Index: file.c
===================================================================
--- file.c	(revision 210862)
+++ file.c	(working copy)
@@ -74,7 +74,7 @@
 
 	switch (filebehave) {
 	case FILE_STDIO:
-		return (fgetc(f->f));
+		return (getc_unlocked(f->f));
 	case FILE_GZIP:
 		return (gzgetc(f->gzf));
 	case FILE_BZIP:
@@ -189,6 +189,7 @@
 	f = grep_malloc(sizeof *f);
 
 	if ((f->f = fdopen(STDIN_FILENO, "r")) != NULL) {
+		flockfile(f->f);
 		f->stdin = true;
 		return (f);
 	}
@@ -238,6 +239,7 @@
 
 	switch (filebehave) {
 	case FILE_STDIO:
+		funlockfile(f->f);
 		fclose(f->f);
 		break;
 	case FILE_GZIP: