Using a DDname

The DD statement enables you to write C or C++ source programs that are independent of the files and input/output devices they use. You can modify the parameters of a file or process different files without recompiling your program.

Use ddnames if you want to use non-DASD devices.

If you specify DISP=MOD on a DD statement and w or wb mode on the fopen() call, z/OS® XL C/C++ treats the file as if you had opened it in append mode instead of write mode.

To open a file by ddname under z/OS batch, you must define the ddname first. You can do this in any of the following ways:

In batch (z/OS, TSO, or IMS), you can write a JCL DD statement. For the declaration shown above for the C or C++ file PARTS.INSTOCK, you write a JCL DD statement similar to the following:
```
   //STOCK   DD DSN=USERID.PARTS.INSTOCK,DISP=SHR
```
When defining DD, do not use DD … FREE=CLOSE for unallocating DD statements. The C library may close files to perform some file operations such as freopen(), and the DD statement will be unallocated.
If you use SPACE=RLSE on a DD statement, z/OS XL C/C++ releases space only if all of the following are true:
- The file is open in w, wb, a, or ab mode
- It is not simultaneously open for read
- No positioning functions (fseek(), ftell(), rewind(), fgetpos(), fsetpos()) have been performed.
For more information on writing DD statements, refer to the job control language (JCL) manuals listed in z/OS Information Roadmap.
Under TSO (interactive and batch), you can issue an ALLOCATE command. The DD definition shown above for the C file STOCK has an equivalent TSO ALLOCATE command, as follows:
```
   ALLOCATE FILE(STOCK) DATASET(PARTS.INSTOCK) SHR
```
See z/OS Information Roadmap for manuals containing information on TSO ALLOCATE.
In the z/OS environment, you can use the svc99() or dynalloc() library functions to define ddnames. For information about these functions, refer to z/OS XL C/C++ Runtime Library Reference.