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

char* substr(const char* src, size_t start, size_t len) {
   while (start && *src) {
      ++start;
      ++src;
   }

   char* dst = malloc(len+1);
   if (!dst)
      return NULL;

   dst[len] = 0;
   return strncpy(dst, src, len);
}

int main() {
   const char* s = "\x90";
   char* first = substr(s, 0, 1);
   printf("%02X\n", first);
   free(first);
   return 0;
}

##</code><code>##

$ gcc -Wall -Wextra -pedantic -std=c99 a.c -o a && a
a.c: In function ‘main’:
a.c:22:4: warning: format ‘%X’ expects argument of type ‘unsigned int’, but argument 2 has type ‘char *’ [-Wformat=]
    printf("%02X\n", first);
    ^
1E25970