232 lines
10 KiB
Plaintext
232 lines
10 KiB
Plaintext
[Quoting from a C/370 manual, courtesy of Carl Forde.]
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C/370 supports three types of input and output: text streams, binary
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streams, and record I/O. Text and binary streams are both ANSI
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standards; record I/O is a C/370 extension.
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[...]
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Record I/O is a C/370 extension to the ANSI standard. For files
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opened in record format, C/370 reads and writes one record at a
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time. If you try to write more data to a record than the record
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can hold, the data is truncated. For record I/O, C/370 only allows
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the use of fread() and fwrite() to read and write to the files. Any
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other functions (such as fprintf(), fscanf(), getc(), and putc())
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fail. For record-orientated files, records do not change size when
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you update them. If the new data has fewer characters than the
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original record, the new data fills the first n characters, where
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n is the number of characters of the new data. The record will
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remain the same size, and the old characters (those after) n are
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left unchanged. A subsequent update begins at the next boundary.
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For example, if you have the string "abcdefgh":
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abcdefgh
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and you overwrite it with the string "1234", the record will look
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like this:
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1234efgh
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C/370 record I/O is binary. That is, it does not interpret any of
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the data in a record file and therefore does not recognize control
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characters.
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The record model consists of:
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* A record, which is the unit of data transmitted to and from a
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program
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* A block, which is the unit of data transmitted to and from a
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device. Each block may contain one or more records.
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In the record model of I/O, records and blocks have the following
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attributes:
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RECFM Specifies the format of the data or how the data is organized
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on the physical device.
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LRECL Specifies the length of logical records (as opposed to
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physical ones).
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BLKSIZE Specifies the length of physical records (blocks on the
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physical device).
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Opening a File by Filename
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The filename that you specify on the call to fopen() or freopen()
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must be in the following format:
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>> ----filename---- ----filetype--------------------
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--.-- -- --filemode--
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--.--
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where
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filename is a 1- to 8-character string of any of the characters,
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A-Z, a-z, 0-9, and +, -, $, #, @, :, and _. You can separate it
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from the filetype with one or more spaces, or with a period.
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[Further note: filenames are fully case-sensitive, as in Unix.]
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filetype is a 1- to 8-character string of any of the characters,
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A-Z, a-z, 0-9, and +, -, $, #, @, :, and _. You can separate it
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from the filemode with one or more spaces, or with a period. The
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separator between filetype and filemode must be the same as the
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one between filename and filetype.
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filemode is a 1- to 2-character string. The first must be any of
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the characters A-Z, a-z, or *. If you use the asis parameter on
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the fopen() or freopen() call, the first character of the filemode
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must be a capital letter or an asterisk. Otherwise, the function
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call fails. The second character of filemode is optional; if you
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specify it, it must be any of the digits 0-6. You cannot specify
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the second character if you have specified * for the first one.
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If you do not use periods as separators, there is no limit to how
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much whitespace you can have before and after the filename, the
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filetype, and filemode.
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Opening a File without a File Mode Specified
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If you omit the file mode or specify * for it, C/370 does one
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of the following when you call fopen() or freopen():
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* If you have specified a read mode, C/370 looks for the named file
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on all the accessed readable disks, in order. If it does not find
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the file, the fopen() or freopen() call fails.
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* If you have specified any of the write modes, C/370 writes the file
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on the first writable disk you have accessed. Specifying a write
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mode on an fopen() or freopen() call that contains the filename of
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an existing file destroys that file. If you do not have any
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writable disks accessed, the call fails.
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fopen() and freopen() parameters
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recfm
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CMS supports only two RECFMs, V and F. [note that MVS supports
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27(!) different RECFMs.] If you do not specify the RECFM for a
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file, C/370 determines whether is is in fixed or variable format.
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lrecl and blksize
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For files in fixed format, CMS allows records to be read and
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written in blocks. To have a fixed format CMS file treated as a
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fixed blocked CMS file, you can open the file with recfm=fb and
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specify the lrecl and blksize. If you do not specify a recfm on
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the open, the blksize can be a multiple of the lrecl, and the
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file is treated as if it were blocked.
