

...

In this noncompliant example, the char * result of the strrchr() function is used to modify the object pointed to by pathname. Because the pointer argument to strrchr() points to a string literal, the effects of the modification are undefined.

#include <stdio.h>
#include <string.h>
 
const char *get_dirname(const char *pathname) {
  char *slash;
  slash = strrchr(pathname, '/');
  if (slash) {
    *slash = '\0'; /* Undefined behavior */
  }
  return pathname;
}

int main(void) {
  puts(get_dirname(__FILE__));
  return 0;
}

...

This compliant solution avoids modifying a const object, even if it is possible to obtain a non-const pointer to such an object by calling a standard C library function, such as strrchr(). To reduce the risk of to callers of get_dirname(), a buffer and length for the directory name are passed into the function.   It is insufficient to change pathname to require a char * instead of a const char * because conforming compilers are not required to diagnose passing a string literal to a function accepting a char *.

#include <stddef.h>
#include <stdio.h>
#include <string.h>
 
char *get_dirname(const char *pathname, char *dirname, size_t size) {
  const char *slash;
  slash = strrchr(pathname, '/');
  if (slash) {
    ptrdiff_t slash_idx = slash - pathname;
    if ((size_t)slash_idx <= size) {
      memcpy(dirname, pathname, slash_idx);
      dirname[slash_idx] = '\0';      
      return dirname;
    }
  }
  return 0;
}
 
int main(void) {
  char dirname[260];
  if (get_dirname(__FILE__, dirname, sizeof(dirname))) {
    puts(dirname);
  }
  return 0;
}

...

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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