Module Iobuf

This type is a compiler witness that 'rw and 'seek do not affect layout; it enables wider use of unboxed GADTs.

The first type parameter controls whether the iobuf can be written to. The second type parameter controls whether the window and limits can be changed.

See the Perms module for information on how the first type parameter is used.

To allow no_seek or seek access, a function's type uses _ rather than no_seek as the type argument to t. Using _ allows the function to be directly applied to either permission. Using a specific permission would require code to use coercion :>.

There is no t_of_sexp. One should use Iobuf.Hexdump.t_of_sexp or @sexp.opaque as desired.

t_with_shallow_sexp has a sexp_of that shows the windows and limits of the underlying bigstring, but no data. We do this rather than deriving sexp_of on t because it is much more likely to be noise than useful information, and so callers should probably not display the iobuf at all.

Provides a Window.Hexdump submodule that renders the contents of t's window.

Provides a Limits.Hexdump submodule that renders the contents of t's limits.

Provides a Debug.Hexdump submodule that renders the contents of t's window, limits, and underlying bigstring using indices relative to the bigstring.

create ~len creates a new iobuf, backed by a bigstring of length len, with the limits and window set to the entire bigstring.

of_bigstring bigstring ~pos ~len returns an iobuf backed by bigstring, with the window and limits specified starting at pos and of length len.

forbid immutable to prevent aliasing

of_string s returns a new iobuf whose contents are s.

sub_shared t ~pos ~len returns a new iobuf with limits and window set to the subrange of t's window specified by pos and len. sub_shared preserves data permissions, but allows arbitrary seek permissions on the resulting iobuf.

copy t returns a new iobuf whose contents are the same as those in the window of t.

clone t returns a new iobuf that is a deep-copy of t including an exact copy of the underlying buffer and bounds. This means data outside the window is copied as well.

set_bounds_and_buffer ~src ~dst copies bounds metadata (i.e., limits and window) and shallowly copies the buffer (data pointer) from src to dst. It does not access data, but does allow access through dst. This makes dst an alias of src.

Because set_bounds_and_buffer creates an alias, we disallow immutable src and dst using [> write]. Otherwise, one of src or dst could be read_write :> read and the other immutable :> read, which would allow you to write the immutable alias's data through the read_write alias.

set_bounds_and_buffer is typically used with a frame iobuf that need only be allocated once. This frame can be updated repeatedly and handed to users, without further allocation. Allocation-sensitive applications need this.

set_bounds_and_buffer_sub ~pos ~len ~src ~dst is a more efficient version of set_bounds_and_buffer ~src:(Iobuf.sub_shared ~pos ~len src) ~dst.

set_bounds_and_buffer ~src ~dst is not the same as set_bounds_and_buffer_sub ~dst ~src ~len:(Iobuf.length src) because the limits are narrowed in the latter case.

~len and ~pos are mandatory for performance reasons, in concert with @@inline. If they were optional, allocation would be necessary when passing a non-default, non-constant value, which is an important use case.

Generalization

One may wonder why you'd want to call no_seek, given that a cast is already possible, e.g., t : (_, seek) t :> (_, no_seek) t. It turns out that if you want to define some f : (_, _) t -> unit of your own that can be conveniently applied to seek iobufs without the user having to cast seek up, you need this no_seek function.

read_only is more of a historical convenience now that read_write is a polymorphic variant, as one can now explicitly specify the general type for an argument with something like t : (_ perms, _) t :> (read, _) t.

capacity t returns the size of t's limits subrange. The capacity of an iobuf can be reduced via narrow.

length t returns the size of t's window.

length_lo t returns the length that t's window would have after calling flip_lo, without actually changing the window. This is the number of bytes between the lower limit and the start of the window.

When you're writing to the window, you can think of this as the number of bytes already written. When reading from the window, this can mean the number of bytes already consumed.

This is equivalent to:

Iobuf.Expert.(lo t - lo_min t)

.

length_hi t returns the length that t's window would have after calling flip_hi, without actually changing the window. This is the number of bytes between the end of the window and the upper limit of the buffer.

