diff --git a/src/xz/coder.c b/src/xz/coder.c
index d26ae6e9..5bca958f 100644
--- a/src/xz/coder.c
+++ b/src/xz/coder.c
@@ -1000,6 +1000,15 @@ coder_run(const char *filename)
 				mytime_set_start_time();
 
 				// Initialize the progress indicator.
+				//
+				// NOTE: When reading from stdin, fstat()
+				// isn't called on it and thus src_st.st_size
+				// is zero. If stdin pointed to a regular
+				// file, it would still be possible to know
+				// the file size but then we would also need
+				// to take into account the current reading
+				// position since with stdin it isn't
+				// necessarily at the beginning of the file.
 				const bool is_passthru = init_ret
 						== CODER_INIT_PASSTHRU;
 				const uint64_t in_size
diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index 61857029..a5841b37 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -768,6 +768,10 @@ io_open_src(const char *src_name)
 	// a statically allocated structure.
 	static file_pair pair;
 
+	// This implicitly also initializes src_st.st_size to zero
+	// which is expected to be <= 0 by default. fstat() isn't
+	// called when reading from standard input but src_st.st_size
+	// is still read.
 	pair = (file_pair){
 		.src_name = src_name,
 		.dest_name = NULL,