LRECL and BLKSIZE defaults

Table 1 shows the defaults for LRECL and BLKSIZE when z/OS® XL C/C++ is creating a file, not appending or updating it. The table assumes that z/OS XL C/C++ has already processed any information from the fopen() statement or ddname. The defaults provide a basis for fopen() to select values for unspecified attributes when you create a file.

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.

In Table 1, the value max represents the maximum block size for the device. These are the current default maximum block sizes for several devices that z/OS XL C/C++ supports:

Device: Block Size
DASD: 6144
3203 Printer: 132
3211 Printer: 132
4245 Printer: 132
2540 Reader: 80
2540 Punch: 80
2501 Reader: 80
3890 Document Processor: 80
TAPE: 32760

For more information about specific default block sizes, refer to z/OS DFSMS Using Data Sets.

For DASD files that do not have recfm=U, if you specify blksize=0 on the call to fopen() or freopen(), the system determines the optimal block size for your file. If you do not have the correct level of DFP or you specify blksize=0 for a ddname instead of specifying it on the fopen() or freopen() call, z/OS XL C/C++ behaves as if you had not specified the blksize parameter at all.

For information about block sizes for different categories of I/O, see the topics listed in Table 1.

You do not have to specify the LRECL and BLKSIZE attributes; however, it is possible to have conflicting attributes when you do specify them. The restrictions are:

For a V file, the LRECL must be greater than 4 bytes and must be at least 4 bytes smaller than the BLKSIZE.
For an F file, the LRECL must be equal to the BLKSIZE, and must be at least 1.
For an FB file, the BLKSIZE must be an integer multiple of the LRECL.
For a U file, the LRECL must be less than or equal to the BLKSIZE and must be greater than or equal to 0. The BLKSIZE must be at least 1.
In spanned files, the LRECL and the BLKSIZE attributes must be greater than 4.
If you specify LRECL=X, the BLKSIZE attribute must be less than or equal to the maximum block size allowed on the device.

To determine the maximum LRECL and BLKSIZE values for the various file types and devices available on your operating system, refer to the topics listed in Table 1.