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These functions provide a VU namespace handler for file names matching the regexp:
             ^-$
When opened for O_RDONLY
 access, "-"
 refers to descriptor 0
.
When opened for O_WRONLY
 access, "-"
 refers to descriptor 1
.
The file "-"
 may not be opened for O_RDWR
 access.
void vu_push_dash_handler (int is_optional);
Establish a VU namespace handler for file-names matching the regexp:
             ^-$
When opened for O_RDONLY
 access, "-"
 refers to descriptor 0
.
When opened for O_WRONLY
 access, "-"
 refers to descriptor 1
.
The file "-"
 may not be opened for O_RDWR
 access.
If the flag is_optional
 is 0
, the namespace handler is simply
installed.  If it is not 0
, the handler is registered under the
name "fd"
, but not installed.  
(See vu_enable_optional_name_handler.)
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