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esballoc() creates a STREAMS message and attaches a user-supplied data buffer in place of a STREAMS data buffer. It calls allocb(9F) to get a message and data block header only. The newly allocated message will have both the b_wptr and b_rptr set to the base of the buffer. As when using allocb(9F), the newly allocated
message will have both b_wptr and b_rptr set to the base of the data buffer. The user-supplied data buffer, pointed to by base, is used as the
data buffer for the message.
When freeb(9F) is called to free the message, the driver's message freeing routine
(referenced through the free_rtn structure) is called, with appropriate arguments, to free the data buffer.
The free_rtn structure includes the following members:
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void (*free_func)(); /* user's freeing routine */
char *free_arg; /* arguments to free_func() */
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Instead of requiring a specific number of arguments, the free_arg field is defined of type char *. This way, the driver can pass a pointer to a structure if
more than one argument is needed.
The method by which free_func is called is implementation-specific. The module writer must not assume that free_func will or will not be called directly from STREAMS utility routines
like freeb(9F) which free a message block.
free_func must not call another modules put procedure nor attempt to acquire a private module lock which may be held by another thread across a call to a STREAMS utility routine
which could free a message block. Otherwise, the possibility for lock recursion and/or deadlock exists.
free_func must not access any dynamically allocated data structure that might no longer exist when it runs.
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