---
name: "Misc.Magic_number"
section: "3o"
platform: "linux"
package: "ocaml"
version: "5.5.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/Misc.Magic_number"
---

# Misc.Magic\_number(3o)

no description

## NAME

Misc.Magic\_number - no description

## Module

Module   Misc.Magic\_number

## Documentation

Module **Magic\_number**\
  :  **sig end**

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.

All magic numbers have the same byte length,  **magic\_length** , 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:

```
        let ic = open_in_bin path in
        let magic =
          try really_input_string ic Magic_number.magic_length
          with End_of_file -> ... in
        match Magic_number.parse magic with
        | Error parse_error -> ...
        | Ok info -> ...

```

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.

An example of code block that expects the "currently supported version" of a given kind of magic numbers, here  **Cmxa** , is as follows:

```
        let ic = open_in_bin path in
        begin
          try Magic_number.(expect_current Cmxa (get_info ic)) with
          | Parse_error error -> ...
          | Unexpected error -> ...
        end;
        ...

```

Parse errors distinguish inputs that are  **Not\_a\_magic\_number str** , which are likely to come from the file being completely different, and  **Truncated str** , raised by headers that are the (possibly empty) prefix of a valid magic number.

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.

The helper functions  **explain\_parse\_error** and  **explain\_unexpected\_error** will generate a textual explanation of each error, for use in error messages.

*type native\_obj\_config* = {\
  flambda :  **bool** ;\
  }

native object files have a format and magic number that depend on certain native-compiler configuration parameters. This configuration space is expressed by the  **native\_obj\_config** type.

*val native\_obj\_config* :  **native\_obj\_config**

the native object file configuration of the active/configured compiler.

*type version* =  **int**

*type kind* =\
  \| Exec\
  \| Cmi\
  \| Cmo\
  \| Cma\
  \| Cmx **of** **native\_obj\_config**\
  \| Cmxa **of** **native\_obj\_config**\
  \| Cmxs\
  \| Cmt\
  \| Ast\_impl\
  \| Ast\_intf

*type info* = {\
  kind :  **kind** ;\
  version :  **version** ;  (\* Note: some versions of the compiler use the same  **version** 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.\
  \*) \
  }

*type raw* =  **string**

the type of raw magic numbers, such as "Caml1999A027" for the .cma files of OCaml 4.10

### Parsing magic numbers

*type parse\_error* =\
  \| Truncated **of** **string**\
  \| Not\_a\_magic\_number **of** **string**

*val explain\_parse\_error* :  **kind option -\> parse\_error -\> string**

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.

*val parse* :  **raw -\>** **(info, parse\_error) result**

Parses a raw magic number

*val read\_info* :  **in\_channel -\>** **(info, parse\_error) result**

Read a raw magic number from an input channel.

If the data read  **str** is not a valid magic number, it can be recovered from the  **Truncated str \| Not\_a\_magic\_number str** payload of the  **Error parse\_error** case.

If parsing succeeds with an  **Ok info** result, we know that exactly  **magic\_length** bytes have been consumed from the input\_channel.

If you also wish to enforce that the magic number is at the current version, see  **Misc.Magic\_number.read\_current\_info** below.

*val magic\_length* :  **int**

all magic numbers take the same number of bytes

### Checking that magic numbers are current

*type* **'a** *unexpected* = {\
  expected :  **'a** ;\
  actual :  **'a** ;\
  }

*type unexpected\_error* =\
  \| Kind **of** **kind unexpected**\
  \| Version **of** **kind** **\* version unexpected**

*val check\_current* :  **kind -\>** **info -\>** **(unit, unexpected\_error) result**

**check\_current kind info** checks that the provided magic  **info** is the current version of  **kind** 's magic header.

*val explain\_unexpected\_error* :  **unexpected\_error -\> string**

Provides an explanation of the  **unexpected\_error** .

*type error* =\
  \| Parse\_error **of** **parse\_error**\
  \| Unexpected\_error **of** **unexpected\_error**

*val read\_current\_info* :  **expected\_kind:kind option -\>** **in\_channel -\>** **(info, error) result**

Read a magic number as  **read\_info** , and check that it is the current version as its kind. If the  **expected\_kind** argument is  **None** , any kind is accepted.

### Information on magic numbers

*val string\_of\_kind* :  **kind -\> string**

a user-printable string for a kind, eg. "exec" or "cmo", to use in error messages.

*val human\_name\_of\_kind* :  **kind -\> string**

a user-meaningful name for a kind, eg. "executable file" or "bytecode object file", to use in error messages.

*val current\_raw* :  **kind -\> raw**

the current magic number of each kind

*val current\_version* :  **kind -\> version**

the current version of each kind

### Raw representations

Mainly for internal usage and testing.

*type raw\_kind* =  **string**

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

*val parse\_kind* :  **raw\_kind -\> kind option**

parse a raw kind into a kind

*val raw\_kind* :  **kind -\> raw\_kind**

the current raw representation of a kind.

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  **parse\_kind** .

*val raw* :  **info -\> raw**

A valid raw representation of the magic number.

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.

*val all\_kinds* :  **kind list**