diff --git a/include/string.h b/include/string.h
index 0fbb790..fe8445b 100644
--- a/include/string.h
+++ b/include/string.h
@@ -12,3 +12,4 @@ void* memset(void* dest, int value, size_t n);
 char* strncpy(char* restrict dst, const char* restrict src, size_t len);
 int strcmp(const char* str1, const char* str2);
 int strncmp(const char* str1, const char* str2, size_t len);
+char* strchr(const char* s, int c);
diff --git a/lib/print.c b/lib/print.c
index 4565965..089a47c 100644
--- a/lib/print.c
+++ b/lib/print.c
@@ -234,6 +234,12 @@ check_fmt:
                 lutoa(va_arg(args, uint64_t), number, 16);
                 break;
             }
+            if (*fmt == 'X') {
+                for (char* c = number; *c; c++) {
+                    if (*c >= 'a' && *c <= 'f')
+                        *c -= 'a' - 'A';
+                }
+            }
 print_num:
             const char* digits = number;
             char sign = 0;
@@ -260,7 +266,7 @@ print_num:
             if (sign)
                 callback(ctx, &sign, 1);
             if (write_prefix)
-                callback(ctx, "0x", 2);
+                callback(ctx, *fmt == 'X' ? "0X" : "0x", 2);
             print_pad(callback, ctx, '0', zeros);
             callback(ctx, digits, len);
             if (left_align)
@@ -285,7 +291,7 @@ print_num:
             callback(ctx, number, len);
 
             fmt++;
-            written += len;
+            written += offset + len;
             break;
         }
         // The number of characters written so far is stored in the pointed location.
diff --git a/lib/string.c b/lib/string.c
index d049058..0a7dd9f 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -5,6 +5,9 @@
 
 [[__hot]]
 size_t strlen(const char* str) {
+    if (!str)
+        return 0;
+
     size_t result = 0;
     while (*str++) {
         result++;
@@ -14,6 +17,9 @@ size_t strlen(const char* str) {
 
 [[__hot]]
 size_t strnlen(const char* str, size_t len) {
+    if (!str || len == 0)
+        return 0;
+
     size_t result = 0;
     while (result < len && *str++) {
         result++;