[14489] | 1 | =head1 NAME
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| 2 |
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| 3 | perlfork - Perl's fork() emulation
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| 4 |
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| 5 | =head1 SYNOPSIS
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| 6 |
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| 7 | NOTE: As of the 5.8.0 release, fork() emulation has considerably
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| 8 | matured. However, there are still a few known bugs and differences
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| 9 | from real fork() that might affect you. See the "BUGS" and
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| 10 | "CAVEATS AND LIMITATIONS" sections below.
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| 11 |
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| 12 | Perl provides a fork() keyword that corresponds to the Unix system call
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| 13 | of the same name. On most Unix-like platforms where the fork() system
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| 14 | call is available, Perl's fork() simply calls it.
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| 15 |
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| 16 | On some platforms such as Windows where the fork() system call is not
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| 17 | available, Perl can be built to emulate fork() at the interpreter level.
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| 18 | While the emulation is designed to be as compatible as possible with the
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| 19 | real fork() at the level of the Perl program, there are certain
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| 20 | important differences that stem from the fact that all the pseudo child
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| 21 | "processes" created this way live in the same real process as far as the
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| 22 | operating system is concerned.
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| 23 |
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| 24 | This document provides a general overview of the capabilities and
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| 25 | limitations of the fork() emulation. Note that the issues discussed here
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| 26 | are not applicable to platforms where a real fork() is available and Perl
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| 27 | has been configured to use it.
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| 28 |
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| 29 | =head1 DESCRIPTION
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| 30 |
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| 31 | The fork() emulation is implemented at the level of the Perl interpreter.
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| 32 | What this means in general is that running fork() will actually clone the
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| 33 | running interpreter and all its state, and run the cloned interpreter in
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| 34 | a separate thread, beginning execution in the new thread just after the
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| 35 | point where the fork() was called in the parent. We will refer to the
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| 36 | thread that implements this child "process" as the pseudo-process.
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| 37 |
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| 38 | To the Perl program that called fork(), all this is designed to be
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| 39 | transparent. The parent returns from the fork() with a pseudo-process
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| 40 | ID that can be subsequently used in any process manipulation functions;
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| 41 | the child returns from the fork() with a value of C<0> to signify that
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| 42 | it is the child pseudo-process.
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| 43 |
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| 44 | =head2 Behavior of other Perl features in forked pseudo-processes
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| 45 |
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| 46 | Most Perl features behave in a natural way within pseudo-processes.
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| 47 |
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| 48 | =over 8
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| 49 |
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| 50 | =item $$ or $PROCESS_ID
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| 51 |
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| 52 | This special variable is correctly set to the pseudo-process ID.
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| 53 | It can be used to identify pseudo-processes within a particular
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| 54 | session. Note that this value is subject to recycling if any
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| 55 | pseudo-processes are launched after others have been wait()-ed on.
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| 56 |
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| 57 | =item %ENV
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| 58 |
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| 59 | Each pseudo-process maintains its own virtual environment. Modifications
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| 60 | to %ENV affect the virtual environment, and are only visible within that
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| 61 | pseudo-process, and in any processes (or pseudo-processes) launched from
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| 62 | it.
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| 63 |
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| 64 | =item chdir() and all other builtins that accept filenames
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| 65 |
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| 66 | Each pseudo-process maintains its own virtual idea of the current directory.
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| 67 | Modifications to the current directory using chdir() are only visible within
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| 68 | that pseudo-process, and in any processes (or pseudo-processes) launched from
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| 69 | it. All file and directory accesses from the pseudo-process will correctly
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| 70 | map the virtual working directory to the real working directory appropriately.
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| 71 |
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| 72 | =item wait() and waitpid()
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| 73 |
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| 74 | wait() and waitpid() can be passed a pseudo-process ID returned by fork().
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| 75 | These calls will properly wait for the termination of the pseudo-process
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| 76 | and return its status.
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| 77 |
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| 78 | =item kill()
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| 79 |
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| 80 | kill() can be used to terminate a pseudo-process by passing it the ID returned
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| 81 | by fork(). This should not be used except under dire circumstances, because
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| 82 | the operating system may not guarantee integrity of the process resources
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| 83 | when a running thread is terminated. Note that using kill() on a
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| 84 | pseudo-process() may typically cause memory leaks, because the thread that
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| 85 | implements the pseudo-process does not get a chance to clean up its resources.
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| 86 |
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| 87 | =item exec()
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| 88 |
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| 89 | Calling exec() within a pseudo-process actually spawns the requested
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| 90 | executable in a separate process and waits for it to complete before
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| 91 | exiting with the same exit status as that process. This means that the
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| 92 | process ID reported within the running executable will be different from
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| 93 | what the earlier Perl fork() might have returned. Similarly, any process
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| 94 | manipulation functions applied to the ID returned by fork() will affect the
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| 95 | waiting pseudo-process that called exec(), not the real process it is
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| 96 | waiting for after the exec().
