Unix from assembler (2) - Read a Unix File

Read a Unix File

Files on Unix and z/OS-datasets have complete different design paradigms. Nearly all files on z/OS have records. You read and write records. However, on Unix, you read and write bytes and/or a stream of bytes. When you wish to read, you say to the system how many bytes you want and you will get them until there are no more bytes in the file. Unix has a kind of records though, but they are merely bytes separated by Newline (NL - X’15’).

Unix files on z/OS

When you browse a file on zFS it looks exactly as records on a dataset with the exception that they have variable length. That is because each “record” is suffixed by X’15’ (New Line) and ISPF Browse shows it as a record. The NL is not displayed

********************************* Top of Data *** ------------------------------------------------         MACRO                                    444444444DCCDD                                    00000000041396                                    ------------------------------------------------ &LABEL   USSSERV &SERVICE,&PARMS                  5DCCCD444EEEECDE45ECDECCC65DCDDE                  0312530004222595002595935B071942                  ------------------------------------------------         GBLC  &PARM_LABEL                        444444444CCDC445DCDD6DCCCD                        00000000072330007194D31253                        ------------------------------------------------         AIF   ('&SERVICE' EQ 'PARM').PARM        444444444CCC444475ECDECCC74CD47DCDD754DCDD        000000000196000DD02595935D0580D7194DDB7194

Display of Unix file on zFS. (ISPF Browse)

If you enter the same file in edit mode, you see all lines padded with spaces but that is just for you to be able to type in characters after the last character. When you save the file an NL will be placed right after the last character and the spaces after NL will be removed.

****** ***************************** Top of Data ******* 000001          MACRO                                          444444444DCCDD44444444444444444444444444444444444       0000000004139600000000000000000000000000000000000 -------------------------------------------------------- 000002 &LABEL   USSSERV &SERVICE,&PARMS                        5DCCCD444EEEECDE45ECDECCC65DCDDE44444444444444444       0312530004222595002595935B07194200000000000000000 --------------------------------------------------------

Same file in edit mode

Assembler program to read a Unix file.

I have made a program that reads a file and write the “records” to operator (WTO). Let’s go through the sections in

the program. I use my macro USSSERV to do the more tedious work of calling the USS services. You can find it in the previous article

General storage fields and instructions

PARM     USSSERV PARM,10   RETVAL   DS   F RETCODE  DS   F RSNCODE  DS   F

Definition of storage for all calls.

PARM as parameter list with 10 fullwords followed by three fullwords to return values from the callable service.

You must check for any error after each call.

ICM   R15,B'1111',RETVAL  Test RETVAL         BL    ERROR               Branch if negative * (-1 = failure)

OPEN - BPX1OPN

First of all you must open the file you wish to read. The open will establish a connection between your program and the file. This specific connection is “pointed” to by the result of the open in the fullword FILEDESC which must be used in each of the following commands to the file, - here READ and CLOSE.

FILEDESC DS   F    Result field that contains a “pointer” to the file. BUFLENA  DS   F    Length of Path and name of file BUFFERA  DS   CL13 Path and name of file FLAGS    DS   F    Open option. Read only (O_RDONLY)       MODE     DS   F    Is here zero

Storage fields used by Open

Open the file for input (read) and save the file descriptor

MVC   BUFFERA(14),=CL14'/u/jee/ussserv'         MVC   BUFLENA,=F'14'         XC    S_MODE,S_MODE         XC    O_FLAGS(OPNF_LENGTH),O_FLAGS         MVI   O_FLAGS4,O_RDONLY          Read only         USSSERV OPEN,(BUFLENA,BUFFERA,FLAGS,MODE,               RETVAL,               RETCODE,               RSNCODE)         ICM   R15,B'1111',RETVAL  Test RETVAL         BL    ERROR               Branch if negative * (-1 = failure)         ST    R15,FILEDESC        Store the file descriptor

Call open for read of “/u/jee/ussserv”. The result (File descriptor) is in register 15.

