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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 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 because string literals are typically stored in read-only memory. See also undefined behavior 30 33  of Annex J of C99C11.

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

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char *p  = "string literal";
p[0] = 'S';

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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 rule STR36-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. The string stored in a can be safely modified.

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

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char *fname;
fname = mkstemp("/tmp/edXXXXXX");

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static char fname[] = "/tmp/edXXXXXX";
mkstemp(fname);

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const char* get_dirname(const char* pathname) {
  char* slash;
  slash = strrchr(pathname, '/');
  if (slash)
    *slash = '\0'; /* undefined behavior */
  return pathname;
}

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

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A 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 callers of get_dirname() passing constant objects to the function, the argument is declared to be a non-const pointer. While converting a string literal to non-const char* is permitted by the language, conforming compilers could issue a diagnostic for such code. See also recommendation EXP05-C. Do not cast away a const qualification.

char* get_dirname(char* pathname) {
  char* slash;
  slash = strrchr(pathname, '/');
  if (slash)
    *slash = '\0';
  return pathname;
}

int main() {
  char pathname[] = __FILE__;
  /* calling get_dirname(__FILE__) may be diagnosed */
  puts(get_dirname(pathname));
  return 0;
}

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Related Vulnerabilities

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

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CERT C++ Secure Coding Standard: STR30-CPP. Do not attempt to modify string literals

ISO/IEC 9899:19992011 Section 6.4.5, "String literals"

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

Bibliography

[Summit 1995] comp.lang.c FAQ list - , Question 1.32
[Plum 1991] Topic 1.26, "strings - string literals"

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