Call lseek() at the start if output is appending, so that future lseek()s are relative to the right position, fixing the issue with incorrect truncation size (#45).

src/main/main.c

@@ -448,7 +448,12 @@ int main(int argc, char **argv)
 	 * do this before looking at setting the size, as the size
 	 * calculation looks at the output file if the input size can't be
 	 * calculated (issue #91).
+	 *
+	 * We have to set the sparse output flag before doing this, so that
+	 * in sparse mode the lseek() on O_APPEND can be done (issue #45);
+	 * see the comments in pv_state_output_set() in src/pv/state.c.
 	 */
+	pv_state_sparse_output_set(state, opts->sparse_output);
 	retcode = pv__set_output(state, opts, opts->output);
 	if (0 != retcode) {
 		pv_state_free(state);
@@ -509,7 +514,6 @@ int main(int argc, char **argv)
 	pv_state_error_skip_block_set(state, opts->error_skip_block);
 	pv_state_sync_after_write_set(state, opts->sync_after_write);
 	pv_state_direct_io_set(state, opts->direct_io);
-	pv_state_sparse_output_set(state, opts->sparse_output);
 	pv_state_discard_input_set(state, opts->discard_input);
 	pv_state_rate_limit_set(state, opts->rate_limit);
 	pv_state_target_buffer_size_set(state, opts->buffer_size);

src/pv/state.c

@@ -272,15 +272,20 @@ static void pv_truncate_output(pvstate_t state)
 	if (state->control.output_fd < 0)
 		return;
 
-	current_offset = lseek(state->control.output_fd, (off_t) 0, SEEK_CUR);
+	current_offset = (off_t) lseek(state->control.output_fd, (off_t) 0, SEEK_CUR);
 	if (current_offset == (off_t) - 1)
 		return;
 
 	debug("%s: %ld", "truncating to current offset", (long) current_offset);
 
+	/*@+longintegral@ */
+	/*
+	 * splint has trouble with off_t / __off_t.
+	 */
 	if (0 != ftruncate(state->control.output_fd, current_offset)) {
 		debug("%s: %s", "output ftruncate() failed", strerror(errno));
 	}
+	/*@-longintegral@ */
 }
 
 
@@ -680,6 +685,22 @@ void pv_state_output_set(pvstate_t state, int fd, const char *name)
 	 */
 	fcntl(state->control.output_fd, F_SETFL, O_NONBLOCK | fcntl(state->control.output_fd, F_GETFL));
 #endif				/* MAKE_OUTPUT_NONBLOCKING */
+
+	/*
+	 * In sparse output mode, if the output is in append mode (>>),
+	 * explicitly lseek() to the end of the file.  Otherwise, the file
+	 * offset is not set until the first write(), which means that if
+	 * the input starts with null bytes, when we lseek() past them
+	 * relative to the current position, the "current position" is 0
+	 * rather than the end of the file, and the file gets truncated on
+	 * exit to the wrong size.
+	 */
+	if (state->control.sparse_output && 0 != (fcntl(fd, F_GETFL) & O_APPEND)) {
+		debug("%s", "sparse output mode, and appending - seeking output to the end");
+		if ((off_t) lseek(fd, 0, SEEK_END) == (off_t) - 1) {
+			debug("%s: %s", "lseek failed", strerror(errno));
+		}
+	}
 }
 
 void pv_state_average_rate_window_set(pvstate_t state, unsigned int val)