technical/api-run-command.html - pub/scm/git/git-htmldocs - Git at Google

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 <body class="article">
 <div id="header">
 <h1>run-command API</h1>
 </div>
 <div id="content">
 <div id="preamble">
 <div class="sectionbody">
 <div class="paragraph"><p>The run-command API offers a versatile tool to run sub-processes with
 redirected input and output as well as with a modified environment
 and an alternate current directory.</p></div>
 <div class="paragraph"><p>A similar API offers the capability to run a function asynchronously,
 which is primarily used to capture the output that the function
 produces in the caller in order to process it.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_functions">Functions</h2>
 <div class="sectionbody">
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>child_process_init</code>
 </dt>
 <dd>
 <p>
         Initialize a struct child_process variable.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>start_command</code>
 </dt>
 <dd>
 <p>
         Start a sub-process. Takes a pointer to a <code>struct child_process</code>
         that specifies the details and returns pipe FDs (if requested).
         See below for details.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>finish_command</code>
 </dt>
 <dd>
 <p>
         Wait for the completion of a sub-process that was started with
         start_command().
 </p>
 </dd>
 <dt class="hdlist1">
 <code>run_command</code>
 </dt>
 <dd>
 <p>
         A convenience function that encapsulates a sequence of
         start_command() followed by finish_command(). Takes a pointer
         to a <code>struct child_process</code> that specifies the details.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>run_command_v_opt</code>, <code>run_command_v_opt_cd_env</code>
 </dt>
 <dd>
 <p>
         Convenience functions that encapsulate a sequence of
         start_command() followed by finish_command(). The argument argv
         specifies the program and its arguments. The argument opt is zero
         or more of the flags <code>RUN_COMMAND_NO_STDIN</code>, <code>RUN_GIT_CMD</code>,
         <code>RUN_COMMAND_STDOUT_TO_STDERR</code>, or <code>RUN_SILENT_EXEC_FAILURE</code>
         that correspond to the members .no_stdin, .git_cmd,
         .stdout_to_stderr, .silent_exec_failure of <code>struct child_process</code>.
         The argument dir corresponds the member .dir. The argument env
         corresponds to the member .env.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>child_process_clear</code>
 </dt>
 <dd>
 <p>
         Release the memory associated with the struct child_process.
         Most users of the run-command API don&#8217;t need to call this
         function explicitly because <code>start_command</code> invokes it on
         failure and <code>finish_command</code> calls it automatically already.
 </p>
 </dd>
 </dl></div>
 <div class="paragraph"><p>The functions above do the following:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 If a system call failed, errno is set and -1 is returned. A diagnostic
   is printed.
 </p>
 </li>
 <li>
 <p>
 If the program was not found, then -1 is returned and errno is set to
   ENOENT; a diagnostic is printed only if .silent_exec_failure is 0.
 </p>
 </li>
 <li>
 <p>
 Otherwise, the program is run. If it terminates regularly, its exit
   code is returned. No diagnostic is printed, even if the exit code is
   non-zero.
 </p>
 </li>
 <li>
 <p>
 If the program terminated due to a signal, then the return value is the
   signal number + 128, ie. the same value that a POSIX shell&#8217;s $? would
   report.  A diagnostic is printed.
 </p>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 <code>start_async</code>
 </dt>
 <dd>
 <p>
         Run a function asynchronously. Takes a pointer to a <code>struct
         async</code> that specifies the details and returns a set of pipe FDs
         for communication with the function. See below for details.
 </p>
 </dd>
 <dt class="hdlist1">
 <code>finish_async</code>
 </dt>
 <dd>
 <p>
         Wait for the completion of an asynchronous function that was
         started with start_async().
 </p>
 </dd>
 <dt class="hdlist1">
 <code>run_hook</code>
 </dt>
 <dd>
 <p>
         Run a hook.
         The first argument is a pathname to an index file, or NULL
         if the hook uses the default index file or no index is needed.
