[spyrja]

More than a few C library functions do that kind of thing. Like `strstr`, which takes const strings as arguments but returns a readily modifiable pointer to char. Const-correctness just wasn't on the top of the list when they standardized this stuff, I guess. (Heck, back in those days, most PROGRAMS for that matter weren't written with much care for it.)

[wahern]

It's a consequence of the peculiarity of C type semantics, which disallows implicit conversions of pointer-to-pointer to pointer-to-pointer-to-const. C23 6.5.16.1 EXAMPLE 3 explains why:

  const char **cpp; char *p;
  const char c = ’A’;
  cpp = &p;   // constraint violation
  *cpp = &c;  // valid
  *p = 0;     // valid

  The first assignment is unsafe because it would allow the 
  following valid code to attempt to change the value of the 
  const object c.

There are proposals on the table for C2y to redefine various APIs, including strtol, strchr, memcpy, etc, to preserve const correctness. Implementations might make use of _Generic (there are some issues there, though), newly specified language features, or possibly use internal extensions not available in the language proper, to accomplish this.