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Why 45 in char str[ 45 ];

bool overflow = false;
int digit = *us;
const unsigned char *start = us;
while (digit >= '0' && digit <= '9') { // or isdigit()
  // Will appending this digit overflow the result?
  if (val > INT128_MIN_DIV10 || (val >= INT128_MIN_DIV10 && digit <= -INT128_MIN_MOD10)) {
    val = val * 10 - digit;
  } else {
    overflow = true;
    val = INT128_MIN;
  }
  digit = *us++;
}

...

if (!neg) {
  if (val < -INT128_MAX) {
    overflow = true;
    val = INT128_MAX);
  }
  else {
    val = -val;
  }
}

I know, early on, it seems all that extra ornamentation makes for for code-bloat. In my experience a function that is robust in error handling is well worth the effort. Further, 128-bit values are optically long and can be far more useful in hex and/or with separators.

Why 45 in char str[ 45 ];?

bool overflow = false;
int digit = *us;
const unsigned char *start = us;
while (digit >= '0' && digit <= '9') { // or isdigit()
  // Will appending this digit overflow the result?
  if (val > INT128_MIN_DIV10 || (val >= INT128_MIN_DIV10 && digit <= -INT128_MIN_MOD10)) {
    val = val * 10 - digit;
  } else {
    overflow = true;
    val = INT128_MIN;
  }
  digit = *us++;
}

...

if (!neg) {
  if (val < -INT128_MAX) {
    overflow = true;
    val = INT128_MAX;
  }
  else {
    val = -val;
  }
}

I know, early on, it seems all that extra ornamentation makes for code-bloat. In my experience a function that is robust in error handling is well worth the effort. Further, 128-bit values are optically long and can be far more useful in hex and/or with separators.

Documentation tip

Even when a function like strtol() has short comings, adding comments that say this int128_t to/from string function works:

just like strtol(), but for int128_t

conveys a lot of information to a user of your new function, making it more easy to use and reduces cognitive effort. Of course it then needs to perform jut like that strtol() too.

Good luck.

my_itoa(size_t n, char *s, int i, int base) which can serfe as a basis for my_int128toa(size_t n, char *s, __int128_t i, int base).

my_itoa(size_t n, char *s, int i, int base) which can serve as a basis for my_int128toa(size_t n, char *s, __int128_t i, int base).

In general, int128 to/from string

Rather than re-write similar code, consider making more full-featured functions that handle other bases and perform robust error checking.

I know, early on, it seems all that extra ornamentation makes for for code-bloat. In my experience a function that is robust in error handling is well worth the effort. Further, 128-bit values are optically long and can be far more useful in hex and/or with separators.

Good engineering programming is about balance. Code what your need that functionally works for all input, include modest error checking, add a tad more functionality if simple to do so, but then pause and wait for a driving need to expanded functionality/performance.