diff -Naur a/jpegoptim.c b/jpegoptim.c
--- a/jpegoptim.c	2023-03-27 15:19:15.047509310 +0800
+++ b/jpegoptim.c	2023-03-27 15:20:14.408374405 +0800
@@ -1028,7 +1028,7 @@
 				fprintf(LOG_FH,csv ? "skipped\n" : "skipped.\n");
 			if (stdout_mode) {
 				set_filemode_binary(stdout);
-				if (fwrite(inbuffer,insize,1,stdout) != 1)
+				if (fwrite(inbuffer, inbufferused, 1, stdout) != 1)
 					fatal("%s, write failed to stdout",(stdin_mode?"stdin":argv[i]));
 			}
 		}