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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 33  of Annex J of the C Standard [ISO/IEC 9899:2011].

The returned value of the library functions strpbrk(), strchr(), strrchr(), wcspbrk(), wcschr(), and wcsrchr() and pointer to (or array of) const characters shall be treated as a string literal.

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#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() {
  puts(get_dirname(__FILE__));
  return 0;
}

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#include <stdio.h>
#include <string.h>
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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Bibliography

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