This patch adds the --write-devices option, which will try to write
data into a device when used as a destination.

To use this patch, run these commands for a successful build:

    patch -p1 <patches/write-devices.diff
    ./configure                      (optional if already run)
    make

This patch has not yet been tested by me (Wayne), but was provided
Darryl Dixon.  Thanks!

based-on: d73762eea3f15f2c56bb3fa9394ad1883c25c949
diff --git a/generator.c b/generator.c
--- a/generator.c
+++ b/generator.c
@@ -39,6 +39,7 @@ extern int preserve_acls;
 extern int preserve_xattrs;
 extern int preserve_links;
 extern int preserve_devices;
+extern int write_devices;
 extern int preserve_specials;
 extern int preserve_hard_links;
 extern int preserve_executability;
@@ -1693,7 +1694,7 @@ static void recv_generator(char *fname, struct file_struct *file, int ndx,
 
 	fnamecmp_type = FNAMECMP_FNAME;
 
-	if (statret == 0 && !S_ISREG(sx.st.st_mode)) {
+	if (statret == 0 && !(S_ISREG(sx.st.st_mode) || (write_devices && IS_DEVICE(sx.st.st_mode)))) {
 		if (delete_item(fname, sx.st.st_mode, del_opts | DEL_FOR_FILE) != 0)
 			goto cleanup;
 		statret = -1;
diff --git a/options.c b/options.c
--- a/options.c
+++ b/options.c
@@ -50,6 +50,7 @@ int append_mode = 0;
 int keep_dirlinks = 0;
 int copy_dirlinks = 0;
 int copy_links = 0;
+int write_devices = 0;
 int preserve_links = 0;
 int preserve_hard_links = 0;
 int preserve_acls = 0;
@@ -705,6 +706,7 @@ void usage(enum logcode F)
   rprintf(F," -o, --owner                 preserve owner (super-user only)\n");
   rprintf(F," -g, --group                 preserve group\n");
   rprintf(F,"     --devices               preserve device files (super-user only)\n");
+  rprintf(F," -w  --write-devices         write to devices as regular files (implies --inplace)\n");
   rprintf(F,"     --specials              preserve special files\n");
   rprintf(F," -D                          same as --devices --specials\n");
   rprintf(F," -t, --times                 preserve modification times\n");
@@ -887,6 +889,7 @@ static struct poptOption long_options[] = {
   {"no-D",             0,  POPT_ARG_NONE,   0, OPT_NO_D, 0, 0 },
   {"devices",          0,  POPT_ARG_VAL,    &preserve_devices, 1, 0, 0 },
   {"no-devices",       0,  POPT_ARG_VAL,    &preserve_devices, 0, 0, 0 },
+  {"write-devices",   'w', POPT_ARG_NONE,   0, 'w', 0, 0 },
   {"specials",         0,  POPT_ARG_VAL,    &preserve_specials, 1, 0, 0 },
   {"no-specials",      0,  POPT_ARG_VAL,    &preserve_specials, 0, 0, 0 },
   {"links",           'l', POPT_ARG_VAL,    &preserve_links, 1, 0, 0 },
@@ -1808,6 +1811,11 @@ int parse_arguments(int *argc_p, const char ***argv_p)
 			return 0;
 #endif
 
+		case 'w':
+			write_devices = 1;
+			inplace = 1;
+			break;
+
 		default:
 			/* A large opt value means that set_refuse_options()
 			 * turned this option off. */
@@ -2792,6 +2800,9 @@ void server_options(char **args, int *argc_p)
 	if (relative_paths && !implied_dirs && (!am_sender || protocol_version >= 30))
 		args[ac++] = "--no-implied-dirs";
 
