Add format support to date(1).

utils/date.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2013 Jonas 'Sortie' Termansen.
+ * Copyright (c) 2013, 2021 Jonas 'Sortie' Termansen.
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -17,24 +17,59 @@
  * Print or set system date and time.
  */
 
-#include <errno.h>
-#include <error.h>
-#include <stdio.h>
+#include <err.h>
+#include <stdbool.h>
 #include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 #include <time.h>
 
-int main(void)
+// Note: There's no way to discern whether the format string was too long for
+// the output buffer or if the output was simply empty, so don't call this
+// function with a format that could produce an empty string. E.g. use a prefix
+// like '+' and skip it when using the string.
+static char* astrftime(const char* format, const struct tm* tm)
 {
+	size_t format_size = strlen(format) + 1;
+	size_t buffer_size = format_size;
+	while ( true )
+	{
+		char* buffer = calloc(buffer_size, 2);
+		if ( !buffer )
+			return NULL;
+		buffer_size *= 2;
+		if ( strftime(buffer, buffer_size, format, tm) )
+			return buffer;
+		free(buffer);
+	}
+}
+
+int main(int argc, char* argv[])
+{
+	const char* format = "+%a %b %e %H:%M:%S %Z %Y";
+
+	if ( 2 <= argc )
+	{
+		if ( argv[1][0] != '+' )
+			errx(1, "setting the system time is not implemented");
+		format = argv[1];
+	}
+	if ( 3 <= argc )
+		errx(1, "unexpected extra operand: %s", argv[2]);
+
 	time_t current_time = time(NULL);
 
 	struct tm tm;
 	if ( !localtime_r(&current_time, &tm) )
-		error(1, errno, "time(%ji)", (intmax_t) current_time);
-
-	const size_t BUFFER_SIZE = 256;
-	char buffer[BUFFER_SIZE];
-	strftime(buffer, BUFFER_SIZE, "%a %b %d %H:%M:%S %Y", &tm);
-	printf("%s\n", buffer);
+		err(1, "localtime_r(%ji)", (intmax_t) current_time);
 
+	char* string = astrftime(format, &tm);
+	if ( !string )
+		err(1, "malloc");
+	if ( printf("%s\n", string + 1) == EOF )
+		err(1, "stdout");
+	if ( ferror(stdout) || fflush(stdout) == EOF )
+		err(1, "stdout");
 	return 0;
 }