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Documentation: seq_file: Document seq_open_private(), seq_release_private()

Despite the fact that these functions have been around for years, they
are little used (only 15 uses in 13 files at the preseht time) even
though many other files use work-arounds to achieve the same result.

By documenting them, hopefully they will become more widely used.

Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Rob Jones and committed by
Linus Torvalds 77be4daf 6fef37c9

+33
+33
Documentation/filesystems/seq_file.txt
··· 235 235 private field of the seq_file structure; that value can then be retrieved 236 236 by the iterator functions. 237 237 238 + There is also a wrapper function to seq_open() called seq_open_private(). It 239 + kmallocs a zero filled block of memory and stores a pointer to it in the 240 + private field of the seq_file structure, returning 0 on success. The 241 + block size is specified in a third parameter to the function, e.g.: 242 + 243 + static int ct_open(struct inode *inode, struct file *file) 244 + { 245 + return seq_open_private(file, &ct_seq_ops, 246 + sizeof(struct mystruct)); 247 + } 248 + 249 + There is also a variant function, __seq_open_private(), which is functionally 250 + identical except that, if successful, it returns the pointer to the allocated 251 + memory block, allowing further initialisation e.g.: 252 + 253 + static int ct_open(struct inode *inode, struct file *file) 254 + { 255 + struct mystruct *p = 256 + __seq_open_private(file, &ct_seq_ops, sizeof(*p)); 257 + 258 + if (!p) 259 + return -ENOMEM; 260 + 261 + p->foo = bar; /* initialize my stuff */ 262 + ... 263 + p->baz = true; 264 + 265 + return 0; 266 + } 267 + 268 + A corresponding close function, seq_release_private() is available which 269 + frees the memory allocated in the corresponding open. 270 + 238 271 The other operations of interest - read(), llseek(), and release() - are 239 272 all implemented by the seq_file code itself. So a virtual file's 240 273 file_operations structure will look like: