strcmp.1: update docs to match utility

I could trace strcmp(1) as far back as
<293436c5ad/src/streq.c>
in my repo.

docs/strcmp.1

@@ -4,7 +4,7 @@
 .\" This work is licensed under CC BY-SA 4.0. To see a copy of this license,
 .\" visit <http://creativecommons.org/licenses/by-sa/4.0/>.
 .\"
-.TH STRCMP 1 2024-06-17 "Harakit X.X.X"
+.TH STRCMP 1 2024-07-15 "Harakit X.X.X"
 .SH NAME
 strcmp \(en compare strings
 .\"
@@ -20,15 +20,15 @@ Check whether string arguments are the same.
 .SH DIAGNOSTICS
 
 The program will exit successfully if the strings are identical. Otherwise, it
-will exit with an error code of 1 if a string passed has a lesser byte value
-than one of the prior strings:
+will exit with an error code less than 128 if a string passed has a lesser byte
+value than one of the prior strings:
 
 .RS
 strcmp b a
 .RE
 
-or with an error code of 255 if it has a greater byte value than one of the
-prior strings:
+or with an error code greater than 128 if it has a greater byte value than one
+of the prior strings:
 
 .RS
 strcmp a b

src/strcmp.c

@@ -1,24 +1,42 @@
-#include <stdio.h> /* fprintf(3), stderr */
-#include <stdlib.h> /* EXIT_FAILURE */
-#include <sysexits.h>
+/*
+ * Copyright (c) 2022–2024 DTB <trinity@trinity.moe>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU Affero General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option) any
+ * later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see https://www.gnu.org/licenses/.
+ */
 
-static char *program_name = "strcmp";
+#include <stdio.h> /* fprintf(3), stderr */
+#include <stdlib.h> /* size_t */
+#include <sysexits.h> /* EX_USAGE */
+
+static *program_name = "strcmp";
 
 int main(int argc, char *argv[]){
-	int i;
-
-	if(argc < 3){
+	if (argc < 3) {
 		fprintf(stderr, "Usage: %s string string...\n",
-			argv[0] == NULL ? program_name : argv[0]);
+			argv[0] == NULL ? program_name : argv[0]
+		);
 		return EX_USAGE;
 	}
 
-	for(; *argv[1] != '\0'; ++argv[1])
-		for(i = 2; i < argc; ++i)
-			if(*argv[i-1] > *argv[i])
-				return 1;
-			else if(*argv[i-1] < *argv[i]++)
-				return -1; /* actually 255 */
+	for (; *argv[1] != '\0'; ++argv[1]) { /* iterate chars in ref */
+		/* iterate argc */
+		for (size_t i = 2 /* ref cmp */; i < argc; ++argv[i], ++i) {
+			/* this doesn't overrun because of nul termination */
+			if (*argv[i-1] != *argv[i]) { return *argv[i-1] - *argv[i]; }
+		}
+	}
 
 	return 0;
 }