In discard mode, if splice() is available, open a file descriptor to /dev/null and splice the input to that, to improve performance (#191).

docs/pv.1

@@ -356,7 +356,6 @@ seekable, this option will have no effect other than to turn on
 Instead of transferring input data to standard output, discard it.
 This is equivalent to redirecting standard output to \fI/dev/null\fR,
 except that \fBwrite\fR(2) is never called.
-Implies \*(lq\fB\-\-no\-splice\fR\*(rq.
 .TP
 .BI \-U\  FILE \fR,\ \fB\-\-store\-and\-forward\  FILE
 Instead of passing data through immediately, do it in two stages - first

docs/pv.1.md

@@ -369,8 +369,7 @@ are explicitly switched on will be shown.
 
 :   Instead of transferring input data to standard output, discard it.
     This is equivalent to redirecting standard output to */dev/null*,
-    except that **write**(2) is never called. Implies
-    "**\--no-splice**".
+    except that **write**(2) is never called.
 
 **-U FILE, \--store-and-forward FILE**
 

src/include/pv-internal.h

@@ -467,6 +467,8 @@ struct pvstate_s {
 		int intermediate_pipe[2];
 		int intermediate_pipe_buffer_size;
 		int intermediate_pipe_buffer_used;
+		/* File descriptor to /dev/null for splicing with -X. */
+		int discard_fd;
 		/*
 		 * These variables are used to keep track of whether
 		 * splice() was used; splice_failed_fd is the file

src/main/options.c

@@ -1084,7 +1084,6 @@ opts_t opts_parse(unsigned int argc, char **argv)
 			break;
 		case 'X':
 			opts->discard_input = true;
-			opts->no_splice = true;
 			break;
 		case 'U':
 			opts->store_and_forward_file = pv_strdup(optarg);

src/pv/file.c

@@ -462,12 +462,26 @@ int pv_next_file(pvstate_t state, unsigned int filenum, int oldfd)
 
 				state->transfer.intermediate_pipe_buffer_size = new_size;
 			}
-#endif
+#endif				/* defined F_SETPIPE_SZ && defined F_GETPIPE_SZ */
 		}
 		debug("%s: [%d,%d]", "intermediate pipe fds", state->transfer.intermediate_pipe[0],
 		      state->transfer.intermediate_pipe[1]);
 		debug("%s: %d", "intermediate pipe buffer size", state->transfer.intermediate_pipe_buffer_size);
 	}
+
+	if (state->control.discard_input && !state->control.no_splice && state->transfer.discard_fd < 0) {
+		/*
+		 * Open a file descriptor to /dev/null, so that input can be
+		 * spliced to it to implement -X.
+		 */
+		state->transfer.discard_fd = open("/dev/null", O_WRONLY);	/* flawfinder: ignore */
+		/* flawfinder: /dev/null is trusted. */
+		if (state->transfer.discard_fd < 0) {
+			pv_perror("%s", "/dev/null");
+			(void) close(fd);
+			fd = -1;
+		}
+	}
 #endif				/* HAVE_SPLICE */
 
 	return fd;

src/pv/state.c

@@ -133,6 +133,9 @@ void pv_reset_transfer(pvtransferstate_t transfer)
 	transfer->last_read_skip_fd = 0;
 #ifdef HAVE_SPLICE
 	transfer->splice_failed_fd = -1;
+	transfer->intermediate_pipe[0] = -1;
+	transfer->intermediate_pipe[1] = -1;
+	transfer->discard_fd = -1;
 #endif				/* HAVE_SPLICE */
 
 	transfer->line_positions_length = 0;
@@ -208,11 +211,6 @@ pvstate_t pv_state_alloc(void)
 #endif				/* HAVE_IPC */
 	state->cursor.lock_fd = -1;
 
-#ifdef HAVE_SPLICE
-	state->transfer.intermediate_pipe[0] = -1;
-	state->transfer.intermediate_pipe[1] = -1;
-#endif				/* HAVE_SPLICE */
-
 	pv_state_reset(state);
 
 #ifdef HAVE_GETCWD
@@ -287,6 +285,12 @@ void pv_freecontents_transfer(pvtransferstate_t transfer)
 			}
 		}
 	}
+
+	/* Close the discard file descriptor, if there was one. */
+	if (transfer->discard_fd >= 0) {
+		(void) close(transfer->discard_fd);
+		transfer->discard_fd = -1;
+	}
 #endif				/* HAVE_SPLICE */
 
 	if (NULL != transfer->line_positions)

src/pv/transfer.c

@@ -341,6 +341,13 @@ static bool pv__transfer_read(pvstate_t state, int fd, bool *eof_in, bool *eof_o
 	size_t bytes_can_read;
 	off_t amount_to_skip, amount_skipped, orig_offset, skip_offset;
 	ssize_t nread;
+	int output_fd;
+
+	output_fd = state->control.output_fd;
+#ifdef HAVE_SPLICE
+	if (state->control.discard_input && !state->control.no_splice)
+		output_fd = state->transfer.discard_fd;
+#endif				/* HAVE_SPLICE */
 
 	do_not_skip_errors = false;
 	if (0 == state->control.skip_errors)
@@ -437,7 +444,7 @@ static bool pv__transfer_read(pvstate_t state, int fd, bool *eof_in, bool *eof_o
 					bytes_to_splice_out = bytes_to_splice;
 
 				spliced_out =
-				    splice(state->transfer.intermediate_pipe[0], NULL, state->control.output_fd, NULL,
+				    splice(state->transfer.intermediate_pipe[0], NULL, output_fd, NULL,
 					   bytes_to_splice_out, SPLICE_F_MORE);
 				if (spliced_out > 0) {
 					state->transfer.intermediate_pipe_buffer_used -= (int) spliced_out;
@@ -446,7 +453,7 @@ static bool pv__transfer_read(pvstate_t state, int fd, bool *eof_in, bool *eof_o
 			}
 		} else {
 			/* Normal splice() from input to output. */
-			nread = splice(fd, NULL, state->control.output_fd, NULL, bytes_to_splice, SPLICE_F_MORE);
+			nread = splice(fd, NULL, output_fd, NULL, bytes_to_splice, SPLICE_F_MORE);
 		}
 		/*@+type@ */
 		/*@+nullpass@ */
@@ -474,7 +481,7 @@ static bool pv__transfer_read(pvstate_t state, int fd, bool *eof_in, bool *eof_o
 				 * error, it can't be skipped, so set
 				 * "do_not_skip_errors".
 				 */
-				if ((fdatasync(state->control.output_fd) < 0)
+				if ((fdatasync(output_fd) < 0)
 				    && (EIO == errno)) {
 					nread = -1;
 					do_not_skip_errors = true;