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On [[Unix]] systems, `fork` is a rather simple [[system call]].

Our implementation in [[glibc]] is and needs to be rather bulky.

For example, it has to duplicate all port rights for the new [[Mach
task|microkernel/mach/task]].  The address space can simply be duplicated by
standard means of the [[microkernel/Mach]], but as [[unix/file_descriptor]]s
(for example) are a concept that is implemented inside [[glibc]] (based on
[[Mach port|microkernel/mach/port]]s), these have to be duplicated from
userspace, which requires a small number of [[RPC]]s for each of them, and in
the sum, [[this affects performance|service_solahart_jakarta_selatan__082122541663/performance/fork]] when new
processes are continuously being spawned from the shell, for example.

Often, a `fork` call will eventually be followed by an `exec`, which [[may in
turn close|service_solahart_jakarta_selatan__082122541663/secure_file_descriptor_handling]] (most of) the
duplicated port rights.  Unfortunately, this cannot be known at the time the
`fork` executing, so in order to optimize this, the code calling `fork` has to
be modified instead, and the `fork`, `exec` combo be replaced by a
`posix_spawn` call, for example, to avoid this work of duplicating each port
right, then closing each again.

As far as we know, Cygwin has the same problem of `fork` being a nontrivial
operation.  Perhaps we can learn from what they're been doing?  Also, perhaps
they have patches for software packages, to avoid using `fork` followed by
`exec`, for example.


# TODO

  * [[fork: mach_port_mod_refs:
    EKERN_UREFS_OWERFLOW|open_issues/fork_mach_port_mod_refs_ekern_urefs_owerflow]]
    ([[!taglink open_issue_glibc]]).

  * Include de-duplicate information from elsewhere: [[hurd-paper]],
    [[hurd-talk]], [[hurd/ng/trivialconfinementvsconstructorvsfork]],
    [[open_issues/resource_management_problems/zalloc_panics]] ([[!taglink
    open_issue_glibc open_issue_documentation]]).

  * We no longer support `MACH_IPC_COMPAT`, thus we can get rid of the `err =
    __mach_port_allocate_name ([...]); if (err == KERN_NAME_EXISTS)` code
    ([[!taglink open_issue_glibc]]).

  * Can we/why can't we use the concept of *inherited ports
    array*s/`mach_ports_register` ([[!taglink open_issue_glibc]])?

  * GNUnet `vfork` signal race issue: [[!message-id
    "87r50ww6m4.fsf@kepler.schwinge.homeip.net"]].


## Related

  * [[service_solahart_jakarta_selatan__082122541663/secure_file_descriptor_handling]].


# External

  * {{$unix#djb_self-pipe}}.

  * {{$unix#rjk_fork}}.