[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).          */       };