Parse the monitoring side setting from "--monitor" (#67).

src/include/options.h

@@ -36,6 +36,15 @@ typedef enum {
 	PV_ACTION_MONITOR		/* run a process, watch its stdin/out */
 } pvaction_t;
 
+/*
+ * Sides of a monitored command to monitor with PV_ACTION_MONITOR.
+ */
+typedef enum {
+	PV_SIDE_IN,			/* monitor only the input side */
+	PV_SIDE_OUT,			/* monitor only the output side */
+	PV_SIDE_BOTH			/* monitor both sides */
+} pvside_t;
+
 /*
  * Structure describing run-time options.
  *
@@ -71,6 +80,7 @@ struct opts_s {
 	unsigned int watchfd_count;	       /* number of watchfd items */
 	unsigned int watchfd_length;	       /* allocated array size */
 	pvaction_t action;	       /* the program action to perform */
+	pvside_t side;		       /* which side of the monitored command to monitor */
 	bool progress;                 /* progress bar flag */
 	bool timer;                    /* timer flag */
 	bool eta;                      /* ETA flag */

src/main/options.c

@@ -416,6 +416,27 @@ static bool opts_watchfd_parse(opts_t opts, const char *argument, /*@null@ */ co
 }
 
 
+/*
+ * Return true if the first string is at least as long as the second, and
+ * its start matches the entirety of the second.  Both strings must be
+ * null-terminated.
+ */
+static bool string_starts_with(const char *string, const char *match)
+{
+	size_t string_length, match_length;
+
+	string_length = strlen(string);	    /* flawfinder: ignore */
+	match_length = strlen(match);	    /* flawfinder: ignore */
+	/* flawfinder - these are null-terminated strings. */
+
+	if (string_length < match_length)
+		return false;
+	if (0 == strncmp(string, match, match_length))
+		return true;
+	return false;
+}
+
+
 #ifdef HAVE_NFTW
 /*
  * Callback function for nftw() to add the size of the given file to the
@@ -1107,7 +1128,30 @@ opts_t opts_parse(unsigned int argc, char **argv)
 			break;
 		case 'M':
 			opts->action = PV_ACTION_MONITOR;
-			/* TODO: parse optarg to set the side */
+			if (string_starts_with(optarg, "0")) {
+				opts->side = PV_SIDE_IN;
+			} else if (string_starts_with(optarg, "in")) {
+				opts->side = PV_SIDE_IN;
+			} else if (string_starts_with(optarg, "stdin")) {
+				opts->side = PV_SIDE_IN;
+			} else if (string_starts_with(optarg, "1")) {
+				opts->side = PV_SIDE_OUT;
+			} else if (string_starts_with(optarg, "out")) {
+				opts->side = PV_SIDE_OUT;
+			} else if (string_starts_with(optarg, "stdout")) {
+				opts->side = PV_SIDE_OUT;
+			} else if (string_starts_with(optarg, "2")) {
+				opts->side = PV_SIDE_BOTH;
+			} else if (string_starts_with(optarg, "both")) {
+				opts->side = PV_SIDE_BOTH;
+			} else {
+				/*@-mustfreefresh@ *//* see above */
+				fprintf(stderr, "%s: -M: %s: %s\n",
+					opts->program_name, optarg, _("invalid side specification"));
+				opts_free(opts);
+				return NULL;
+				/*@+mustfreefresh@ */
+			}
 			break;
 #ifdef ENABLE_DEBUGGING
 		case '!':