1Misc.Magic_number(3)             OCaml library            Misc.Magic_number(3)
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NAME

6       Misc.Magic_number - no description
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Module

9       Module   Misc.Magic_number
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Documentation

12       Module Magic_number
13        : sig end
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23       a  typical  magic number is "Caml1999I011"; it is formed of an alphanu‐
24       meric prefix, here Caml1990I, followed by a version, here 011. The pre‐
25       fix  identifies  the  kind  of the versioned data: here the I indicates
26       that it is the magic number for .cmi files.
27
28       All magic numbers have the same byte length, magic_length , and this is
29       important for users as it gives them the number of bytes to read to ob‐
30       tain the byte sequence that should be a magic number. Typical user code
31       will look like:
32               let ic = open_in_bin path in
33               let magic =
34                 try really_input_string ic Magic_number.magic_length
35                 with End_of_file -> ... in
36               match Magic_number.parse magic with
37               | Error parse_error -> ...
38               | Ok info -> ...
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40
41       A  given compiler version expects one specific version for each kind of
42       object file, and will fail if given  an  unsupported  version.  Because
43       versions  grow  monotonically,  you can compare the parsed version with
44       the expected  "current  version"  for  a  kind,  to  tell  whether  the
45       wrong-magic object file comes from the past or from the future.
46
47       An example of code block that expects the "currently supported version"
48       of a given kind of magic numbers, here Cmxa , is as follows:
49               let ic = open_in_bin path in
50               begin
51                 try Magic_number.(expect_current Cmxa (get_info ic)) with
52                 | Parse_error error -> ...
53                 | Unexpected error -> ...
54               end;
55               ...
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57
58       Parse errors distinguish inputs that are Not_a_magic_number str , which
59       are  likely to come from the file being completely different, and Trun‐
60       cated str , raised by headers that are the (possibly empty) prefix of a
61       valid magic number.
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63       Unexpected  errors  correspond  to valid magic numbers that are not the
64       one expected, either because it corresponds to a different kind, or  to
65       a newer or older version.
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67       The  helper  functions explain_parse_error and explain_unexpected_error
68       will generate a textual explanation of each error,  for  use  in  error
69       messages.
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71       type native_obj_config = {
72        flambda : bool ;
73        }
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76       native  object files have a format and magic number that depend on cer‐
77       tain native-compiler configuration parameters. This configuration space
78       is expressed by the native_obj_config type.
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82       val native_obj_config : native_obj_config
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84       the native object file configuration of the active/configured compiler.
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87       type version = int
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92       type kind =
93        | Exec
94        | Cmi
95        | Cmo
96        | Cma
97        | Cmx of native_obj_config
98        | Cmxa of native_obj_config
99        | Cmxs
100        | Cmt
101        | Ast_impl
102        | Ast_intf
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107       type info = {
108        kind : kind ;
109        version  :  version  ;  (* Note: some versions of the compiler use the
110       same version suffix for all kinds, but others  use  different  versions
111       counters  for  different  kinds.  We  may only assume that versions are
112       growing monotonically (not necessarily always by one) between  compiler
113       versions.
114        *)
115        }
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120       type raw = string
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123       the  type  of  raw  magic  numbers, such as "Caml1999A027" for the .cma
124       files of OCaml 4.10
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129   Parsing magic numbers
130       type parse_error =
131        | Truncated of string
132        | Not_a_magic_number of string
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138       val explain_parse_error : kind option -> parse_error -> string
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140       Produces an explanation for a parse error. If no kind is  provided,  we
141       use an unspecific formulation suggesting that any compiler-produced ob‐
142       ject file would have been satisfying.
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146       val parse : raw -> (info, parse_error) result
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148       Parses a raw magic number
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152       val read_info : in_channel -> (info, parse_error) result
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154       Read a raw magic number from an input channel.
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156       If the data read str is not a valid magic number, it can  be  recovered
157       from  the  Truncated  str | Not_a_magic_number str payload of the Error
158       parse_error case.
159
160       If parsing succeeds with an  Ok  info  result,  we  know  that  exactly
161       magic_length bytes have been consumed from the input_channel.
162
163       If  you  also  wish  to enforce that the magic number is at the current
164       version, see Misc.Magic_number.read_current_info below.
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168       val magic_length : int
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170       all magic numbers take the same number of bytes
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175   Checking that magic numbers are current
176       type 'a unexpected = {
177        expected : 'a ;
178        actual : 'a ;
179        }
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184       type unexpected_error =
185        | Kind of kind unexpected
186        | Version of kind * version unexpected
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192       val check_current : kind -> info -> (unit, unexpected_error) result
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195       check_current kind info checks that the provided magic info is the cur‐
196       rent version of kind 's magic header.
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200       val explain_unexpected_error : unexpected_error -> string
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202       Provides an explanation of the unexpected_error .
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205       type error =
206        | Parse_error of parse_error
207        | Unexpected_error of unexpected_error
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213       val  read_current_info  :  expected_kind:kind  option  -> in_channel ->
214       (info, error) result
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216       Read a magic number as read_info , and check that  it  is  the  current
217       version  as its kind.  If the expected_kind argument is None , any kind
218       is accepted.
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223   Information on magic numbers
224       val string_of_kind : kind -> string
225
226       a user-printable string for a kind, eg. "exec" or "cmo", to use in  er‐
227       ror messages.
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231       val human_name_of_kind : kind -> string
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233       a  user-meaningful  name for a kind, eg. "executable file" or "bytecode
234       object file", to use in error messages.
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238       val current_raw : kind -> raw
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240       the current magic number of each kind
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244       val current_version : kind -> version
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246       the current version of each kind
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251   Raw representations
252       Mainly for internal usage and testing.
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254       type raw_kind = string
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257       the type of raw magic numbers kinds, such as "Caml1999A" for .cma files
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261       val parse_kind : raw_kind -> kind option
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263       parse a raw kind into a kind
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267       val raw_kind : kind -> raw_kind
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269       the current raw representation of a kind.
270
271       In some cases the raw representation of a kind has  changed  over  com‐
272       piler  versions, so other files of the same kind may have different raw
273       kinds.  Note that all currently known cases  are  parsed  correctly  by
274       parse_kind .
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278       val raw : info -> raw
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280       A valid raw representation of the magic number.
281
282       Due  to  past  and future changes in the string representation of magic
283       numbers, we cannot guarantee that the raw strings returned for past and
284       future versions actually match the expectations of those compilers. The
285       representation is accurate for current versions, and  it  is  correctly
286       parsed back into the desired version by the parsing functions above.
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292OCamldoc                          2022-07-22              Misc.Magic_number(3)
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