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