+	if (write_devices)
+		args[ac++] = "--write-devices";
+
 	if (remove_source_files == 1)
 		args[ac++] = "--remove-source-files";
 	else if (remove_source_files)
diff --git a/receiver.c b/receiver.c
--- a/receiver.c
+++ b/receiver.c
@@ -39,6 +39,7 @@ extern int protocol_version;
 extern int relative_paths;
 extern int preserve_hard_links;
 extern int preserve_perms;
+extern int write_devices;
 extern int preserve_xattrs;
 extern int basis_dir_cnt;
 extern int make_backups;
@@ -231,7 +232,7 @@ int open_tmpfile(char *fnametmp, const char *fname, struct file_struct *file)
 }
 
 static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
-			const char *fname, int fd, OFF_T total_size)
+			const char *fname, int fd, struct file_struct *file)
 {
 	static char file_sum1[MAX_DIGEST_LEN];
 	struct map_struct *mapbuf;
@@ -245,10 +246,10 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 	char *map = NULL;
 
 #ifdef SUPPORT_PREALLOCATION
-	if (preallocate_files && fd != -1 && total_size > 0 && (!inplace || total_size > size_r)) {
+	if (preallocate_files && fd != -1 && F_LENGTH(file) > 0 && (!inplace || F_LENGTH(file) > size_r)) {
 		/* Try to preallocate enough space for file's eventual length.  Can
 		 * reduce fragmentation on filesystems like ext4, xfs, and NTFS. */
-		if ((preallocated_len = do_fallocate(fd, 0, total_size)) < 0)
+		if ((preallocated_len = do_fallocate(fd, 0, F_LENGTH(file))) < 0)
 			rsyserr(FWARNING, errno, "do_fallocate %s", full_fname(fname));
 	} else
 #endif
@@ -285,7 +286,7 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 		if (append_mode == 2 && mapbuf) {
 			for (j = CHUNK_SIZE; j < sum.flength; j += CHUNK_SIZE) {
 				if (INFO_GTE(PROGRESS, 1))
-					show_progress(offset, total_size);
+					show_progress(offset, F_LENGTH(file));
 				sum_update(map_ptr(mapbuf, offset, CHUNK_SIZE),
 					   CHUNK_SIZE);
 				offset = j;
@@ -293,7 +294,7 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 			if (offset < sum.flength) {
 				int32 len = (int32)(sum.flength - offset);
 				if (INFO_GTE(PROGRESS, 1))
-					show_progress(offset, total_size);
+					show_progress(offset, F_LENGTH(file));
 				sum_update(map_ptr(mapbuf, offset, len), len);
 			}
 		}
@@ -307,7 +308,7 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 
 	while ((i = recv_token(f_in, &data)) != 0) {
 		if (INFO_GTE(PROGRESS, 1))
-			show_progress(offset, total_size);
+			show_progress(offset, F_LENGTH(file));
 
 		if (allowed_lull)
 			maybe_send_keepalive(time(NULL), MSK_ALLOW_FLUSH | MSK_ACTIVE_RECEIVER);
@@ -377,16 +378,16 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 
 #ifdef HAVE_FTRUNCATE
 	/* inplace: New data could be shorter than old data.
-	 * preallocate_files: total_size could have been an overestimate.
+	 * preallocate_files: F_LENGTH(file) could have been an overestimate.
 	 *     Cut off any extra preallocated zeros from dest file. */
-	if ((inplace || preallocated_len > offset) && fd != -1 && do_ftruncate(fd, offset) < 0) {
+	if ((inplace || preallocated_len > offset) && fd != -1 && !IS_DEVICE(file->mode) && do_ftruncate(fd, offset) < 0) {
 		rsyserr(FERROR_XFER, errno, "ftruncate failed on %s",
 			full_fname(fname));
 	}
 #endif
 
 	if (INFO_GTE(PROGRESS, 1))
-		end_progress(total_size);
+		end_progress(F_LENGTH(file));
 
 	sum_len = sum_end(file_sum1);
 
@@ -402,9 +403,9 @@ static int receive_data(int f_in, char *fname_r, int fd_r, OFF_T size_r,
 }
 
