1Proc::Fork(3)         User Contributed Perl Documentation        Proc::Fork(3)
2
3
4

NAME

6       Proc::Fork - simple, intuitive interface to the fork() system call
7

SYNOPSIS

9        use Proc::Fork;
10
11        run_fork {
12            child {
13                # child code goes here.
14            }
15            parent {
16                my $child_pid = shift;
17                # parent code goes here.
18                waitpid $child_pid, 0;
19            }
20            retry {
21                my $attempts = shift;
22                # what to do if fork() fails:
23                # return true to try again, false to abort
24                return if $attempts > 5;
25                sleep 1, return 1;
26            }
27            error {
28                # Error-handling code goes here
29                # (fork() failed and the retry block returned false)
30            }
31        };
32

DESCRIPTION

34       This module provides an intuitive, Perl-ish way to write forking
35       programs by letting you use blocks to illustrate which code section
36       executes in which fork. The code for the parent, child, retry handler
37       and error handler are grouped together in a "fork block". The clauses
38       may appear in any order, but they must be consecutive (without any
39       other statements in between).
40
41       All four clauses need not be specified. If the retry clause is omitted,
42       only one fork will be attempted. If the error clause is omitted the
43       program will die with a simple message if it can't retry. If the parent
44       or child clause is omitted, the respective (parent or child) process
45       will start execution after the final clause. So if one or the other
46       only has to do some simple action, you need only specify that one. For
47       example:
48
49        # spawn off a child process to do some simple processing
50        run_fork { child {
51            exec '/bin/ls', '-l';
52            die "Couldn't exec ls: $!\n";
53        } };
54        # Parent will continue execution from here
55        # ...
56
57       If the code in any of the clauses does not die or exit, it will
58       continue execution after the fork block.
59

INTERFACE

61       All of the following functions are exported by default:
62
63   run_fork
64        run_fork { ... }
65
66       Performs the fork operation configured in its block.
67
68   child
69        child { ... }
70
71       Declares the block that should run in the child process.
72
73   parent
74        parent { ... }
75
76       Declares the block that should run in the parent process. The child's
77       PID is passed as an argument to the block.
78
79   retry
80        retry { ... }
81
82       Declares the block that should run in case of an error, ie. if "fork"
83       returned "undef". If the code returns true, another "fork" is
84       attempted. The number of fork attempts so far is passed as an argument
85       to the block.
86
87       This can be used to implement a wait-and-retry logic that may be
88       essential for some applications like daemons.
89
90       If a "retry" clause is not used, no retries will be attempted and a
91       fork failure will immediately lead to the "error" clause being called.
92
93   error
94        error { ... }
95
96       Declares the block that should run if there was an error, ie when
97       "fork" returns "undef" and the "retry" clause returns false. The number
98       of forks attempted is passed as an argument to the block.
99
100       If an "error" clause is not used, errors will raise an exception using
101       "die".
102

EXAMPLES

104       The distribution includes the following examples as separate files in
105       the eg/ directory:
106
107   Simple example with IPC via pipe
108        use strict;
109        use Proc::Fork;
110
111        use IO::Pipe;
112        my $p = IO::Pipe->new;
113
114        run_fork {
115            parent {
116                my $child = shift;
117                $p->reader;
118                print while <$p>;
119                waitpid $child,0;
120            }
121            child {
122                $p->writer;
123                print $p "Line 1\n";
124                print $p "Line 2\n";
125                exit;
126            }
127            retry {
128                if( $_[0] < 5 ) {
129                    sleep 1;
130                    return 1;
131                }
132                return 0;
133            }
134            error {
135                die "That's all folks\n";
136            }
137        };
138
139   Multi-child example
140        use strict;
141        use Proc::Fork;
142        use IO::Pipe;
143
144        my $num_children = 5;    # How many children we'll create
145        my @children;            # Store connections to them
146        $SIG{CHLD} = 'IGNORE';   # Don't worry about reaping zombies
147
148        # Spawn off some children
149        for my $num ( 1 .. $num_children ) {
150            # Create a pipe for parent-child communication
151            my $pipe = IO::Pipe->new;
152
153            # Child simply echoes data it receives, until EOF
154            run_fork { child {
155                $pipe->reader;
156                my $data;
157                while ( $data = <$pipe> ) {
158                    chomp $data;
159                    print STDERR "child $num: [$data]\n";
160                }
161                exit;
162            } };
163
164            # Parent here
165            $pipe->writer;
166            push @children, $pipe;
167        }
168
169        # Send some data to the kids
170        for ( 1 .. 20 ) {
171            # pick a child at random
172            my $num = int rand $num_children;
173            my $child = $children[$num];
174            print $child "Hey there.\n";
175        }
176
177   Daemon example
178        use strict;
179        use Proc::Fork;
180        use POSIX;
181
182        # One-stop shopping: fork, die on error, parent process exits.
183        run_fork { parent { exit } };
184
185        # Other daemon initialization activities.
186        $SIG{INT} = $SIG{TERM} = $SIG{HUP} = $SIG{PIPE} = \&some_signal_handler;
187        POSIX::setsid() == -1 and die "Cannot start a new session: $!\n";
188        close $_ for *STDIN, *STDOUT, *STDERR;
189
190        # rest of daemon program follows
191
192   Forking socket-based network server example
193        use strict;
194        use IO::Socket::INET;
195        use Proc::Fork;
196
197        $SIG{CHLD} = 'IGNORE';
198
199        my $server = IO::Socket::INET->new(
200            LocalPort => 7111,
201            Type      => SOCK_STREAM,
202            Reuse     => 1,
203            Listen    => 10,
204        ) or die "Couln't start server: $!\n";
205
206        my $client;
207        while ($client = $server->accept) {
208            run_fork { child {
209                # Service the socket
210                sleep(10);
211                print $client "Ooga! ", time % 1000, "\n";
212                exit; # child exits. Parent loops to accept another connection.
213            } }
214        }
215

AUTHOR

217       Aristotle Pagaltzis <pagaltzis@gmx.de>
218
219       Documentation by Eric J. Roode.
220
222       This documentation is copyright (c) 2002 by Eric J. Roode.
223
224       This software is copyright (c) 2018 by Aristotle Pagaltzis.
225
226       This is free software; you can redistribute it and/or modify it under
227       the same terms as the Perl 5 programming language system itself.
228
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231perl v5.36.0                      2022-08-10                     Proc::Fork(3)
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