sort: Implement -c and -C flags

sort.1

@@ -15,9 +15,20 @@ writes the sorted concatenation of the given files to stdout.  If no file is
 given, sort reads from stdin.
 .SH OPTIONS
 .TP
+.B \-C
+check that the concatenation of the given files is sorted rather than sorting
+them.  In this mode, no output is printed to stdout, and the exit status
+indicates the result of the check.
+.TP
 .B \-b
 skip leading whitespace of columns when sorting.
 .TP
+.B \-c
+the same as
+.B \-C
+except that when disorder is detected, a message is printed to stderr
+indicating the location of the disorder.
+.TP
 .BI \-k \ key
 specifies a key definition of the form
 .BR S [. s ][ f ][, E [. e ][ f ]]

sort.c

@@ -32,6 +32,7 @@ static struct kdlist *head = NULL;
 static struct kdlist *tail = NULL;
 
 static void addkeydef(char *, int);
+static void check(FILE *);
 static void freelist(void);
 static int linecmp(const char **, const char **);
 static char *skipblank(char *);
@@ -40,13 +41,13 @@ static int parse_keydef(struct keydef *, char *, int);
 static char *nextcol(char *);
 static char *columns(char *, const struct keydef *);
 
-static int uflag = 0;
+static int Cflag = 0, cflag = 0, uflag = 0;
 static char *fieldsep = NULL;
 
 static void
 usage(void)
 {
-	enprintf(2, "usage: %s [-bnru] [-t delim] [-k def]... [file...]\n", argv0);
+	enprintf(2, "usage: %s [-Cbcnru] [-t delim] [-k def]... [file...]\n", argv0);
 }
 
 int
@@ -58,9 +59,15 @@ main(int argc, char *argv[])
 	int global_flags = 0;
 
 	ARGBEGIN {
+	case 'C':
+		Cflag = 1;
+		break;
 	case 'b':
 		global_flags |= MOD_STARTB | MOD_ENDB;
 		break;
+	case 'c':
+		cflag = 1;
+		break;
 	case 'k':
 		addkeydef(EARGF(usage()), global_flags);
 		break;
@@ -87,15 +94,25 @@ main(int argc, char *argv[])
 	addkeydef("1", global_flags & MOD_R);
 
 	if (argc == 0) {
+		if (Cflag || cflag) {
+			check(stdin);
+		} else {
 			getlines(stdin, &linebuf);
+		}
 	} else for (; argc > 0; argc--, argv++) {
 		if (!(fp = fopen(argv[0], "r"))) {
 			enprintf(2, "fopen %s:", argv[0]);
 			continue;
 		}
+		if (Cflag || cflag) {
+			check(fp);
+		} else {
 			getlines(fp, &linebuf);
+		}
 		fclose(fp);
 	}
+
+	if (!Cflag && !cflag) {
 		qsort(linebuf.lines, linebuf.nlines, sizeof *linebuf.lines,
 				(int (*)(const void *, const void *))linecmp);
 
@@ -105,6 +122,7 @@ main(int argc, char *argv[])
 				fputs(linebuf.lines[i], stdout);
 			}
 		}
+	}
 
 	freelist();
 	return 0;
@@ -128,6 +146,25 @@ addkeydef(char *def, int flags)
 	tail = node;
 }
 
+static void
+check(FILE *fp)
+{
+	static struct { char *buf; size_t size; } prev, cur, tmp;
+
+	if (!prev.buf)
+		getline(&prev.buf, &prev.size, fp);
+	while (getline(&cur.buf, &cur.size, fp) != -1) {
+		if (uflag > linecmp((const char **) &cur.buf, (const char **) &prev.buf)) {
+			if (!Cflag)
+				weprintf("disorder: %s", cur.buf);
+			exit(1);
+		}
+		tmp = cur;
+		cur = prev;
+		prev = tmp;
+	}
+}
+
 static void
 freelist(void)
 {