DCB (Data Control Block) attributes

For OS files, the C runtime library creates a skeleton data control block (DCB) for the file when you open it. File attributes are determined from the following sources in this order:

1. The fopen() or freopen() function call
2. Attributes for a ddname specified previously (if you are opening by ddname)
3. Existing file attributes (if you specify recfm=* or you are opening an existing file for read or append)
4. Defaults from fopen() or freopen() for creating a new file.

If recfm is not specified for a record I/O or blocked I/O file, you will get the default of recfm=VB. Table 1 shows the defaults for LRECL and BLKSIZE when the z/OS XL C/C++ compiler creates an OS file. Information from the C or C++ program overrides that from the DD statement and the tape label. Information from the DD statement overrides that from the data set label.

Figure 1. How the operating system completes the DCB

Table 1. fopen() defaults for LRECL and BLKSIZE when creating OS files

lrecl specified?	blksize specified?	RECFM	LRECL	BLKSIZE
no	no	All F	80	80
		All FB	80	maximum integral multiple of 80 less than or equal to max
		All V, VB, VS, or VBS	minimum of 1028 or max–4	max
		All U	0	max
yes	no	All F	lrecl	lrecl
		All FB	lrecl	maximum integral multiple of lrecl less than or equal to max
		All V	lrecl	lrecl+4
		All U	0	lrecl
no	yes	All F or FB	blksize	blksize
		All V, VB, VS, or VBS	minimum of 1028 or blksize–4	blksize
		All U	0	blksize
Note: All includes the standard (S) specifier for fixed formats, the ASA (A) specifier, and the machine control character (M) specifier.

For more information about specific default block sizes as returned by the DEVTYPE macro, refer to z/OS DFSMS Using Data Sets.

You can perform multiple buffering under z/OS. See Multiple buffering for details.