Remember, that path and file names are case sensitive.

READ - BPX1RED

You can now issue READ as many times you like until End-Of-File. The READ-command takes some fields.

READ_COUNT     DS F      Saves the result from R15 of bytes read BUFFER         DS CL40   The area with the read bytes BUFFER_ADR     DS F      The address of the area above BUFFER_LENGTH  DS F      The length of the buffer area B_ALET         DS F      Not used. Must be zero

READ fields

Before we call the READ service we must initiate some of the fields. The buffer (BUFFER) must be filled with binary zeroes in case we do not get all the bytes into the buffer. This will typically be at the end of the file.

Please note, that we supply the ADDRESS of the buffer rather than the buffer itself. It might be because we could issue another read right after adding the number of bytes read to the buffer address.  That would be convenient if the buffer is greater than the number we supply in the BUFFER_LENGTH. We do not do it here!

LA    R4,WTOTEXT LOOP_READ DS   0H         MVC   BUFFER_LENGTH,=A(L'BUFFER)         XC    B_ALET,B_ALET         XC    BUFFER,BUFFER         LA    1,BUFFER         ST    1,BUFFER_ADR                                                                             USSSERV READ,(FILEDESC,               BUFFER_ADR,               B_ALET,               BUFFER_LENGTH,               RETVAL,               RETCODE,               RSNCODE)                                                                             ICM   R15,B'1111',RETVAL  Test RETVAL         BM    ERROR               Branch if negative * (-1 = failure)         BZ    CLOSE               End-Of-File         ST    R15,READ_COUNT      Store the number of bytes read

READ code

We check for any errors after the read and saves the number of bytes read in READ_COUNT.

Splitting a buffer into records

We must have a routine to convert the stream input to the record output.

WTO      DS    0F   WTOLGD   DS    H    WTOTEXT  DS    CL80

WTO fields

This routine takes the buffer and separates it into records that can be written to operator (WTO). One buffer can contain several records and a record can be split into two buffers. The routine checks for NL (X’15’) and when it is found it writes the record. However, when the routine comes to the end of the buffer without encountering an NL it goes to read a new buffer full of bytes.

L     R6,READ_COUNT                LA    R2,BUFFER            NEXT_BYTE DS   0H                           CLI   0(R2),X'15'                  BE    WRITE_BYTES                  MVC   0(1,R4),0(R2)                LA    R2,1(,R2)                    LA    R4,1(,R4)                    BCT   R6,NEXT_BYTE                 B     LOOP_READ                                               WRITE_BYTES DS 0H                           LA    R2,1(,R2)                    MVC   WTOLGD,=Y(L'WTOTEXT)         WTO   TEXT=WTO,ROUTCDE=11         MVI   WTOTEXT,C' '                 MVC   WTOTEXT+1(l'WTOTEXT-1),WTOTEXT           LA    R4,WTOTEXT                               BCT   R6,NEXT_BYTE                             B     LOOP_READ

Use of general registers:

R2 Points to the byte in the input buffer

R4 Points to the byte in the output buffer (WTOTEXT)

R6 Counts down the number of bytes left in the input buffer.

The number of bytes in the input buffer is limited to the input buffer length set on the READ command (BUFFER_LENGTH)

Close BPX1CLO

Nothing special with the CLOSE. It takes the FILEDESC and disconnects the program from the file.

CLOSE    DS    0h         USSSERV CLOSE,(FILEDESC,               RETVAL,               RETCODE,               RSNCODE)                                                                             ICM   R15,B'1111',RETVAL  Test RETVAL         BM    ERROR       Branch if negative (-1 = failure)         BZ    RETURN

Manuals

UNIX System Services Programming: Assembler Callable Services Reference (SA22-7803-14)

EBCDIC Code Page: https://en.wikipedia.org/wiki/EBCDIC_037