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

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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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 = mkstemp("/tmp/edXXXXXX");

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Instead of passing a string literal, use a named array:

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

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In this noncompliant example, the non-const char* result of the strrchr() function is used to modify the object pointed to by pathname. Since the pointer 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.

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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