A seq_file API tweak

The seq_file mechanism is a helper for kernel subsystems wanting to create lengthy virtual files, usually in /proc. 2.6.15 will include a small enhancement which may prove helpful for some users.

When user space opens a virtual file, the kernel must, in turn, call seq_open() to set things up. On return, the file structure passed to seq_open() will have, in its private_data field, a pointer to the seq_file structure created at open time. That is the same structure which will be passed to the seq_file iterator functions, and which must be used when actually generating output.

Traditionally, seq_open() has always allocated the seq_file structure itself. In 2.6.15, however, it will examine the private_data field first, and, if that field is non-NULL, it will assume that the seq_file has already been allocated by the caller. This change allows seq_file users to embed the structure within something larger. It is worth noting, though, that seq_release() still frees the seq_file structure regardless of who created it. Among other things, that implies that, if the caller allocates a seq_file structure within a larger structure, the seq_file structure must appear at the beginning.

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