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-rw-r--r--doc/ref/api-io.texi3
1 files changed, 3 insertions, 0 deletions
diff --git a/doc/ref/api-io.texi b/doc/ref/api-io.texi
index 4a38a8378..ac9bb3e77 100644
--- a/doc/ref/api-io.texi
+++ b/doc/ref/api-io.texi
@@ -812,6 +812,9 @@ used only during port creation are not retained.
Return the filename associated with @var{port}. This function returns
the strings "standard input", "standard output" and "standard error"
when called on the current input, output and error ports respectively.
+
+@var{port} must be open, @code{port-filename} cannot be used once the
+port is closed.
@end deffn
@deffn {Scheme Procedure} set-port-filename! port filename