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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 UTF-8 string literal is the same, except prefixed by u8. A wchar_t string literal is the same, except prefixed by L. A UTF-16 string literal is the same, except prefixed by

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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 and of sufficient length to contain the character sequence and a terminating null termination charactercharacter. 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 , as because string literals are typically stored in read-only memory. (See undefined behavior 33.)

Do not attempt to modify a string literal. Use a named array of characters to obtain a modifiable string.

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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 returned value of the following library functions must be treated as a 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 modify constant objects.

Noncompliant Code Example

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

char *str  = "string literal";
str[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 str. The string stored in str can be modified safely.

char str[] = "string literal";
str[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:

#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 (POSIX)

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

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

Noncompliant Code Example (Result of strrchr())

In this noncompliant example, the char * result of the strrchr() function is used to modify the object pointed to by pathname. Because the 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;
}

Compliant Solution (Result of strrchr())

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 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;
}

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Risk Assessment

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

Automated Detection

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Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V	string-literal-modfication write-to-string-literal	Fully checked
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-STR30	Fully implemented
Compass/ROSE			Can detect simple violations of this rule
Coverity	Include Page Coverity_V Coverity_V	PW	Deprecates conversion from a string literal to "char *"
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C0556, C0752, C0753, C0754 C++3063, C++3064, C++3605, C++3606, C++3607
Klocwork	Include Page Klocwork_V Klocwork_V	CERT.STR.ARG.CONST_TO_NONCONST CERT.STR.ASSIGN.CONST_TO_NONCONST
LDRA tool suite	Include Page LDRA_V LDRA_V	157 S	Partially implemented
Parasoft C/C++test	Include Page Parasoft_V Parasoft_V	CERT_C-STR30-a CERT_C-STR30-b	A string literal shall not be modified Do not modify string literals
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	489, 1776	Partially supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rule STR30-C	Checks for writing to const qualified object (rule fully covered)
PVS-Studio	Include Page PVS-Studio_V PVS-Studio_V	V675
RuleChecker	Include Page RuleChecker_V RuleChecker_V	string-literal-modfication	Partially checked
Splint	Include Page Splint_V Splint_V
TrustInSoft Analyzer	Include Page TrustInSoft Analyzer_V TrustInSoft Analyzer_V	mem_access	Exhaustively verified (see one compliant and one non-compliant example).

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

References

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Image Added Image Added FAQ list - Question 1.32 \[[Plum 91|AA. C References#Plum 91]\] Topic 1.26, "strings - string literals"STR07-A. Use plain char for character data      07. Characters and Strings (STR)       Image Modified