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POChild process is blocked by full pipe, cannot read in parent process
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  1. POChild process is blocked by full pipe, cannot read in parent process
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    copied!<p>I have roughly created the following code to call a child process:</p> <pre><code>// pipe meanings const int READ = 0; const int WRITE = 1; int fd[2]; // Create pipes if (pipe(fd)) { throw ... } p_pid = fork(); if (p_pid == 0) // in the child { close(fd[READ]); if (dup2(fd[WRITE], fileno(stdout)) == -1) { throw ... } close(fd[WRITE]); // Call exec execv(argv[0], const_cast&lt;char*const*&gt;(&amp;argv[0])); _exit(-1); } else if (p_pid &lt; 0) // fork has failed { throw } else // in th parent { close(fd[WRITE]); p_stdout = new std::ifstream(fd[READ])); } </code></pre> <p>Now, if the subprocess does not write too much to stdout, I can wait for it to finish and then read the stdout from <code>p_stdout</code>. If it writes too much, the write blocks and the parent waits for it forever. To fix this, I tried to wait with <code>WNOHANG</code> in the parent, if it is not finished, read all available output from p_stdout using <code>readsome</code>, sleep a bit and try again. Unfortunately, <code>readsome</code> never reads anything:</p> <pre><code>while (true) { if (waitid(P_PID, p_pid, &amp;info, WEXITED | WNOHANG) != 0) throw ...; else if (info.si_pid != 0) // waiting has succeeded break; char tmp[1024]; size_t sizeRead; sizeRead = p_stdout-&gt;readsome(tmp, 1024); if (sizeRead &gt; 0) s_stdout.write(tmp, sizeRead); sleep(1); } </code></pre> <p>The question is: Why does this not work and how can I fix it?</p> <p><b>edit:</b> If there is only child, simply using <code>read</code> instead of <code>readsome</code> would probably work, but the process has multiple children and needs to react as soon as one of them terminates.</p>
 

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