diff --git a/kyaa.h b/kyaa.h
index 5b58b9b..5efdda8 100644
--- a/kyaa.h
+++ b/kyaa.h
@@ -16,7 +16,7 @@
 #endif
 
 #define KYAA_SETUP \
-    /* dumb sanity checks */ \
+    /* sanity checks */ \
     if (argc <= 0 || argv == NULL || argv[0] == NULL) { \
         fprintf(stderr, "You've met with a terrible fate.\n"); \
         return KYAA_ERROR; \
@@ -104,15 +104,6 @@
         kyaa_flag = c; \
         kyaa_any = true; \
 
-#define KYAA_FLAG_LONG(c, name, description) \
-    KYAA_FLAG_ARG(c, name, description) \
-        errno = 0; \
-        long kyaa_flag_arg = strtol(kyaa_etc, NULL, 0); \
-        if (errno) { \
-            perror(NULL); \
-            return KYAA_ERROR; \
-        } \
-
 #define KYAA_HELP(description) \
     } \
     if (kyaa_helping) { \
diff --git a/kyaa_extra.h b/kyaa_extra.h
new file mode 100644
index 0000000..420dd67
--- /dev/null
+++ b/kyaa_extra.h
@@ -0,0 +1,204 @@
+static char *kyaa_skip_spaces(char *str) {
+    /* iterates str to first non-space character according to the C locale */
+    while (*str != '\0') {
+        switch (*str) {
+        case ' ':
+        case '\f':
+        case '\n':
+        case '\r':
+        case '\t':
+        case '\v': { str++; } break;
+        default: return str;
+        }
+    }
+    return str;
+}
+
+static const char *kyaa__base_2(char **p_str, long *p_out) {
+    char *str = *p_str;
+    long out = *p_out;
+    for (char c; (c = *str) != '\0'; str++) {
+        switch (c) {
+        case '0': case '1': {
+            long digit = (long)(c - '0');
+            if (out < (LONG_MIN + digit) / 2) {
+                return "out of range for long integer";
+            }
+            out = out * 2 - digit;
+        } break;
+
+        case '2': case '3': case '4': case '5':
+        case '6': case '7': case '8': case '9':
+        case 'A': case 'B': case 'C': case 'D': case 'E': case 'F':
+        case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
+        case '.': return "invalid character for base 2 integer";
+
+        default: goto exit;
+        }
+    }
+exit:
+    *p_str = str;
+    *p_out = out;
+    return NULL;
+}
+
+static const char *kyaa__base_8(char **p_str, long *p_out) {
+    char *str = *p_str;
+    long out = *p_out;
+    for (char c; (c = *str) != '\0'; str++) {
+        switch (c) {
+        case '0': case '1': case '2': case '3':
+        case '4': case '5': case '6': case '7': {
+            long digit = (long)(c - '0');
+            if (out < (LONG_MIN + digit) / 8) {
+                return "out of range for long integer";
+            }
+            out = out * 8 - digit;
+        } break;
+
+        case '8': case '9':
+        case 'A': case 'B': case 'C': case 'D': case 'E': case 'F':
+        case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
+        case '.': return "invalid character for base 8 integer";
+
+        default: goto exit;
+        }
+    }
+exit:
+    *p_str = str;
+    *p_out = out;
+    return NULL;
+}
+
+static const char *kyaa__base_10(char **p_str, long *p_out) {
+    char *str = *p_str;
+    long out = *p_out;
+    for (char c; (c = *str) != '\0'; str++) {
+        switch (c) {
+        case '0': case '1': case '2': case '3': case '4':
+        case '5': case '6': case '7': case '8': case '9': {
+            long digit = (long)(c - '0');
+            if (out < (LONG_MIN + digit) / 10) {
+                return "out of range for long integer";
+            }
+            out = out * 10 - digit;
+        } break;
+
+        case 'A': case 'B': case 'C': case 'D': case 'E': case 'F':
+        case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
+        case '.': return "invalid character for base 10 integer";
+
+        default: goto exit;
+        }
+    }
+exit:
+    *p_str = str;
+    *p_out = out;
+    return NULL;
+}
+
+static const char *kyaa__base_16(char **p_str, long *p_out) {
+    char *str = *p_str;
