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+    <title>skalibs: the cspawn library interface</title>
+    <meta name="Description" content="skalibs: the cspawn library interface" />
+    <meta name="Keywords" content="skalibs c unix cspawn fork posix_spawn child process exec library libstddjb" />
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+<p>
+<a href="index.html">libstddjb</a><br />
+<a href="../libskarnet.html">libskarnet</a><br />
+<a href="../index.html">skalibs</a><br />
+<a href="//skarnet.org/software/">Software</a><br />
+<a href="//skarnet.org/">skarnet.org</a>
+</p>
+
+<h1> The <tt>cspawn</tt> library interface </h1>
+
+<p>
+ The following functions are declared in the <tt>skalibs/cspawn.h</tt> header,
+and implemented in the <tt>libskarnet.a</tt> or <tt>libskarnet.so</tt> library.
+</p>
+
+<h2> General information </h2>
+
+<p>
+ <tt>cspawn</tt> is a unifier API to spawn child processes with
+<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawn.html">posix_spawn()</a>
+as a backend if supported by the system, falling back on
+<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html">fork()</a> +
+<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/execve.html">execve()</a>
+otherwise.
+</p>
+
+<h2> Functions </h2>
+
+<h3> Primitive </h3>
+
+<p>
+<code> pid_t cspawn (char const *file, char const *const *argv, char const *const *envp, uint32_t flags, cspawn_fileaction const *fa, size_t n) </code> <br />
+Forks and execs a child as with <tt>exec_ae(file, argv, envp)</tt>.
+Returns 0 if it fails, and the pid of the child if it succeeds.
+Before execing, the following operations are performed in the child:
+</p>
+
+<ul>
+ <li> If <em>flags</em> contains (the or-able value) CSPAWN_FLAGS_SIGBLOCKNONE,
+the child's signal processing mask is reset - no signal will be blocked. </li>
+ <li> Alternatively, if <em>flags</em> contains (the or-able value) CSPAWN_FLAGS_SELFPIPE_FINISH,
+the child's signal state is reset to what it was before the invocation of
+<tt>selfpipe_init()</tt> in the parent. </li>
+ <li> The file actions described in the array <em>fa</em>, which must have at least <em>n</em> elements,
+are processed in order. For every <em>i</em> from 0 to <em>n</em>-1:
+  <ul>
+    <li> if <tt>fa[i].type</tt> is CSPAWN_FA_CLOSE, then file descriptor <tt>fa[i].x.fd</tt> is closed </li>
+    <li> else if <tt>fa[i].type</tt> is CSPAWN_FA_MOVE, then file descriptor <tt>fa[i].x.fd2[1]</tt>
+is moved to <tt>fa[i].x.fd2[0]</tt> </li>
+    <li> else if <tt>fa[i].type</tt> is CSPAWN_FA_COPY, then file descriptor <tt>fa[i].x.fd2[1]</tt>
+is copied to <tt>fa[i].x.fd2[0]</tt>, as with <tt>dup2(fa[i].x.fd2[1], fa[i].x.fd2[0])</tt> </li>
+    <li> else if <tt>fa[i].type</tt> is CSPAWN_FA_OPEN, then file <tt>fa[i].x.openinfo.file</tt> is
+opened with flags <tt>fa[i].x.openinfo.oflag</tt> and mode <tt>fa[i].x.openinfo.mode</tt>, and file
+descriptor <tt>fa[i].x.openinfo.fd</tt> points to it. </li>
+  </ul> </li>
+</ul>
+
+<h3> Higher level interfaces </h3>
+
+<p>
+<code> pid_t child_spawn1_pipe (char const *file, char const *const *argv, char const *const *envp, int *fd, int w) </code> <br />
+Spawns <em>file</em> as with <tt>cspawn()</tt> with a <em>flags</em> value of CSPAWN_FLAGS_SIGBLOCKNONE;
+a pipe is created between the child's
+stdin (if <em>w</em> is 0) or stdout (if <em>w</em> is nonzero) and the parent.
+The parent's end of the pipe will be stored in *<em>fd</em>.
+</p>
+
+<p>
+<code> pid_t child_spawn1_socket (char const *file, char const *const *argv, char const *const *envp, int *fd) </code> <br />
+Like <tt>child_spawn1</tt>, except that a socket, not a pipe is created between the parent
+and the child. Both the child's stdin and stdout point to the socket; the parent has
+its end of the socket available in *<em>fd</em>.
+</p>
+
+<p>
+<code> pid_t child_spawn2 (char const *file, char const *const *argv, char const *const *envp, int *fds) </code> <br />
+Two pipes are created between the
+parent and the child. <em>fds</em> must point to an array of 2 ints: this
+array is read as well as written by <tt>child_spawn2()</tt>. On function
+call, the numbers in <em>fds</em> must describe what fds should be used
+in the child to contain the communication pipes (for instance, if the child
+will read from its parent on stdin and write to its parent on stdout, <em>fds</em>
+must contain 0 and 1). On function return, <em>fds</em> contains the
+descriptors for the pipes that read from / write to the child.
+</p>
+
+<p>
+<code> pid_t child_spawn3 (char const *file, char const *const *argv, char const *const *envp, int *fds) </code> <br />
+Three pipes are created between the
+parent and the child. <em>fds</em> must point to an array of 3 ints: this
+array is read as well as written by <tt>child_spawn2()</tt>. On function
+call, the numbers in <em>fds[0]</em> and <em>fds[1]</em> must describe what fds should be used
+in the child to read from (resp. write to) the parent.
+On function return, <em>fds</em> contains the
+descriptors for the pipes that read from / write to the child. <em>fds[2]</em> is
+the reading end of a pipe; the writing end is present in the child, and its number
+is available as the value of the <tt>SKALIBS_CHILD_SPAWN_FDS</tt> environment variable.
+</p>
+
+
+<p>
+<code> pid_t child_spawn (char const *file, char const *const *argv, char const *const *envp, int *fds, unsigned int nfds) </code> <br />
+More generic spawning function. <em>fds</em> must point to an array of at least <em>nfds</em> ints;
+file descriptors reading from or writing to the child will be stored there. The function returns
+0 on failure or the pid of the child on success.
+</p>
+<ul>
+ <li> If <em>nfds</em> is 0, then the function behaves like <tt>cspawn</tt> with a flags value
+of CSPAWN_FLAGS_SIGBLOCKNONE. </li>
+ <li> If <em>nfds</em> is 1 or more, then <em>fds</em> will contain pipes between the
+child and the parent. The parent will read on even-numbered ones
+and write on odd-numbered ones. The child will read on <em>fds[0]</em>, write on <em>fds[1]</em>,
+then any additional fds are available to it in the <tt>SKALIBS_CHILD_SPAWN_FDS</tt> environment
+variable as a comma-separated list of integers. </li>
+</ul>
+
+</body>
+</html>