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PONonblocking Get Character
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copied!<ul> <li>Platform: Linux 3.2.0 x86 (Debian 7)</li> <li>Compiler: GCC 4.7.2 (Debian 4.7.2-5)</li> </ul> <p>I am writing a function that reads a single character from stdin if a character is already present in stdin. If stdin is empty the function is suppose to do nothing and return -1. I googled nonblocking input and was pointed to <a href="http://pubs.opengroup.org/onlinepubs/009695399/functions/poll.html">poll()</a> or <a href="http://pubs.opengroup.org/onlinepubs/009695399/functions/select.html">select()</a>. First I tried to use select() but I could not get it to work so I tried poll() and reached the same conclusion. I am not sure what these functions do exactly but from what I understand of poll()'s documentation if I call it like so:</p> <pre><code>struct pollfd pollfds; pollfds = STDIN_FILENO; pollfds.events = POLLIN; poll(pollfds, 1, 0); </code></pre> <p>if(pollfds.revents & POLLIN) will be true if "Data other than high-priority data may be read without blocking.". But poll() always times out in my test situation. How I test the function could be the problem but the functionality I want is exactly what I am testing for. Here is the function currently and the test situation as well.</p> <pre><code>#include <poll.h> #include <stdio.h> #include <unistd.h> int ngetc(char *c) { struct pollfd pollfds; pollfds.fd = STDIN_FILENO; pollfds.events = POLLIN; poll(&pollfds, 1, 0); if(pollfds.revents & POLLIN) { //Bonus points to the persons that can tell me if //read() will change the value of '*c' if an error //occurs during the read read(STDIN_FILENO, c, 1); return 0; } else return -1; } //Test Situation: //Try to read a character left in stdin by an fgets() call int main() { int ret = 0; char c = 0; char str[256]; //Make sure to enter more than 2 characters so that the excess //is left in stdin by fgets() fgets(str, 2, stdin); ret = ngetc(&c); printf("ret = %i\nc = %c\n", ret, c); return 0; } </code></pre>

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