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POPlumbing pipes with pipe() in c++
 

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  1. POPlumbing pipes with pipe() in c++
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    <p>I am currently trying to connect pipes between a parent and its children. The children are execing <code>sort</code> and are sorting input they receive from the parent. The children then write to a separate pipe. There are two pipes for each process. One so the parent may send input to child. Another so the parent may receive the result of sort.</p> <p>My problem thus far has been reading the input. I have been getting confirmation from <code>fputs()</code> that I am successfully writing to the childs input. After that I <code>fflush(NULL)</code> and try to read the child's output. The read blocks, as in it never returns or reaches a statement after <code>fputs</code>. Which is rather odd, as I believe that I have set the reads to <code>O_NONBLOCK</code>. The output is listed below. </p> <pre><code>line 174 line 176 line 178 </code></pre> <p>Here is a snippet of code:</p> <pre><code>int sort_writes[num_sort][2]; int sort_reads[num_sort][2]; int i; int *status; int flags; char buffer[BUFFER_SIZE]; // this should contain a bunch of write fds wrapped in fdopen FILE* to_sort[num_sort]; // the same except with reads FILE* from_sort[num_sort]; //this only include for simplicity and so that exec will happen proper char *sort_argv[2]; sort_argv[0]=(char*)"sort"; sort_argv[1]= (char *)NULL; // This will create all of the pipes for the sorts. // The parent will read 0 and the even pipes. it will write to the odd. for(i=0; i&lt; num_sort; i++){ //parent reads from this pipe. child writes to it. assert(pipe(sort_writes[i]) == 0); //parent write to this pipe. child reads from it. assert(pipe(sort_reads[i]) ==0); switch(fork()){ case 0: //this is the child //this closes unnecessary fds _close_less_than(i, sort_writes); _close_less_than(i, sort_reads); dup2(sort_reads[i][0], STDIN_FILENO); // standard out becomes parent pipe in dup2(sort_writes[i][1], STDOUT_FILENO); execv(SORT_LOC.c_str(), sort_argv); default: // this the parent. //clean up. close unused. close(sort_writes[i][1]); close(sort_reads[i][0]); } } //Creates a file pointer for all of the fds I will use to communicate with my sorts //It also sets all reads to nonblock and the parent write stdio buffers to zero. for(i=0; i&lt; num_sort; i++){ assert((from_sort[i]= fdopen(sort_writes[i][0] ,"r")) != NULL); assert((to_sort[i]= fdopen(sort_reads[i][1] , "w")) != NULL); //pipes ignore truncate flags = fcntl(sort_writes[i][0], F_GETFL); flags |= O_NONBLOCK; fcntl(sort_writes[i][0], F_SETFL, flags); } for(i=0; i&lt;(int)theArray.size(); i++){ fputs(theArray.back().c_str(), to_sort[i % num_sort]); theArray.pop_back(); fflush(NULL); // so that the data gets from stdio buffers to pipe buffers. } cout &lt;&lt; "line 174 \n"; for(i=0; i &lt;1; i++){ cout &lt;&lt; "line 176 \n"; while(!feof(from_sort[i])){ cout &lt;&lt; "line 178 \n"; cout &lt;&lt; fgets(buffer, BUFFER_SIZE, from_sort[i]); cout &lt;&lt; buffer; cout &lt;&lt; "at least i tried \n"; } </code></pre>
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    1. COThe whole `dup2()` and `close()` problem is more complex than it seems at first glance. See a [detailed discussion of pathological cases](http://www.unixwiz.net/techtips/remap-pipe-fds.html) and [complete sample code](http://www.unixwiz.net/techtips/remap-pipe-fds.c.txt).
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      1. POPlumbing pipes with pipe() in c++
      1. USAndré Caron
 

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