Parameter 2 specifies the address of a buffer and parameter 3 specifies the buffer length. These two parameters are not used for the INIT, TERM, and CLOSE functions.

On input for a WRITE function, the buffer address points to a buffer that contains the record to be written. The buffer length parameter specifies the length of the data to be written from the buffer. The caller must provide the buffer address and length.

For the WRITE function, if data is truncated during the write operation, the I/O routine returns the length of the data that was actually written in the buffer length parameter. A return code of 16 is also returned.

On output for a READ or READX function, the buffer address points to a buffer that contains the record that was read. The buffer length parameter specifies the length of the data being returned in the buffer.

For a READ or READX function, the I/O routine obtains the buffer needed to store the record. The caller must copy the data that is returned into its own storage before calling the I/O routine again for another request. The buffers are reused for subsequent I/O requests.

On output for an OPENR, OPENW, or OPENX function, the buffer address points to the data set information block, which is an area in storage that contains information about the file. The buffer length parameter returns the length of the data set information block (DSIB) whose address is being returned. Data set information block describes the format of this area. TSO/E provides a mapping macro, IRXDSIB, that you can use to map the buffer area returned for an open request.

For an OPENR, OPENW, or OPENX function, all of the information in the data set information block does not have to be returned. The buffer length must be large enough for all of the information being returned about the file or unpredictable results can occur. The data set information block buffer must be large enough to contain the flags field and any fields that have been set, as indicated by the flags field (see Data set information block).

REXX does not check the content of the buffer for valid or printable characters. Any hexadecimal characters may be passed.

Any replaceable I/O routine must conform to this procedure to ensure that the exec that is currently running accesses valid data.

If you provide your own replaceable I/O routines, your routine must support all of the functions that the system-supplied I/O routine performs. All open requests must open the specified file. However, for an open request, your replaceable I/O routine need only fill in the data set information block fields for the logical record length (LRECL) and its corresponding flag bit. These fields are DSIB_LRECL and DSIB_LRECL_FLAG. The language processor needs these two fields to determine the line length being used for its write operations. The language processor will format all of its output lines to the width that is specified by the LRECL field. Your routine can specify a LRECL (DSIB_LRECL field) of 0, which means that the language processor will format its output using a width of 80 characters, which is the default.

When the I/O routine is called with the TERM function, all buffers are freed.