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For files in variable format, the CMS LRECL is different from the
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LRECL for the record model. In the record model, the LRECL is
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equal to the data length plus 4 bytes (for the record descriptor
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word), and the BLKSIZE is equal to the LRECL plus 4 bytes (for
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the block descriptor word). In CMS, BDWs and RDWs do not exist,
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but because CMS follows the record model, you must still account
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for them. When you specify V, you must still allocate the record
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descriptor word and block descriptor word. That is, if you want
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a maximum of n bytes per record, you must specify a minimum LRECL
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of n+4 and a minimum BLKSIZE of n+8.
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When you are appending to V files, you can enlarge the record size
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dynamically, but only if you have not specified LRECL or BLKSIZE
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on the fopen() or freopen() command that opened the file.
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type
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If you specify this parameter, the only valid value for CMS disk
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files is type =record. This opens a file for record I/O.
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asis
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If you use this parameter, you can open files with mixed-case
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filenames such as JaMeS dAtA or pErCy.FILE. If you specify this
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parameter, the file mode that you specify must be a capital letter
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(if it is not an asterisk); otherwise; the function call fails and
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the value returned is NULL.
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Reading from Record I/O Files
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fread() is the only interface allowed for reading record I/O files.
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Each time you call fread() for a record I/O file, fread() reads
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one record from the system. If you call fread() with a request for
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less than a complete record, the requested bytes are copied to your
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buffer, and the file position is set to the start fo the next
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record. If the request is for more bytes that are in the record,
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one record is read and the position is set to the start of the next
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record. C/370 does not strip any blank characters or interpret any
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data.
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fread() returns the number of items read successfully, so if you
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pass a size argument equal to 1 and a count argument equal to the
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maximum expected length of the record, fread() returns the length,
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in bytes, of the record read. If you pass a size argument equal
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to the maximum expected length of the record, and a count argument
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equal to 1, fread() returns either 0 or 1, indicating whether a
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record of length size read. If a record is read successfully but
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is less than size bytes long, fread() returns 0.
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Writing to Record I/O Files
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fwrite() is the only interface allowed for writing to a file
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opened for record I/O. Only one record is written at a time. If
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you attempt to write more new data than a full record can hold or
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try to update a record with more data than it currently has, C/370
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truncates your output at the record boundary. When C/370 performs
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a truncation, it sets errno and raises SIGIOERR, if SIGIOERR is not
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set to SIG_IGN.
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When you are writing new records to a fixed-record I/O file, if you
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try to write a short record, C/370 pads the record with nulls out
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to LRECL.
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At the completion of an fwrite(), the file position is at the start
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of the next record. For new data, the block is flushed out to the
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system as soon as it is full.
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fldata() Behavior
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When you call the fldata() function for an open CMS minidisk file,
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it returns a data structure that looks like this:
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struct __filedata {
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unsigned int __recfmF : 1, /* fixed length records */
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__recfmV : 1, /* variable length records */
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__recfmU : 1, /* n/a */
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__recfmS : 1, /* n/a */
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__recfmBlk : 1, /* n/a */
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__recfmASA : 1, /* text mode and ASA */
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__recfmM : 1, /* n/a */
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__dsorgPO : 1, /* n/a */
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__dsorgPDSmem : 1, /* n/a */
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__dsorgPDSdir : 1, /* n/a */
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__dsorgPS : 1, /* sequential data set */
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__dsorgConcat : 1, /* n/a */
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__dsorgMem : 1, /* n/a */
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__dsorgHiper : 1, /* n/a */
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__dsorgTemp : 1, /* created with tmpfile() */
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__dsorgVSAM : 1, /* n/a */
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__reserve1 : 1, /* n/a */
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__openmode : 2, /* see below 1 */
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__modeflag : 4, /* see below 2 */
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__reserve2 : 9, /* n/a */
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char __device; __DISK
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unsigned long __blksize, /* see below 3 */
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__maxreclen; /* see below 4 */
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unsigned short __vsamtype; /* n/a */
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unsigned long __vsamkeylen; /* n/a */
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unsigned long __vsamRKP; /* n/a */
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char * __dsname; /* fname ftype fmode */
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unsigned int __reserve4; /* n/a */
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/* note 1: values are: __TEXT, __BINARY, __RECORD
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note 2: values are: __READ, __WRITE, __APPEND, __UPDATE
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these values can be added together to determine
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the return value; for example, a file opened with
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a+ will have the value __READ + __APPEND.
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note 3: total block size of the file, including ASA
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characters as well as RDW information
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note 4: maximum record length of the data only (includes
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ASA characters but excludes RDW information).
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*/
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};
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