         The second argument is the name of the hook.
         The further arguments correspond to the hook arguments.
         The last argument has to be NULL to terminate the arguments list.
         If the hook does not exist or is not executable, the return
         value will be zero.
         If it is executable, the hook will be executed and the exit
         status of the hook is returned.
         On execution, .stdout_to_stderr and .no_stdin will be set.
         (See below.)
 </p>
 </dd>
 </dl></div>
 </li>
 </ol></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_data_structures">Data structures</h2>
 <div class="sectionbody">
 <div class="ulist"><ul>
 <li>
 <p>
 <code>struct child_process</code>
 </p>
 </li>
 </ul></div>
 <div class="paragraph"><p>This describes the arguments, redirections, and environment of a
 command to run in a sub-process.</p></div>
 <div class="paragraph"><p>The caller:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 allocates and clears (using child_process_init() or
    CHILD_PROCESS_INIT) a struct child_process variable;
 </p>
 </li>
 <li>
 <p>
 initializes the members;
 </p>
 </li>
 <li>
 <p>
 calls start_command();
 </p>
 </li>
 <li>
 <p>
 processes the data;
 </p>
 </li>
 <li>
 <p>
 closes file descriptors (if necessary; see below);
 </p>
 </li>
 <li>
 <p>
 calls finish_command().
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>The .argv member is set up as an array of string pointers (NULL
 terminated), of which .argv[0] is the program name to run (usually
 without a path). If the command to run is a git command, set argv[0] to
 the command name without the <em>git-</em> prefix and set .git_cmd = 1.</p></div>
 <div class="paragraph"><p>Note that the ownership of the memory pointed to by .argv stays with the
 caller, but it should survive until <code>finish_command</code> completes. If the
 .argv member is NULL, <code>start_command</code> will point it at the .args
 <code>argv_array</code> (so you may use one or the other, but you must use exactly
 one). The memory in .args will be cleaned up automatically during
 <code>finish_command</code> (or during <code>start_command</code> when it is unsuccessful).</p></div>
 <div class="paragraph"><p>The members .in, .out, .err are used to redirect stdin, stdout,
 stderr as follows:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 Specify 0 to request no special redirection. No new file descriptor
   is allocated. The child process simply inherits the channel from the
   parent.
 </p>
 </li>
 <li>
 <p>
 Specify -1 to have a pipe allocated; start_command() replaces -1
   by the pipe FD in the following way:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>.in: Returns the writable pipe end into which the caller writes;
         the readable end of the pipe becomes the child's stdin.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>.out, .err: Returns the readable pipe end from which the caller
         reads; the writable end of the pipe end becomes child's
         stdout/stderr.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>The caller of start_command() must close the so returned FDs
 after it has completed reading from/writing to it!</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 Specify a file descriptor &gt; 0 to be used by the child:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>.in: The FD must be readable; it becomes child's stdin.
 .out: The FD must be writable; it becomes child's stdout.
 .err: The FD must be writable; it becomes child's stderr.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>The specified FD is closed by start_command(), even if it fails to
 run the sub-process!</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 Special forms of redirection are available by setting these members
   to 1:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>.no_stdin, .no_stdout, .no_stderr: The respective channel is
         redirected to /dev/null.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>.stdout_to_stderr: stdout of the child is redirected to its
         stderr. This happens after stderr is itself redirected.
         So stdout will follow stderr to wherever it is
         redirected.</code></pre>
 </div></div>
 </li>
 </ol></div>
 <div class="paragraph"><p>To modify the environment of the sub-process, specify an array of
 string pointers (NULL terminated) in .env:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 If the string is of the form "VAR=value", i.e. it contains <em>=</em>
   the variable is added to the child process&#8217;s environment.
 </p>
 </li>
 <li>
 <p>
 If the string does not contain <em>=</em>, it names an environment
   variable that will be removed from the child process&#8217;s environment.
