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    <p>You cannot do what you want in the general case (unbuffering after <a href="http://man7.org/linux/man-pages/man2/execve.2.html" rel="nofollow noreferrer"><code>execve(2)</code></a> of arbitrary executables...)</p> <p>Buffering is done by code (e.g. by some <code>libc</code> code related to <code>&lt;stdio.h&gt;</code>). And the code is defined by the program being <code>execve</code>-ed.</p> <p>You might perhaps play with <a href="http://www.catonmat.net/blog/simple-ld-preload-tutorial/" rel="nofollow noreferrer">LD_PRELOAD tricks</a> which might call <code>setvbuf(stdin, NULL, _IONBF, BUFSIZ);</code> <em>after</em> the <code>execve</code> (but before the <code>main</code>....); but this would work only with dynamically linked executables.</p> <p>Perhaps using some <code>constructor</code> <a href="http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html" rel="nofollow noreferrer">function attribute</a> in some initialization function of your <code>LD_PRELOAD</code>-ed shared object might <em>sometimes</em> do the trick. Or redefine <code>printf</code>, <code>fopen</code>, .... in that shared object...</p> <h3>addenda</h3> <p>You commented that you do :</p> <blockquote> <p>two-way communication with a sub-process using two pipes.</p> </blockquote> <p>Then your approach is wrong. The parent process should monitor the two pipes, probably with a multiplexing call like <a href="http://man7.org/linux/man-pages/man2/poll.2.html" rel="nofollow noreferrer">poll(2)</a>, then (according to the result of that multiplexing) decide to read or to write to the child process. In reality, you want some <a href="https://en.wikipedia.org/wiki/Event_loop" rel="nofollow noreferrer">event loop</a>: either implement a simple <strong>event <em>loop</em></strong> yourself (with e.g. <code>poll</code> [iteratively called many times] inside a repeated <em>loop</em>) or use some existing one (see <a href="http://libevent.org/" rel="nofollow noreferrer">libevent</a> or <a href="http://software.schmorp.de/pkg/libev.html" rel="nofollow noreferrer">libev</a>, or the one provided by some toolkit like <a href="http://www.gtk.org/" rel="nofollow noreferrer">GTK</a> or <a href="http://qt-project.org/qt5" rel="nofollow noreferrer">Qt</a> etc...)</p> <p><sup>You might also multiplex with <a href="http://man7.org/linux/man-pages/man2/select.2.html" rel="nofollow noreferrer">select(2)</a> but I recommend <code>poll</code> because of the <a href="https://en.wikipedia.org/wiki/C10k_problem" rel="nofollow noreferrer">C10K problem</a></sup></p> <p>You won't lose your time by reading <a href="http://advancedlinuxprogramming.com/" rel="nofollow noreferrer">Advanced Linux Programming</a> ...</p> <p>See also <a href="https://stackoverflow.com/a/20582916/841108">this answer</a> to your next related question.</p>
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    1. POForce a program created using `exec` to perform unbuffered I/O
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    1. COThank you for the addendum! Is the poll function available in GNU C? Why does it not appear in the documentation of GNU C?
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    2. COWhat do you call GNU C? Is it [GCC](http://gcc.gnu.org/) the compiler (which is unrelated to your question) or [Glibc](http://www.gnu.org/software/libc/) (a.k.a. GNU libc) the standard C library? `poll` is documented in POSIX [poll](http://pubs.opengroup.org/onlinepubs/009695399/functions/poll.html) and in the man pages!! And GNU libc does not document all of POSIX (but assumes it is known). BTW, GNU libc documents [select](http://www.gnu.org/software/libc/manual/html_node/Waiting-for-I_002fO.html#Waiting-for-I_002fO) -which is an old-fashion way to multiplex.
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    3. COI meant glibc. I couldn't find the poll function documented on this page: http://www.gnu.org/software/libc/manual/html_mono/libc.html
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