Parameters

Register 1 contains the address of a parameter list, which consists of a list of addresses. Each address in the parameter list points to a parameter. The high-order bit of the last address in the parameter list must be set to 1 to indicate the end of the parameter list. For more information about passing parameters, see Parameter lists for TSO/E REXX routines.

Table 1 describes the parameters for the user ID routine.

Table 1. Parameters for the user ID replaceable routine
Parameter	Number of bytes	Description
Parameter 1	8	The function to be performed. The function name is left justified, in uppercase, and padded to the right with blanks. The valid functions are USERID and TSOID. Functions supported for the user ID routine describes the functions in detail.
Parameter 2	4	An address of storage into which the routine places the user ID. On output, the area that this address points to contains a character representation of the user ID.
Parameter 3	4	The length of storage pointed to by the address in parameter 2. On input, this value is the maximum length of the area that is available to contain the ID. The length supplied is 160 bytes. The routine must change this parameter and return the actual length of the character string it returns. If the routine returns a 0, the USERID built-in function returns a null value. If the routine copies more characters into the storage area than the storage provided, REXX may abend and any results will be unpredictable.
Parameter 4	4	The address of the environment block that represents the environment in which you want the user ID replaceable routine to run. This parameter is optional. If you specify a non-zero value for the environment block address parameter, the user ID routine uses the value you specify and ignores register 0. However, the routine does not check whether the address is valid. Therefore, you must ensure the address you specify is correct or unpredictable results can occur. For more information, see Using the environment block address.
Parameter 5	4	A 4-byte field that the user ID replaceable routine uses to return the return code. The return code parameter is optional. If you use this parameter, the user ID routine returns the return code in the parameter and also in register 15. Otherwise, the routine uses register 15 only. If the parameter list is invalid, the return code is returned in register 15 only. Return codes describes the return codes. If you do not want to use the return code parameter, you can end the parameter list at a preceding parameter. Set the high-order bit on in the preceding parameter's address. For more information about parameter lists, see Parameter lists for TSO/E REXX routines.