Comtrol eCos - Manual (Page 421)

Chapter 22. File Table
Once a file has been opened it is represented by an open file object. These are allocated from an array of avail-
able file objects. User code accesses these open file objects via a second array of pointers which is indexed by
small integer offsets. This gives the usual Unix file descriptor functionality, complete with the various duplication
mechanisms.
A file table entry has the following structure:
struct CYG_FILE_TAG
{
cyg_uint32 f_flag; /* file state */
cyg_uint16 f_ucount; /* use count */
cyg_uint16 f_type; /* descriptor type */
cyg_uint32 f_syncmode; /* synchronization protocol */
struct CYG_FILEOPS_TAG *f_ops; /* file operations */
off_t f_offset; /* current offset */
CYG_ADDRWORD f_data; /* file or socket */
CYG_ADDRWORD f_xops; /* extra type specific ops */
cyg_mtab_entry *f_mte; /* mount table entry */
};
The f_flag field contains some FILEIO control bits and some bits propagated from the flags argument of the
open() call (defined by CYG_FILE_MODE_MASK ).
The f_ucount field contains a use count that controls when a file will be closed. Each duplicate in the file
descriptor array counts for one reference here. It is also incremented around each I/O operation to ensure that the
file cannot be closed while it has current I/O operations.
The f_type field indicates the type of the underlying file object. Some of the possible values here are
CYG_FILE_TYPE_FILE , CYG_FILE_TYPE_SOCKET or CYG_FILE_TYPE_DEVICE .
The f_syncmode field is copied from the syncmode field of the implementing filesystem. Its use is described
in Chapter 24 .
The f_offset field records the current file position. It is the responsibility of the file operation functions to keep
this field up to date.
The f_data field contains private data placed here by the underlying filesystem. Normally this will be a pointer
to, or handle on, the filesystem object that implements this file.
The f_xops field contains a pointer to any extra type specific operation functions. For example, the socket I/O
system installs a pointer to a table of functions that implement the standard socket operations.
The f_mte field contains a pointer to the parent mount table entry for this file. It is used mainly to implement the
synchronization protocol. This may contain a pointer to some other data structure in file objects not derived from a
filesystem.
The f_ops field contains a pointer to a table of file I/O operations. This has the following structure:
struct CYG_FILEOPS_TAG
{
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