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| 97 |
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| 98 | =item exit()
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| 99 |
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| 100 | exit() always exits just the executing pseudo-process, after automatically
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| 101 | wait()-ing for any outstanding child pseudo-processes. Note that this means
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| 102 | that the process as a whole will not exit unless all running pseudo-processes
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| 103 | have exited.
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| 104 |
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| 105 | =item Open handles to files, directories and network sockets
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| 106 |
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| 107 | All open handles are dup()-ed in pseudo-processes, so that closing
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| 108 | any handles in one process does not affect the others. See below for
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| 109 | some limitations.
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| 110 |
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| 111 | =back
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| 112 |
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| 113 | =head2 Resource limits
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| 114 |
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| 115 | In the eyes of the operating system, pseudo-processes created via the fork()
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| 116 | emulation are simply threads in the same process. This means that any
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| 117 | process-level limits imposed by the operating system apply to all
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| 118 | pseudo-processes taken together. This includes any limits imposed by the
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| 119 | operating system on the number of open file, directory and socket handles,
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| 120 | limits on disk space usage, limits on memory size, limits on CPU utilization
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| 121 | etc.
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| 122 |
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| 123 | =head2 Killing the parent process
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| 124 |
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| 125 | If the parent process is killed (either using Perl's kill() builtin, or
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| 126 | using some external means) all the pseudo-processes are killed as well,
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| 127 | and the whole process exits.
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| 128 |
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| 129 | =head2 Lifetime of the parent process and pseudo-processes
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| 130 |
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| 131 | During the normal course of events, the parent process and every
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| 132 | pseudo-process started by it will wait for their respective pseudo-children
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| 133 | to complete before they exit. This means that the parent and every
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| 134 | pseudo-child created by it that is also a pseudo-parent will only exit
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| 135 | after their pseudo-children have exited.
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| 136 |
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| 137 | A way to mark a pseudo-processes as running detached from their parent (so
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| 138 | that the parent would not have to wait() for them if it doesn't want to)
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| 139 | will be provided in future.
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| 140 |
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| 141 | =head2 CAVEATS AND LIMITATIONS
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| 142 |
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| 143 | =over 8
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| 144 |
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| 145 | =item BEGIN blocks
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| 146 |
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| 147 | The fork() emulation will not work entirely correctly when called from
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| 148 | within a BEGIN block. The forked copy will run the contents of the
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| 149 | BEGIN block, but will not continue parsing the source stream after the
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| 150 | BEGIN block. For example, consider the following code:
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| 151 |
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| 152 | BEGIN {
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| 153 | fork and exit; # fork child and exit the parent
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| 154 | print "inner\n";
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| 155 | }
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| 156 | print "outer\n";
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| 157 |
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| 158 | This will print:
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| 159 |
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| 160 | inner
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| 161 |
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| 162 | rather than the expected:
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| 163 |
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| 164 | inner
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| 165 | outer
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| 166 |
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| 167 | This limitation arises from fundamental technical difficulties in
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| 168 | cloning and restarting the stacks used by the Perl parser in the
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| 169 | middle of a parse.
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| 170 |
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| 171 | =item Open filehandles
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| 172 |
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| 173 | Any filehandles open at the time of the fork() will be dup()-ed. Thus,
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| 174 | the files can be closed independently in the parent and child, but beware
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| 175 | that the dup()-ed handles will still share the same seek pointer. Changing
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| 176 | the seek position in the parent will change it in the child and vice-versa.
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| 177 | One can avoid this by opening files that need distinct seek pointers
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| 178 | separately in the child.
