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<h4 class="subsection">27.6.1 Data Structures for the Shell</h4>

<p>All of the program examples included in this chapter are part of
a simple shell program.  This section presents data structures
and utility functions which are used throughout the example.

   <p>The sample shell deals mainly with two data structures.  The
<code>job</code> type contains information about a job, which is a
set of subprocesses linked together with pipes.  The <code>process</code> type
holds information about a single subprocess.  Here are the relevant
data structure declarations:

<pre class="smallexample">     /* <span class="roman">A process is a single process.</span>  */
     typedef struct process
     {
       struct process *next;       /* <span class="roman">next process in pipeline</span> */
       char **argv;                /* <span class="roman">for exec</span> */
       pid_t pid;                  /* <span class="roman">process ID</span> */
       char completed;             /* <span class="roman">true if process has completed</span> */
       char stopped;               /* <span class="roman">true if process has stopped</span> */
       int status;                 /* <span class="roman">reported status value</span> */
     } process;
     
     /* <span class="roman">A job is a pipeline of processes.</span>  */
     typedef struct job
     {
       struct job *next;           /* <span class="roman">next active job</span> */
       char *command;              /* <span class="roman">command line, used for messages</span> */
       process *first_process;     /* <span class="roman">list of processes in this job</span> */
       pid_t pgid;                 /* <span class="roman">process group ID</span> */
       char notified;              /* <span class="roman">true if user told about stopped job</span> */
       struct termios tmodes;      /* <span class="roman">saved terminal modes</span> */
       int stdin, stdout, stderr;  /* <span class="roman">standard i/o channels</span> */
     } job;
     
     /* <span class="roman">The active jobs are linked into a list.  This is its head.</span>   */
     job *first_job = NULL;
</pre>
   <p>Here are some utility functions that are used for operating on <code>job</code>
objects.

<pre class="smallexample">     /* <span class="roman">Find the active job with the indicated </span><var>pgid</var><span class="roman">.</span>  */
     job *
     find_job (pid_t pgid)
     {
       job *j;
     
       for (j = first_job; j; j = j-&gt;next)
         if (j-&gt;pgid == pgid)
           return j;
       return NULL;
     }
     
     /* <span class="roman">Return true if all processes in the job have stopped or completed.</span>  */
     int
     job_is_stopped (job *j)
     {
       process *p;
     
       for (p = j-&gt;first_process; p; p = p-&gt;next)
         if (!p-&gt;completed &amp;&amp; !p-&gt;stopped)
           return 0;
       return 1;
     }
     
     /* <span class="roman">Return true if all processes in the job have completed.</span>  */
     int
     job_is_completed (job *j)
     {
       process *p;
     
       for (p = j-&gt;first_process; p; p = p-&gt;next)
         if (!p-&gt;completed)
           return 0;
       return 1;
     }
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