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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "IPC::Open2 3" | |
132 | .TH IPC::Open2 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | IPC::Open2, open2 \- open a process for both reading and writing | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 1 | |
138 | \& use IPC::Open2; | |
139 | .Ve | |
140 | .PP | |
141 | .Vb 3 | |
142 | \& $pid = open2(\e*CHLD_OUT, \e*CHLD_IN, 'some cmd and args'); | |
143 | \& # or without using the shell | |
144 | \& $pid = open2(\e*CHLD_OUT, \e*CHLD_IN, 'some', 'cmd', 'and', 'args'); | |
145 | .Ve | |
146 | .PP | |
147 | .Vb 5 | |
148 | \& # or with handle autovivification | |
149 | \& my($chld_out, $chld_in); | |
150 | \& $pid = open2($chld_out, $chld_in, 'some cmd and args'); | |
151 | \& # or without using the shell | |
152 | \& $pid = open2($chld_out, $chld_in, 'some', 'cmd', 'and', 'args'); | |
153 | .Ve | |
154 | .SH "DESCRIPTION" | |
155 | .IX Header "DESCRIPTION" | |
156 | The \fIopen2()\fR function runs the given \f(CW$cmd\fR and connects \f(CW$chld_out\fR for | |
157 | reading and \f(CW$chld_in\fR for writing. It's what you think should work | |
158 | when you try | |
159 | .PP | |
160 | .Vb 1 | |
161 | \& $pid = open(HANDLE, "|cmd args|"); | |
162 | .Ve | |
163 | .PP | |
164 | The write filehandle will have autoflush turned on. | |
165 | .PP | |
166 | If \f(CW$chld_out\fR is a string (that is, a bareword filehandle rather than a glob | |
167 | or a reference) and it begins with \f(CW\*(C`>&\*(C'\fR, then the child will send output | |
168 | directly to that file handle. If \f(CW$chld_in\fR is a string that begins with | |
169 | \&\f(CW\*(C`<&\*(C'\fR, then \f(CW$chld_in\fR will be closed in the parent, and the child will | |
170 | read from it directly. In both cases, there will be a \fIdup\fR\|(2) instead of a | |
171 | \&\fIpipe\fR\|(2) made. | |
172 | .PP | |
173 | If either reader or writer is the null string, this will be replaced | |
174 | by an autogenerated filehandle. If so, you must pass a valid lvalue | |
175 | in the parameter slot so it can be overwritten in the caller, or | |
176 | an exception will be raised. | |
177 | .PP | |
178 | \&\fIopen2()\fR returns the process \s-1ID\s0 of the child process. It doesn't return on | |
179 | failure: it just raises an exception matching \f(CW\*(C`/^open2:/\*(C'\fR. However, | |
180 | \&\f(CW\*(C`exec\*(C'\fR failures in the child are not detected. You'll have to | |
181 | trap \s-1SIGPIPE\s0 yourself. | |
182 | .PP | |
183 | \&\fIopen2()\fR does not wait for and reap the child process after it exits. | |
184 | Except for short programs where it's acceptable to let the operating system | |
185 | take care of this, you need to do this yourself. This is normally as | |
186 | simple as calling \f(CW\*(C`waitpid $pid, 0\*(C'\fR when you're done with the process. | |
187 | Failing to do this can result in an accumulation of defunct or \*(L"zombie\*(R" | |
188 | processes. See \*(L"waitpid\*(R" in perlfunc for more information. | |
189 | .PP | |
190 | This whole affair is quite dangerous, as you may block forever. It | |
191 | assumes it's going to talk to something like \fBbc\fR, both writing | |
192 | to it and reading from it. This is presumably safe because you | |
193 | \&\*(L"know\*(R" that commands like \fBbc\fR will read a line at a time and | |
194 | output a line at a time. Programs like \fBsort\fR that read their | |
195 | entire input stream first, however, are quite apt to cause deadlock. | |
196 | .PP | |
197 | The big problem with this approach is that if you don't have control | |
198 | over source code being run in the child process, you can't control | |
199 | what it does with pipe buffering. Thus you can't just open a pipe to | |
200 | \&\f(CW\*(C`cat \-v\*(C'\fR and continually read and write a line from it. | |
201 | .PP | |
202 | The IO::Pty and Expect modules from \s-1CPAN\s0 can help with this, as they | |
203 | provide a real tty (well, a pseudo\-tty, actually), which gets you | |
204 | back to line buffering in the invoked command again. | |
205 | .SH "WARNING" | |
206 | .IX Header "WARNING" | |
207 | The order of arguments differs from that of \fIopen3()\fR. | |
208 | .SH "SEE ALSO" | |
209 | .IX Header "SEE ALSO" | |
210 | See IPC::Open3 for an alternative that handles \s-1STDERR\s0 as well. This | |
211 | function is really just a wrapper around \fIopen3()\fR. |