Unix.open_process does not expose pid of child

smolkaj · November 13, 2017, 5:56pm

I need to run a process in parallel with OCaml. Unix.open_process provides a nice high-level API for doing so, but unfortunately it does not expose the PID of the forked child process:

github.com

ocaml/ocaml/blob/c5fe6932b2151d0e4426072b4df3510318bc4edc/otherlibs/unix/unix.mli#L784-L788


val open_process : string -> in_channel * out_channel
(** Same as {!Unix.open_process_out}, but redirects both the standard input
and standard output of the command to pipes connected to the two
returned channels.  The input channel is connected to the output
of the command, and the output channel to the input of the command. *)

I am wondering if there is a deeper reason for this, or if this is simply a design flaw? Where would I suggest a change of the API?

I would like the PID so I can check periodically from the parent process if the child is still running. In my context, some work done by the parent thread becomes obsolete in case the child terminates before the parent. It seems that the current API does not allow me to implement that pattern.

vinoski · November 13, 2017, 6:04pm

Use create_process instead.

github.com

ocaml/ocaml/blob/c5fe6932b2151d0e4426072b4df3510318bc4edc/otherlibs/unix/unix.mli#L745-L758


val create_process :
string -> string array -> file_descr -> file_descr -> file_descr -> int
(** [create_process prog args new_stdin new_stdout new_stderr]
 forks a new process that executes the program
 in file [prog], with arguments [args]. The pid of the new
 process is returned immediately; the new process executes
 concurrently with the current process.
 The standard input and outputs of the new process are connected
 to the descriptors [new_stdin], [new_stdout] and [new_stderr].
 Passing e.g. [stdout] for [new_stdout] prevents the redirection
 and causes the new process to have the same standard output
 as the current process.
 The executable file [prog] is searched in the path.
 The new process has the same environment as the current process. *)

–steve

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Unix.open_process does not expose pid of child

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