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>If the .env member is NULL, <code>start_command</code> will point it at the
 .env_array <code>argv_array</code> (so you may use one or the other, but not both).
 The memory in .env_array will be cleaned up automatically during
 <code>finish_command</code> (or during <code>start_command</code> when it is unsuccessful).</p></div>
 <div class="paragraph"><p>To specify a new initial working directory for the sub-process,
 specify it in the .dir member.</p></div>
 <div class="paragraph"><p>If the program cannot be found, the functions return -1 and set
 errno to ENOENT. Normally, an error message is printed, but if
 .silent_exec_failure is set to 1, no message is printed for this
 special error condition.</p></div>
 <div class="ulist"><ul>
 <li>
 <p>
 <code>struct async</code>
 </p>
 </li>
 </ul></div>
 <div class="paragraph"><p>This describes a function to run asynchronously, whose purpose is
 to produce output that the caller reads.</p></div>
 <div class="paragraph"><p>The caller:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 allocates and clears (memset(&amp;asy, 0, sizeof(asy));) a
    struct async variable;
 </p>
 </li>
 <li>
 <p>
 initializes .proc and .data;
 </p>
 </li>
 <li>
 <p>
 calls start_async();
 </p>
 </li>
 <li>
 <p>
 processes communicates with proc through .in and .out;
 </p>
 </li>
 <li>
 <p>
 closes .in and .out;
 </p>
 </li>
 <li>
 <p>
 calls finish_async().
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>The members .in, .out are used to provide a set of fd&#8217;s for
 communication between the caller and the callee as follows:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 Specify 0 to have no file descriptor passed.  The callee will
   receive -1 in the corresponding argument.
 </p>
 </li>
 <li>
 <p>
 Specify &lt; 0 to have a pipe allocated; start_async() replaces
   with the pipe FD in the following way:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>.in: Returns the writable pipe end into which the caller
 writes; the readable end of the pipe becomes the function's
 in argument.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>.out: Returns the readable pipe end from which the caller
 reads; the writable end of the pipe becomes the function's
 out argument.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>The caller of start_async() must close the returned FDs after it
 has completed reading from/writing from them.</code></pre>
 </div></div>
 </li>
 <li>
 <p>
 Specify a file descriptor &gt; 0 to be used by the function:
 </p>
 <div class="literalblock">
 <div class="content">
 <pre><code>.in: The FD must be readable; it becomes the function's in.
 .out: The FD must be writable; it becomes the function's out.</code></pre>
 </div></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>The specified FD is closed by start_async(), even if it fails to
 run the function.</code></pre>
 </div></div>
 </li>
 </ol></div>
 <div class="paragraph"><p>The function pointer in .proc has the following signature:</p></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>int proc(int in, int out, void *data);</code></pre>
 </div></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 in, out specifies a set of file descriptors to which the function
   must read/write the data that it needs/produces.  The function
   <strong>must</strong> close these descriptors before it returns.  A descriptor
   may be -1 if the caller did not configure a descriptor for that
   direction.
 </p>
 </li>
 <li>
 <p>
 data is the value that the caller has specified in the .data member
   of struct async.
 </p>
 </li>
 <li>
 <p>
 The return value of the function is 0 on success and non-zero
   on failure. If the function indicates failure, finish_async() will
   report failure as well.
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>There are serious restrictions on what the asynchronous function can do
 because this facility is implemented by a thread in the same address
 space on most platforms (when pthreads is available), but by a pipe to
 a forked process otherwise:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 It cannot change the program&#8217;s state (global variables, environment,
   etc.) in a way that the caller notices; in other words, .in and .out
   are the only communication channels to the caller.
 </p>
 </li>
 <li>
 <p>
 It must not change the program&#8217;s state that the caller of the
   facility also uses.
 </p>
 </li>
 </ol></div>
 </div>
 </div>
 </div>
 <div id="footnotes"><hr /></div>
 <div id="footer">
 <div id="footer-text">
 Last updated
  2018-01-26 15:11:04 PST
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