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According to the C Standard, 6.4.5, paragraph 3 [ISO/IEC 9899:2024]:

A character string literal is a sequence of zero or more multibyte characters enclosed in double-quotes

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, as in "xyz"

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. A

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UTF-8 string literal is the same, except prefixed by

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u8. A wchar_t string literal is the same, except prefixed by 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 -termination character. 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 string literals but 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 also undefined behavior 33  of Annex J of the C standard [ISO/IEC 9899:2011]. undefined behavior 32.)

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, a pointer to (or array of) const characters must be treated as a string literal. Similarly, the The returned value of the following library functions must be treated as a string literal if the first argument is a string literal:

• strpbrk(),

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•  strchr(), strrchr(), strstr()
• wcspbrk(),

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•  wcschr(), wcsrchr(),

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• wcsstr()
• memchr

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• (), wmemchr()

This rule is a specific instance of EXP40-C. Do not modify constant objects and pointer to (or array of) const characters shall be treated as a string literal.Do not attempt to modify a string literal. Use a named array of characters to obtain a modifiable string.

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)

In this noncompliant code example, a string literal is passed to the (pointer to non-const) parameter of the POSIX function mkstemp(), which then modifies the characters of the string literal.:

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

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

Compliant Solution

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(POSIX)

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

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

Noncompliant Code Example (Result of strrchr())

In this noncompliant example, the non-const 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 and are likely to cause a signal, such as SIGSEGV, to be generated for the process if the object is stored in read-only memory.

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

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

Compliant Solution (Result of strrchr())

A compliant 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() passing constant objects to the function, the argument is declared to be a non-const pointer. Although converting a string literal to non-const char* is permitted by the language, conforming compilers could issue a diagnostic for such code. See also EXP05-C. Do not cast away a const qualification, 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 * pathnamepathname, 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,  *slashslash_idx);
      dirname[slash_idx] = '\0';      
      return pathname dirname;
    }
  }
  return 0;
}
 
int main(void) {
   char pathnamedirname[] = __FILE__260];
  if /* calling (get_dirname(__FILE__), may be diagnosed */
  puts(get_dirname(pathname));
 dirname, sizeof(dirname))) {
    puts(dirname);
  }
  return 0;
}

Risk Assessment

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)

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Bibliography

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ISO/IEC TR 17961 (Draft) Modifying string literals [strmod]

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