$ cat parse256.c
  #include <stdio.h>
  #include <stdint.h>
  #include <string.h>

  int parse_uint8_fastswar(const char *str, size_t len, uint8_t *num) {
    union {
      uint8_t as_str[4];
      uint32_t as_int;
    } digits;
    memcpy(&digits.as_int, str, sizeof(digits));
    digits.as_int ^= 0x30303030lu;
    digits.as_int <<= (4 - (len & 0x3)) * 8;
    uint32_t y = ((UINT64_C(0x640a0100) * digits.as_int)>>32)&0xff;
    *num = (uint8_t)(y);
    return (digits.as_int & 0xf0f0f0f0) == 0 && y < 256 && len != 0
            && len < 4;
  }

  int main(int argc, char **argv)
  {
    if (argv[1]) {
      uint8_t val = 0;
      if (parse_uint8_fastswar(argv[1], strlen(argv[1]), &val)) {
        printf("value of %s is %d\n", argv[1], (int) val);
      } else {
        printf("%s is invalid, value stored %d\n", argv[1], (int) val);
      }
    }

    return 0;
  }
  $ ./parse256 123
  123 is invalid, value stored 225
  $ uname -a
  Linux gcc1-power7.osuosl.org 3.10.0-862.14.4.el7.ppc64 #1 SMP Wed Sep 26 20:38:32 GMT 2018 ppc64 ppc64 ppc64 GNU/Linux

Oops!