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<H2><A NAME="s9">9.</A> <A HREF="slshfun.html#toc9">Subprocess Functions</A></H2>

These functions in <CODE>process.sl</CODE> facilitate the creation of
subprocesses and pipelines.
<H2><A NAME="new_process"></A> <A NAME="ss9.1">9.1</A> <A HREF="slshfun.html#toc9.1">new_process</A>
</H2>

<DL>
<DT> Synopsis <DD>
Create a subprocess object
<DT> Usage <DD>
<CODE>Struct_Type new_process (String_Type argv[]; qualifiers)</CODE>
<DT> Description <DD>
This function executes the program specified by the <CODE>argv</CODE>
parameter in a subprocess. If <CODE>argv</CODE> is an array, the first
element (<CODE>argv[0]</CODE>) of the array gives the name of the program
to be executed, and the remaining elements serve as arguments passed
to the program. The program returns a structure that may be used to
interact with the process. Upon error, an exception will be thrown.
The calling program may interact with the subprocess by reading from
or writing to the file descriptor fields of the structure returned
by the <CODE>new_process</CODE> function. The specific file descriptors
are dictated via the <CODE>read</CODE>, <CODE>write</CODE>, and <CODE>dupN</CODE>
qualifiers, as described in detail below.
The function returns a structure containing zero or more fields of the form
<CODE>fdN</CODE> where <CODE>N</CODE> is an integer derived from the qualifiers,
e.g., <CODE>fd0</CODE> and <CODE>fd1</CODE> correspond to the child's stdin and
stdout, respectively. The structure also contains fields of the
form <CODE>fpN</CODE> whose values are stdio <CODE>File_Type</CODE> objects
obtained using <CODE>fdopen</CODE> with the corresponding <CODE>fdN</CODE> value.
Other important fields include <CODE>pid</CODE> whose value is
the process-id of the newly created process.
The status of the process may be checked or collected using the
<CODE>wait</CODE> method. It is very important to call this method to
avoid the creation of zombie processes.

<DT> Qualifiers <DD>
The following qualifiers are supported:
<BLOCKQUOTE><CODE>
<PRE>
 read=fds
</PRE>
</CODE></BLOCKQUOTE>

fds is a list of integer file descriptors that are open for read
access in the subprocess, and may be written to by the calling
process using the fdN or fpN fields of the structure.
<BLOCKQUOTE><CODE>
<PRE>
 write=fds
</PRE>
</CODE></BLOCKQUOTE>

fds is a list of integer file descriptors that are open for write
access in the subprocess, and may be read to by the calling
process using the fdN or fpN fields of the structure.
<BLOCKQUOTE><CODE>
<PRE>
 stdin=filename
 stdout=filename
 stderr=filename
</PRE>
</CODE></BLOCKQUOTE>

These qualifiers allow the stdin, stdout, and stderr file
descriptors in the subprocess to be redirected to a file. Note:
The filenames are interpreted relative to the value of the
<CODE>dir</CODE> qualifier.
<BLOCKQUOTE><CODE>
<PRE>
 fdN=string
</PRE>
</CODE></BLOCKQUOTE>

This qualifier will cause the integer file descriptor N to be open
in the subprocess and redirected to the filename represented by
the string, which is interpreted relative to the value of the <CODE>dir</CODE>
qualifier. The access mode is dictated by the first few
characters of the string as described in more detail below.
<BLOCKQUOTE><CODE>
<PRE>
 stdin=File_Type|FD_Type
 stdout=File_Type|FD_Type
 stderr=File_Type|FD_Type
 fdN=FD_Type|FD_Type
</PRE>
</CODE></BLOCKQUOTE>

If the stdin, stdout, stderr, or fdN qualifiers have File_Type or
FD_Type values, then corresponding file descriptors in the
subprocess will be dup'd to FD_Type or FP_Type file descriptor.
This form of the qualifier may be used to setup pipelines.
<BLOCKQUOTE><CODE>
<PRE>
 dupN=int
</PRE>
</CODE></BLOCKQUOTE>

The file descriptor corresponding to the integer N in the
subprocess is created by duping the descriptor given by the
integer value of the qualifier. For example, dup2=1 would cause
stderr (fd=2) in the subprocess to be redirected to stdout (fd=1).
<BLOCKQUOTE><CODE>
<PRE>
 dir=string
</PRE>
</CODE></BLOCKQUOTE>

Change to the specified directory in the child process. This
will happen after the child process is started, but before any
files have been opened. Hence, files attached to <CODE>stdin</CODE>,
<CODE>stdout</CODE>, etc will be opened relative to this directory.
<BLOCKQUOTE><CODE>
<PRE>
 pre_exec_hook=&amp;func
</PRE>
</CODE></BLOCKQUOTE>

This qualifier will cause the function corresponding to <CODE>func</CODE> to
be called prior to closing unused file descriptors and invoking
the executable. The function will be passed a list of integer
valued file descriptors that will be kept open. Additional
integers may be added to the list by the function. If the
qualifier <CODE>pre_exec_hook_optarg</CODE> exists, it will also be
passed as an addtional argument.
<BLOCKQUOTE><CODE>
<PRE>
 pre_exec_hook_optarg=VALUE
</PRE>
</CODE></BLOCKQUOTE>

If this qualifier exists, its value will be passed as the second
argument to the <CODE>pre_exec_hook</CODE> callback function.
Note that the read and write qualifiers specify the nature of the
file descriptors from the child process's view. That is, those
opened in the child process using the read qualifier, may be written
to by the parent. Similarly, those opened using the write qualifier
may be read by the parent.

