diff --git a/Makefile b/Makefile
index 683a306..ec4156c 100644
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,7 @@ SOURCE.d  := source
 OBJECT.d  := object
 LIB.d     := library/
 
-SOURCE    := main.c
+SOURCE    := main.c cli.c
 OBJECT    := $(addprefix ${OBJECT.d}/,${SOURCE})
 OBJECT    := ${OBJECT:.c=.o}
 
diff --git a/source/cli.c b/source/cli.c
new file mode 100644
index 0000000..365a808
--- /dev/null
+++ b/source/cli.c
@@ -0,0 +1,71 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <getopt.h>
+
+extern void yyerror(const char * const fmt, ...);
+
+char * output_file_name = NULL;
+char * input_file_name  = NULL;
+
+static
+const char * help_message = "\
+tbsp [options] <file>  : convert tbsp source file to C/C++ source file\n\
+    -h          : show help\n\
+    -o <string> : specify output file\
+";
+
+static
+char * default_output_name(const char * const name) {
+    static const char default_extension[] = ".tb.c";
+    char * r;
+
+    const int len = strlen(name);
+
+    int i = len;
+    for (; i != 0; i--) {
+        if (name[i] == '.') {
+            break;
+        }
+    }
+
+    if (i == 0) {
+        i = len;
+    }
+
+    r = (char *)malloc(i + sizeof(default_extension));
+    memcpy(r, name, i);
+    memcpy(r + i, default_extension, sizeof(default_extension));
+
+    return r;
+}
+
+int handle_arguments(const int argc, const char * const * const argv) {
+    if (argc < 2) {
+        puts(help_message);
+        return 1;
+    }
+
+    int opt;
+    while ((opt = getopt(argc, (char * const *)argv, "ho:")) != -1) {
+        switch (opt) {
+            case 'h': {
+                puts(help_message);
+            } exit(0);
+            case 'o': {
+                output_file_name = optarg;
+            } break;
+            default: {
+                yyerror("unknown option '%s'", argv[optind]);
+            } return 1;
+        }
+    }
+
+    input_file_name = strdup(argv[argc-1]);
+
+    if (!output_file_name) {
+        output_file_name = default_output_name(input_file_name);
+    }
+
+    return 0;
+}
diff --git a/source/cli.h b/source/cli.h
new file mode 100644
index 0000000..5dd9c8f
--- /dev/null
+++ b/source/cli.h
@@ -0,0 +1,7 @@
+#ifndef CLI_H
+#define CLI_H
+extern char * const output_file_name;
+extern char * const input_file_name;
+
+int handle_arguments(const int argc, const char * const * const argv);
+#endif
diff --git a/source/main.c b/source/main.c
index 24d6214..2002b1c 100644
--- a/source/main.c
+++ b/source/main.c
@@ -1,12 +1,23 @@
 #define _GNU_SOURCE
 #include <stdio.h>
+#include <stdarg.h>
 
 #include "tbsp.yy.h"
 #include "tbsp.tab.h"
 
+#include "cli.h"
+
 // XXX i am so desperate for #embed, you would not believe
 #include "TBSP_strings.inc"
 
+#define CHECKED_FOPEN(target, filename, mode) do {\
+    target = fopen(filename, mode);\
+    if (!target) {\
+        yyerror("failed to open '%s'", filename);\
+        return 1;\
+    }\
+} while (0)
+
 extern int tbsp_yy_init(void);
 extern int tbsp_yy_deinit(void);
 
@@ -26,6 +37,7 @@ void yyerror(const char * const fmt, ...) {
     va_end(args);
 }
 
+static
 void put_rule_table(const char * const name, rule_type_t type_mask) {
     char * sprint_buffer;
     int sprint_r;
@@ -47,32 +59,8 @@ void put_rule_table(const char * const name, rule_type_t type_mask) {
     fputs("};\n\n", yyout);
 }
 
-signed main(const int argc, const char * const * const argv) {
-  #ifdef DEBUG
-    yydebug = 1;
-  #endif
-
-    if (argc < 2) {
-        printf("%s <file>", argv[0]);
-    }
-
-    tbsp_yy_init();
-    tbsp_tab_init();
-
-    yyin = fopen(argv[1], "r");
-    if (!yyin) {
-        puts("Failed to open file");
-        return 1;
-    }
-
-    //yyout = fopen("tbsp.c", "w");
-    yyout = stdout;
-
-    int yyparse_r = yyparse();
-    if (yyparse_r) {
-        return 1;
-    }
-
+static
+void put_output(void) {
     char * sprint_buffer;
     int sprint_r;
     (void)sprint_r;
@@ -108,6 +96,25 @@ signed main(const int argc, const char * const * const argv) {
 
     // Code section
     fputs(verbatim, yyout);
+}
+
+signed main(const int argc, const char * const * const argv) {
+  #ifdef DEBUG
+    yydebug = 1;
+  #endif
+
+    if (handle_arguments(argc, argv)) { return 1; }
+
+    tbsp_yy_init();
+    tbsp_tab_init();
+
+    CHECKED_FOPEN(yyin,  input_file_name,  "r");
+    CHECKED_FOPEN(yyout, output_file_name, "w");
+
+    int yyparse_r = yyparse();
+    if (yyparse_r) { return yyparse_r; }
+
+    put_output();
 
     // Deinit
     for (int i = 0; i < kv_size(rules); i++) {
@@ -116,6 +123,8 @@ signed main(const int argc, const char * const * const argv) {
     }
 
     tbsp_yy_deinit();
+    free(output_file_name);
+    free(input_file_name);
     free(verbatim);
     free(language);
     free(top);