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| 179 |
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| 180 | =item Forking pipe open() not yet implemented
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| 181 |
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| 182 | The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
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| 183 | implemented. This limitation can be easily worked around in new code
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| 184 | by creating a pipe explicitly. The following example shows how to
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| 185 | write to a forked child:
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| 186 |
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| 187 | # simulate open(FOO, "|-")
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| 188 | sub pipe_to_fork ($) {
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| 189 | my $parent = shift;
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| 190 | pipe my $child, $parent or die;
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| 191 | my $pid = fork();
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| 192 | die "fork() failed: $!" unless defined $pid;
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| 193 | if ($pid) {
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| 194 | close $child;
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| 195 | }
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| 196 | else {
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| 197 | close $parent;
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| 198 | open(STDIN, "<&=" . fileno($child)) or die;
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| 199 | }
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| 200 | $pid;
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| 201 | }
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| 202 |
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| 203 | if (pipe_to_fork('FOO')) {
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| 204 | # parent
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| 205 | print FOO "pipe_to_fork\n";
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| 206 | close FOO;
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| 207 | }
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| 208 | else {
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| 209 | # child
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| 210 | while (<STDIN>) { print; }
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| 211 | exit(0);
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| 212 | }
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| 213 |
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| 214 | And this one reads from the child:
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| 215 |
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| 216 | # simulate open(FOO, "-|")
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| 217 | sub pipe_from_fork ($) {
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| 218 | my $parent = shift;
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| 219 | pipe $parent, my $child or die;
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| 220 | my $pid = fork();
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| 221 | die "fork() failed: $!" unless defined $pid;
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| 222 | if ($pid) {
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| 223 | close $child;
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| 224 | }
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| 225 | else {
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| 226 | close $parent;
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| 227 | open(STDOUT, ">&=" . fileno($child)) or die;
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| 228 | }
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| 229 | $pid;
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| 230 | }
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| 231 |
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| 232 | if (pipe_from_fork('BAR')) {
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| 233 | # parent
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| 234 | while (<BAR>) { print; }
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| 235 | close BAR;
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| 236 | }
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| 237 | else {
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| 238 | # child
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| 239 | print "pipe_from_fork\n";
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| 240 | exit(0);
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| 241 | }
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| 242 |
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| 243 | Forking pipe open() constructs will be supported in future.
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| 244 |
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| 245 | =item Global state maintained by XSUBs
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| 246 |
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| 247 | External subroutines (XSUBs) that maintain their own global state may
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| 248 | not work correctly. Such XSUBs will either need to maintain locks to
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| 249 | protect simultaneous access to global data from different pseudo-processes,
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| 250 | or maintain all their state on the Perl symbol table, which is copied
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| 251 | naturally when fork() is called. A callback mechanism that provides
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| 252 | extensions an opportunity to clone their state will be provided in the
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| 253 | near future.
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| 254 |
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| 255 | =item Interpreter embedded in larger application
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| 256 |
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| 257 | The fork() emulation may not behave as expected when it is executed in an
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| 258 | application which embeds a Perl interpreter and calls Perl APIs that can
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| 259 | evaluate bits of Perl code. This stems from the fact that the emulation
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| 260 | only has knowledge about the Perl interpreter's own data structures and
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| 261 | knows nothing about the containing application's state. For example, any
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| 262 | state carried on the application's own call stack is out of reach.
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| 263 |
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| 264 | =item Thread-safety of extensions
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| 265 |
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| 266 | Since the fork() emulation runs code in multiple threads, extensions
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| 267 | calling into non-thread-safe libraries may not work reliably when
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| 268 | calling fork(). As Perl's threading support gradually becomes more
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| 269 | widely adopted even on platforms with a native fork(), such extensions
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| 270 | are expected to be fixed for thread-safety.
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| 271 |
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| 272 | =back
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| 273 |
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| 274 | =head1 BUGS
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| 275 |
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| 276 | =over 8
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| 277 |
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| 278 | =item *
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| 279 |
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| 280 | Having pseudo-process IDs be negative integers breaks down for the integer
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| 281 | C<-1> because the wait() and waitpid() functions treat this number as
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| 282 | being special. The tacit assumption in the current implementation is that
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| 283 | the system never allocates a thread ID of C<1> for user threads. A better
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| 284 | representation for pseudo-process IDs will be implemented in future.
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| 285 |
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| 286 | =item *
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| 287 |
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| 288 | In certain cases, the OS-level handles created by the pipe(), socket(),
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| 289 | and accept() operators are apparently not duplicated accurately in
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| 290 | pseudo-processes. This only happens in some situations, but where it
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| 291 | does happen, it may result in deadlocks between the read and write ends
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| 292 | of pipe handles, or inability to send or receive data across socket
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| 293 | handles.
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| 294 |
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| 295 | =item *
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| 296 |
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| 297 | This document may be incomplete in some respects.
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| 298 |
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| 299 | =back
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| 300 |
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| 301 | =head1 AUTHOR
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| 302 |
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| 303 | Support for concurrent interpreters and the fork() emulation was implemented
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| 304 | by ActiveState, with funding from Microsoft Corporation.
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| 305 |
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| 306 | This document is authored and maintained by Gurusamy Sarathy
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| 307 | E<lt>[email protected]<gt>.
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| 308 |
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| 309 | =head1 SEE ALSO
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| 310 |
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| 311 | L<perlfunc/"fork">, L<perlipc>
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| 312 |
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| 313 | =cut
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