<DT> Methods <DD>
<CODE></CODE>
<BLOCKQUOTE><CODE>
<PRE>
 Struct_Type .wait ( [ options ] )
</PRE>
</CODE></BLOCKQUOTE>

The <CODE>.wait</CODE> method may be used to collect the exist status of
the process. When called without arguments, it will cause the
parent process to wait for the subprocess to exit and return its
exit status in the form of a <CODE>waitpid</CODE> structure. The optional
<CODE>options</CODE> argument corresponds to the options argument of the
<CODE>waitpid</CODE> function. The most common is the WNOHANG option,
which will cause the <CODE>.wait</CODE> method to return immediately if
the process has not exited.
If an error occurs, the function will return NULL and set
<CODE>errno</CODE> accordingly. Otherwise it will return a <CODE>waitpid</CODE>
structure. See the documentation for <CODE>waitpid</CODE> for more
information.

<DT> Example <DD>
In the following examples, <CODE>pgm</CODE> represents the program to be
invoked in the subprocess. For simplicity, no addition arguments are
shown
Create subprocess that inherits stdin, stdout, stderr from the caller:
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm);
</PRE>
</CODE></BLOCKQUOTE>

Create a subprocess that inherits stdin, stdout, and writes stderr
to a file:
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; stderr="/tmp/file"); % form 1
 obj = new_process (pgm; fd2="&gt;/tmp/file"); % form 2
</PRE>
</CODE></BLOCKQUOTE>

Mimic popen(pgm, "r"):
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; write=1); % Read from obj.fp1
</PRE>
</CODE></BLOCKQUOTE>

Mimic popen(pgm, "w"):
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; read=0); % Write to obj.fp0
</PRE>
</CODE></BLOCKQUOTE>

Mimic popen("pgm 2&gt;&amp;1", "r"):
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; write=1, dup2=1); % Read from fp1
</PRE>
</CODE></BLOCKQUOTE>

Send stdout to a file, read from the subprocess's stderr:
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; stdout="/tmp/file", write=2);
 % Read from obj.fp2
</PRE>
</CODE></BLOCKQUOTE>

Create a process with handles to its stdin, stdout, stderr
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; write={1,2}, read=0);
 % Use obj.fp0 for stdin, obj.fp1 for stdout, and obj.fp2 for stderr
</PRE>
</CODE></BLOCKQUOTE>

Create a process with a write handle to the process's fd=27 and a
read handle to the process's stdout.
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; read=27, write=1);
 % write to fp27, read from fp1
</PRE>
</CODE></BLOCKQUOTE>

Create a pipeline: pgm1 | pgm2 &gt; /tmp/log :
<BLOCKQUOTE><CODE>
<PRE>
 obj1 = new_process (pgm1; write=1);
 obj2 = new_process (pgm2; stdin=obj1.fp1, stdout="/tmp/log");
</PRE>
</CODE></BLOCKQUOTE>

Create a pipeline with fd=27 from pgm1 redirected to stdin of pgm2:
<BLOCKQUOTE><CODE>
<PRE>
 obj1 = new_process (pgm1; write=27);
 obj2 = new_process (pgm2; stdin=obj1.fp27);
</PRE>
</CODE></BLOCKQUOTE>

Create a pipeline with fd=27 from pgm1 redirected to fd=9 of pgm2:
<BLOCKQUOTE><CODE>
<PRE>
 obj1 = new_process (pgm1; write=27);
 obj2 = new_process (pgm2; fp9=obj1.fp27);
</PRE>
</CODE></BLOCKQUOTE>

Mimic: pgm 2&gt;&amp;1 1&gt;/dev/null
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; fp2=1, stdout="/dev/null");
</PRE>
</CODE></BLOCKQUOTE>

Mimic: pgm &gt;/dev/null 2&gt;&amp;1
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; stdout="/dev/null", dup2=1);
</PRE>
</CODE></BLOCKQUOTE>

Append the output of pgm to /tmp/file.log:
<BLOCKQUOTE><CODE>
<PRE>
 obj = new_process (pgm; stdout="&gt;&gt;/tmp/file.log");
</PRE>
</CODE></BLOCKQUOTE>

<DT> Notes <DD>
Care must be exercised when reading or writing to multiple file
descriptors of a subprocess to avoid deadlock. In such cases, the
select module should be used, or the file descriptors could be put in
non-blocking mode via the fcntl module.
It is important to call the <CODE>.wait</CODE> method prevent the process
from becoming a zombie and clogging the process table.
<DT> See Also <DD>
<CODE>popen, system</CODE>
</DL>

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