+    long out = *p_out;
+    for (char c; (c = *str) != '\0'; str++) {
+        switch (c) {
+        case '0': case '1': case '2': case '3': case '4':
+        case '5': case '6': case '7': case '8': case '9': {
+            long digit = (long)(c - '0');
+            if (out < (LONG_MIN + digit) / 16) {
+                return "out of range for long integer";
+            }
+            out = out * 16 - digit;
+        } break;
+
+        case 'A': case 'B': case 'C': case 'D': case 'E': case 'F': {
+            long digit = (long)(c - 'A') + 10;
+            if (out < (LONG_MIN + digit) / 16) {
+                return "out of range for long integer";
+            }
+            out = out * 16 - digit;
+        } break;
+
+        case 'a': case 'b': case 'c': case 'd': case 'e': case 'f': {
+            long digit = (long)(c - 'a') + 10;
+            if (out < (LONG_MIN + digit) / 16) {
+                return "out of range for long integer";
+            }
+            out = out * 16 - digit;
+        } break;
+
+        case '.': return "invalid character for base 16 integer";
+
+        default: goto exit;
+        }
+    }
+exit:
+    *p_str = str;
+    *p_out = out;
+    return NULL;
+}
+
+static const char *kyaa_str_to_long(char *str, long *p_out) {
+    /* returns error message or NULL */
+    long out = 0;
+    int base = 10;
+    bool negated = false;
+
+    str = kyaa_skip_spaces(str);
+
+    switch (*str) {
+    case '-': { negated = true; str++; } break;
+    case '+': { str++; } break;
+    default: break;
+    }
+
+    switch (*str) {
+    case '#': { base = 10; str++; } break;
+    case '$': { base =  8; str++; } break;
+    case '%': { base =  2; str++; } break;
+    case '0': { base = -1; str++; } break;
+    default: break;
+    }
+
+    if (base == -1) {
+        switch (*str) {
+        case '\0': { *p_out = 0; } return NULL;
+        case 'b': { base =  2; str++; } break;
+        case 'h': { base = 16; str++; } break;
+        case 'o': { base =  8; str++; } break;
+        case 'x': { base = 16; str++; } break;
+        default:  { base =  8; } break;
+        }
+    }
+
+    if (*str == '\0') return "no number given";
+
+    // TODO: comment on how we go towards negatives instead of positives
+    const char *err;
+    switch (base) {
+        case  2: { err = kyaa__base_2( &str, &out); } break;
+        case  8: { err = kyaa__base_8( &str, &out); } break;
+        case 10: { err = kyaa__base_10(&str, &out); } break;
+        case 16: { err = kyaa__base_16(&str, &out); } break;
+        default: return "internal error";
+    }
+    if (err != NULL) return err;
+
+    str = kyaa_skip_spaces(str);
+    if (*str != '\0') return "unexpected character";
+
+    // assuming two's complement
+    if (!negated && out == LONG_MIN) return "out of range for long integer";
+    *p_out = negated ? out : -out;
+    return NULL;
+}
+
+#define KYAA_FLAG_LONG(c, name, description) \
+    KYAA_FLAG_ARG(c, name, description) \
+        long kyaa_flag_arg; \
+        const char *err = kyaa_str_to_long(kyaa_etc, &kyaa_flag_arg); \
+        if (err) { \
+            fprintf(stderr, "%s\n", err); \
+            return KYAA_ERROR; \
+        } \
+
diff --git a/resynth.c b/resynth.c
index 70e6fa9..c454596 100644
--- a/resynth.c
+++ b/resynth.c
@@ -11,7 +11,7 @@
     If not, visit <http://gnu.org/licenses/> to obtain one.
 */
 
-#include <errno.h> // for argument parsing with strtol (kyaa.h)
+#include <limits.h>
 #include <math.h> // for log (neglog_cauchy) used in pixel diff. calculations
 #include <stdbool.h> // we're targetting C11 anyway, may as well use it
 #include <stdint.h> // for uint8_t for pixel data
@@ -47,6 +47,7 @@
 
 // for command-line argument parsing
 #include "kyaa.h"
+#include "kyaa_extra.h"
 
 // convenience macros. hopefully these names don't interfere
 // with any defined in the standard library headers on any system.