 
-static void discard_receive_data(int f_in, OFF_T length)
+static void discard_receive_data(int f_in, struct file_struct *file)
 {
-	receive_data(f_in, NULL, -1, 0, NULL, -1, length);
+	receive_data(f_in, NULL, -1, 0, NULL, -1, file);
 }
 
 static void handle_delayed_updates(char *local_name)
@@ -660,7 +661,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 					"(Skipping batched update for%s \"%s\")\n",
 					redoing ? " resend of" : "",
 					fname);
-				discard_receive_data(f_in, F_LENGTH(file));
+				discard_receive_data(f_in, file);
 				file->flags |= FLAG_FILE_SENT;
 				continue;
 			}
@@ -671,13 +672,13 @@ int recv_files(int f_in, int f_out, char *local_name)
 		if (!do_xfers) { /* log the transfer */
 			log_item(FCLIENT, file, iflags, NULL);
 			if (read_batch)
-				discard_receive_data(f_in, F_LENGTH(file));
+				discard_receive_data(f_in, file);
 			continue;
 		}
 		if (write_batch < 0) {
 			log_item(FCLIENT, file, iflags, NULL);
 			if (!am_server)
-				discard_receive_data(f_in, F_LENGTH(file));
+				discard_receive_data(f_in, file);
 			if (inc_recurse)
 				send_msg_int(MSG_SUCCESS, ndx);
 			continue;
@@ -767,7 +768,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 		} else if (do_fstat(fd1,&st) != 0) {
 			rsyserr(FERROR_XFER, errno, "fstat %s failed",
 				full_fname(fnamecmp));
-			discard_receive_data(f_in, F_LENGTH(file));
+			discard_receive_data(f_in, file);
 			close(fd1);
 			if (inc_recurse)
 				send_msg_int(MSG_NO_SEND, ndx);
@@ -782,18 +783,32 @@ int recv_files(int f_in, int f_out, char *local_name)
 			 */
 			rprintf(FERROR_XFER, "recv_files: %s is a directory\n",
 				full_fname(fnamecmp));
-			discard_receive_data(f_in, F_LENGTH(file));
+			discard_receive_data(f_in, file);
 			close(fd1);
 			if (inc_recurse)
 				send_msg_int(MSG_NO_SEND, ndx);
 			continue;
 		}
 
-		if (fd1 != -1 && !S_ISREG(st.st_mode)) {
+		if (fd1 != -1 && !(S_ISREG(st.st_mode) || (write_devices && IS_DEVICE(st.st_mode)))) {
 			close(fd1);
 			fd1 = -1;
 		}
 
+		/* On Linux systems (at least), st_size is typically 0 for devices.
+		 * If so, try to determine the actual device size. */
+		if (fd1 != -1 && IS_DEVICE(st.st_mode) && st.st_size == 0) {
+			OFF_T off = lseek(fd1, 0, SEEK_END);
+			if (off == (OFF_T) -1)
+				rsyserr(FERROR, errno, "failed to seek to end of %s to determine size", fname);
+			else {
+				st.st_size = off;
+				off = lseek(fd1, 0, SEEK_SET);
+				if (off != 0)
+					rsyserr(FERROR, errno, "failed to seek back to beginning of %s to read it", fname);
+			}
+		}
+
 		/* If we're not preserving permissions, change the file-list's
 		 * mode based on the local permissions and some heuristics. */
 		if (!preserve_perms) {
@@ -825,7 +840,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 		}
 
 		if (fd2 == -1) {
-			discard_receive_data(f_in, F_LENGTH(file));
+			discard_receive_data(f_in, file);
 			if (fd1 != -1)
 				close(fd1);
 			if (inc_recurse)
@@ -840,8 +855,7 @@ int recv_files(int f_in, int f_out, char *local_name)
 			rprintf(FINFO, "%s\n", fname);
 
 		/* recv file data */
-		recv_ok = receive_data(f_in, fnamecmp, fd1, st.st_size,
-				       fname, fd2, F_LENGTH(file));
+		recv_ok = receive_data(f_in, fnamecmp, fd1, st.st_size, fname, fd2, file);
 
 		log_item(log_code, file, iflags, NULL);