diff --git a/mmap/mmap_readat.3 b/mmap/mmap_readat.3
index de48a9f..353f550 100644
--- a/mmap/mmap_readat.3
+++ b/mmap/mmap_readat.3
@@ -10,7 +10,8 @@ mmap_readat opens \fIfilename\fR for reading, maps the whole file into
 memory, closes the file, writes the length of the file to \fIfilesize\fR
 and returns a pointer to the mapped file.
 
-If \fIfilename\fR is a relative path and \fIdirfd\fR is not -1,
+If \fIfilename\fR is a relative path and \fIdirfd\fR is not AT_FDCWD
+(defined in fcntl.h),
 \fIfilename\fR is interpreted relative to \fIdirfd\fR, which must be an
 open directory.