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The
function associates a stream with up to four Either or must be specified;
the others can be given as an appropriately-typed pointer. These functions
will be used to read, write, seek and close the new stream. In general,
omitting a function means that any attempt to perform the associated operation
on the resulting stream will fail. If the close function is omitted, closing
the stream will flush any buffered output and then succeed. The calling
conventions of and must match those, respectively, of and with
the single exception that they are passed the argument specified to
in place of the traditional file descriptor argument. Read and write functions
are allowed to change the underlying buffer on fully buffered or line buffered
streams by calling They are also not required to completely fill or empty
the buffer. They are not, however, allowed to change streams from unbuffered
to buffered or to change the state of the line buffering flag. They must
also be prepared to have read or write calls occur on buffers other than
the one most recently specified. All user functions can report an error
by returning -1. Additionally, all of the functions should set the external
variable appropriately if an error occurs. An error on does not keep
the stream open. As a convenience, the include file defines the macros
and as calls to with only a read or write function specified.
Upon successful completion, returns a pointer. Otherwise, is returned
and the global variable is set to indicate the error.
The function
was called without either a read or write function. The function may also
fail and set for any of the errors specified for the routine
The functions first appeared in 4.4BSD.
The function may
not be portable to systems other than
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