.TH "Misc.Magic_number" 3o 2023-09-18 OCamldoc "OCaml library"
.SH NAME
Misc.Magic_number \- no description
.SH Module
Module   Misc.Magic_number
.SH Documentation
.sp
Module
.BI "Magic_number"
 : 
.B sig end

.sp

.sp

.sp
.sp

.PP
a typical magic number is "Caml1999I011"; it is formed of an
alphanumeric prefix, here Caml1990I, followed by a version,
here 011\&. The prefix identifies the kind of the versioned data:
here the I indicates that it is the magic number for \&.cmi files\&.
.sp
All magic numbers have the same byte length, 
.ft B
magic_length
.ft R
, and
this is important for users as it gives them the number of bytes
to read to obtain the byte sequence that should be a magic
number\&. Typical user code will look like:
.EX
.ft B
.br
\&        let ic = open_in_bin path in
.br
\&        let magic =
.br
\&          try really_input_string ic Magic_number\&.magic_length
.br
\&          with End_of_file \-> \&.\&.\&. in
.br
\&        match Magic_number\&.parse magic with
.br
\&        | Error parse_error \-> \&.\&.\&.
.br
\&        | Ok info \-> \&.\&.\&.
.br
\&      
.ft R
.EE
.sp
A given compiler version expects one specific version for each
kind of object file, and will fail if given an unsupported
version\&. Because versions grow monotonically, you can compare
the parsed version with the expected "current version" for
a kind, to tell whether the wrong\-magic object file comes from
the past or from the future\&.
.sp
An example of code block that expects the "currently supported version"
of a given kind of magic numbers, here 
.ft B
Cmxa
.ft R
, is as follows:
.EX
.ft B
.br
\&        let ic = open_in_bin path in
.br
\&        begin
.br
\&          try Magic_number\&.(expect_current Cmxa (get_info ic)) with
.br
\&          | Parse_error error \-> \&.\&.\&.
.br
\&          | Unexpected error \-> \&.\&.\&.
.br
\&        end;
.br
\&        \&.\&.\&.
.br
\&      
.ft R
.EE
.sp
Parse errors distinguish inputs that are 
.ft B
Not_a_magic_number str
.ft R
,
which are likely to come from the file being completely
different, and 
.ft B
Truncated str
.ft R
, raised by headers that are the
(possibly empty) prefix of a valid magic number\&.
.sp
Unexpected errors correspond to valid magic numbers that are not
the one expected, either because it corresponds to a different
kind, or to a newer or older version\&.
.sp
The helper functions 
.ft B
explain_parse_error
.ft R
and 
.ft B
explain_unexpected_error
.ft R
will generate a textual explanation of each error,
for use in error messages\&.
.PP
.I type native_obj_config 
= {
 flambda : 
.B bool
;
 }

.sp
native object files have a format and magic number that depend
on certain native\-compiler configuration parameters\&. This
configuration space is expressed by the 
.ft B
native_obj_config
.ft R
type\&.

.sp

.I val native_obj_config 
: 
.B native_obj_config
.sp
the native object file configuration of the active/configured compiler\&.

.sp
.I type version 
= 
.B int

.sp

.sp
.I type kind 
=
 | Exec
 | Cmi
 | Cmo
 | Cma
 | Cmx
.B of 
.B native_obj_config
 | Cmxa
.B of 
.B native_obj_config
 | Cmxs
 | Cmt
 | Ast_impl
 | Ast_intf
 
.sp

.sp
.I type info 
= {
 kind : 
.B kind
;
 version : 
.B version
;  (* Note: some versions of the compiler use the same 
.ft B
version
.ft R
suffix
for all kinds, but others use different versions counters for different
kinds\&. We may only assume that versions are growing monotonically
(not necessarily always by one) between compiler versions\&.
 *) 
 }

.sp

.sp
.I type raw 
= 
.B string

.sp
the type of raw magic numbers,
such as "Caml1999A027" for the \&.cma files of OCaml 4\&.10

.sp

.PP
.SS Parsing magic numbers

.PP
.I type parse_error 
=
 | Truncated
.B of 
.B string
 | Not_a_magic_number
.B of 
.B string
 
.sp

.sp

.I val explain_parse_error 
: 
.B kind option -> parse_error -> string
.sp
Produces an explanation for a parse error\&. If no kind is provided,
we use an unspecific formulation suggesting that any compiler\-produced
object file would have been satisfying\&.

