File tagging and conversion

In general, the file system knows the contents of a file only as a set of bytes. Applications which create and process bytes in a file know whether these bytes represent binary data, text (character) data, or a mixture of both. File tags are file metadata fields which describe the contents of a file. Enhanced ASCII includes the following file tag fields:

txtflag

A flag indicating if a file consists solely of character data encoded by a single coded character set ID (CCSID).

file ccsid

A 16 bit field specifying the CCSID of characters in the file.

Applications can explicitly tag files using new open() or fcntl() options, or applications can allow the logical file system (LFS) to tag new files on first write, fopen(). A new environment variable, _BPXK_CCSID, is used to assign a program CCSID to an application, which LFS will use to tag new files on first write. LFS also uses the program CCSID derived from _BPXK_CCSID to set up auto-conversion of pure text datastreams. LFS attempts to set up auto-conversion when:

• Auto-conversion is enabled for an application by the _BPXK_AUTOCVT environment variable
• The file txtflag flag is set indicating a pure text file
• The file and program CCSIDs do not match.

Automatic file conversion and file tagging include the following facilities:

• _OPEN_SYS_FILE_EXT feature test macro. For more information, see z/OS XL C/C++ Runtime Library Reference .
• _BPXK_AUTOCVT and _BPXK_CCSIDS environment variables. For more information, see Using environment variables.
• z/OS® Language Environment® FILETAG runtime option. For more information, see z/OS Language Environment Programming Reference.
• __chattr() and __fchattr() functions; F_SETTAG and F_CONTROL_CVT arguments for the fcntl() function; options for the fopen(), popen(), stat(), fstat(), and lstat() functions. For more information, see z/OS XL C/C++ Runtime Library Reference.