This is equivalent to:

Iobuf.Expert.(hi_max t - hi t) 

.

is_empty t is length t = 0.

narrow t sets t's limits to the current window.

narrow_lo t sets t's lower limit to the beginning of the current window.

narrow_hi t sets t's upper limit to the end of the current window.

memcmp a b first compares the length of a and b's windows and then compares the bytes in the windows for equivalence.

advance t amount advances the lower bound of the window by amount. It is an error to advance past the upper bound of the window or the lower limit.

unsafe_advance is like advance but with no bounds checking, so incorrect usage can easily cause segfaults.

resize t sets the length of t's window, provided it does not exceed limits.

unsafe_resize is like resize but with no bounds checking, so incorrect usage can easily cause segfaults.

rewind t sets the lower bound of the window to the lower limit.

reset t sets the window to the limits.

flip_lo t sets the window to range from the lower limit to the lower bound of the old window. This is typically called after a series of Fills, to reposition the window in preparation to Consume the newly written data.

The bounded version narrows the effective limit. This can preserve some data near the limit, such as a hypothetical packet header (in the case of bounded_flip_lo) or unfilled suffix of a buffer (in bounded_flip_hi).

compact t copies data from the window to the lower limit of the iobuf and sets the window to range from the end of the copied data to the upper limit. This is typically called after a series of Consumes to save unread data and prepare for the next series of Fills and flip_lo.

flip_hi t sets the window to range from the the upper bound of the current window to the upper limit. This operation is dual to flip_lo and is typically called when the data in the current (narrowed) window has been processed and the window needs to be positioned over the remaining data in the buffer. For example:

(* ... determine initial_data_len ... *)
Iobuf.resize buf ~len:initial_data_len;
(* ... and process initial data ... *)
Iobuf.flip_hi buf;

Now the window of buf ranges over the remainder of the data.

protect_window_and_bounds t ~f calls f t with t's bounds set to its current window, and restores t's window, bounds, and buffer afterward.

protect_window_and_bounds_1 t x ~f is a more efficient version of protect_window_and_bounds t ~f:(fun t -> f t x).

to_string t returns the bytes in t as a string. It does not alter the window.

Equivalent to Hexdump.to_string_hum. Renders t's windows and limits.

to_bytes t returns the bytes in t as a bytes. It does not alter the window.

of_bytes b returns a new iobuf whose contents is b.

Consume.string t ~len reads len characters (all, by default) from t into a new string and advances the lower bound of the window accordingly.

Fill.bin_prot X.bin_write_t t x writes x to t in bin-prot form, advancing past the bytes written.

Peek and Poke functions access a value at pos from the lower bound of the window and do not advance.

Poke.bin_prot X.bin_write_t t x writes x to the beginning of t in binary form without advancing. You can use X.bin_size_t to tell how long it was. X.bin_write_t is only allowed to write that portion of the buffer you have access to.

Unsafe has submodules that are like their corresponding module, except with no range checks. Hence, mistaken uses can cause segfaults. Be careful!

The number of bytes in the length prefix of consume_bin_prot and fill_bin_prot.

fill_bin_prot writes a bin-prot value to the lower bound of the window, prefixed by its length, and advances by the amount written. fill_bin_prot returns an error if the window is too small to write the value.

consume_bin_prot t reader reads a bin-prot value from the lower bound of the window, which should have been written using fill_bin_prot, and advances the window by the amount read. consume_bin_prot returns an error if there is not a complete message in the window and in that case the window is left unchanged.

Don't use these without a good reason, as they are incompatible with similar functions in Reader and Writer. They use a 4-byte length rather than an 8-byte length.

Blit copies between iobufs and advances neither src nor dst.

Blit_consume copies between iobufs and advances src but does not advance dst.

Blit_fill copies between iobufs and advances dst but does not advance src.

Blit_consume_and_fill copies between iobufs and advances both src and dst.

memset t ~pos ~len c fills t with c within the range [pos, pos + len).

memsets a buffer to zero.

The Expert module is for building efficient out-of-module Iobuf abstractions.

('d, 'w) Iobuf.t accessor function manipulating 'a, either writing it to the iobuf or reading it from the iobuf.