.sp

.I val parse 
: 
.B raw ->
.B   (info, parse_error) result
.sp
Parses a raw magic number

.sp

.I val read_info 
: 
.B in_channel ->
.B   (info, parse_error) result
.sp
Read a raw magic number from an input channel\&.
.sp
If the data read 
.ft B
str
.ft R
is not a valid magic number, it can be
recovered from the 
.ft B
Truncated str | Not_a_magic_number str
.ft R
payload of the 
.ft B
Error parse_error
.ft R
case\&.
.sp
If parsing succeeds with an 
.ft B
Ok info
.ft R
result, we know that
exactly 
.ft B
magic_length
.ft R
bytes have been consumed from the
input_channel\&.
.sp
If you also wish to enforce that the magic number
is at the current version, see 
.ft B
Misc\&.Magic_number\&.read_current_info
.ft R
below\&.

.sp

.I val magic_length 
: 
.B int
.sp
all magic numbers take the same number of bytes

.sp

.PP
.SS Checking that magic numbers are current

.PP
.I type 
.B 'a
.I unexpected 
= {
 expected : 
.B 'a
;
 actual : 
.B 'a
;
 }

.sp

.sp
.I type unexpected_error 
=
 | Kind
.B of 
.B kind unexpected
 | Version
.B of 
.B kind
.B    * version unexpected
 
.sp

.sp

.I val check_current 
: 
.B kind ->
.B   info ->
.B   (unit, unexpected_error) result
.sp

.ft B
check_current kind info
.ft R
checks that the provided magic 
.ft B
info
.ft R
is the current version of 
.ft B
kind
.ft R
\&'s magic header\&.

.sp

.I val explain_unexpected_error 
: 
.B unexpected_error -> string
.sp
Provides an explanation of the 
.ft B
unexpected_error
.ft R
\&.

.sp
.I type error 
=
 | Parse_error
.B of 
.B parse_error
 | Unexpected_error
.B of 
.B unexpected_error
 
.sp

.sp

.I val read_current_info 
: 
.B expected_kind:kind option ->
.B   in_channel ->
.B   (info, error) result
.sp
Read a magic number as 
.ft B
read_info
.ft R
,
and check that it is the current version as its kind\&.
If the 
.ft B
expected_kind
.ft R
argument is 
.ft B
None
.ft R
, any kind is accepted\&.

.sp

.PP
.SS Information on magic numbers

.PP

.I val string_of_kind 
: 
.B kind -> string
.sp
a user\-printable string for a kind, eg\&. "exec" or "cmo", to use
in error messages\&.

.sp

.I val human_name_of_kind 
: 
.B kind -> string
.sp
a user\-meaningful name for a kind, eg\&. "executable file" or
"bytecode object file", to use in error messages\&.

.sp

.I val current_raw 
: 
.B kind -> raw
.sp
the current magic number of each kind

.sp

.I val current_version 
: 
.B kind -> version
.sp
the current version of each kind

.sp

.PP
.SS Raw representations
.sp
Mainly for internal usage and testing\&.
.PP
.I type raw_kind 
= 
.B string

.sp
the type of raw magic numbers kinds,
such as "Caml1999A" for \&.cma files

.sp

.I val parse_kind 
: 
.B raw_kind -> kind option
.sp
parse a raw kind into a kind

.sp

.I val raw_kind 
: 
.B kind -> raw_kind
.sp
the current raw representation of a kind\&.
.sp
In some cases the raw representation of a kind has changed
over compiler versions, so other files of the same kind
may have different raw kinds\&.
Note that all currently known cases are parsed correctly by 
.ft B
parse_kind
.ft R
\&.

.sp

.I val raw 
: 
.B info -> raw
.sp
A valid raw representation of the magic number\&.
.sp
Due to past and future changes in the string representation of
magic numbers, we cannot guarantee that the raw strings returned
for past and future versions actually match the expectations of
those compilers\&. The representation is accurate for current
versions, and it is correctly parsed back into the desired
version by the parsing functions above\&.

.sp