710320e3dc
Although "macOS" is the correct name for Apple's current desktop OS, we've decided to go with "Mac OS" to cover all of Apple's Unix-like desktop OS versions. The label "Mac OS" is more readable, clear in context (i.e., unlikely to be confused with the Mac OSes that proceeded Mac OS X), and as likely to match Apple's future OS names as anything.
40 lines
1.9 KiB
Racket
40 lines
1.9 KiB
Racket
#lang scribble/doc
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@(require "utils.rkt"
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(for-label racket/system))
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@title{Subprocesses}
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On Unix and Mac OS, subprocess handling involves
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@as-index[@cpp{fork}], @as-index[@cpp{waitpid}], and
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@as-index[@cpp{SIGCHLD}], which creates a variety of issues within an
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embedding application. On Windows, subprocess handling is more
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straightforward, since no @cpp{fork} is required, and since Windows
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provides an abstraction that is a close fit to Racket's subprocess
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values.
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After Racket creates a subprocess via @racket[subprocess] (or
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@racket[system], @racket[process], etc.), it periodically polls the
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process status using @cpp{waitpid}. If the process is created as its
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own group, then the call to @cpp{waitpid} uses the created
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subprocess's process ID; for all other subprocesses, polling uses a
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single call to @cpp{waitpid} with the first argument as @cpp{0}. Using
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@cpp{0}, in particular, can interfere with other libraries in an
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embedding context, so Racket refrains from calling @cpp{waitpid} if no
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subprocesses are pending.
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Racket may or may not rely on a @cpp{SIGCHLD} handler, and it may or
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may not block @cpp{SIGCHLD}. Currently, when Racket is compiled to
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support @|tech-place|s, Racket blocks @cpp{SIGCHLD} on start up with
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the expectation that all created threads have @cpp{SIGCHLD} blocked.
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When Racket is not compiled to support @|tech-place|s, then a
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@cpp{SIGCHLD} handler is installed.
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Using @cpp{fork} in an application that embeds Racket is problematic
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for several reasons: Racket may install a @cpp{SIGALRM} handler and
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schedule alarms to implement context switches, it may have file
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descriptors open that should be closed in a child process, and it may
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have changed the disposition of signals such as
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@cpp{SIGCHLD}. Consequently, embedding Racket in a process that
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@cpp{fork}s is technically not supported; in the future, Racket may
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provide better support for such applications.
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