Date: Sun, 15 Jan 2012 23:08:05 +0530 From: Manish Jain <invalid.pointer@gmail.com> To: ss griffon <ssgriffonuser@gmail.com> Cc: freebsd-questions@freebsd.org Subject: Re: Question on select() : why am I getting absurd output ? Message-ID: <4F130EFD.8070306@gmail.com> In-Reply-To: <CAFYJ9ejj=QDh_XDqgohu6oVYpg-Q7FoL7zriyr4D%2B3OrR2wW8Q@mail.gmail.com> References: <4F12F56B.8030904@gmail.com> <CAFYJ9ejj=QDh_XDqgohu6oVYpg-Q7FoL7zriyr4D%2B3OrR2wW8Q@mail.gmail.com>
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Sometimes I do wonder how much stupid I can be.
Thanks
MJ
On 15-Jan-12 22:49, ss griffon wrote:
> On Sun, Jan 15, 2012 at 8:48 AM, Manish Jain<invalid.pointer@gmail.com> wrote:
>>
>> Hi All,
>>
>> I was trying to write a small demo code using the select() system call. Here
>> are the sources :
>>
>> #include<sys/types.h>
>> #include<unistd.h>
>> #include<fcntl.h>
>> #include<iostream>
>> #include<cstring>
>> #include<cassert>
>>
>> int nice_child(int * fd, int * fd_close)
>> {
>> close(fd[0]);
>> close(fd_close[0]);
>> close(fd_close[1]);
>>
>> char buffer[32];
>>
>> while (1)
>> {
>> sleep(3);
>> strcpy(buffer, "I love my wife !");
>> write(fd[1], buffer, strlen(buffer) + 1);
>> }
>>
>> return 0;
>> }
>>
>> int naughty_child(int * fd, int * fd_close)
>> {
>> close(fd[0]);
>> close(fd_close[0]);
>> close(fd_close[1]);
>>
>> char buffer[32];
>>
>> while (1)
>> {
>> sleep(4);
>> strcpy(buffer, "I love your wife !");
>> write(fd[1], buffer, strlen(buffer) + 1);
>> }
>>
>> return 0;
>> }
>>
>> int main()
>> {
>> int fd_nice[2];
>> int fd_naughty[2];
>>
>> pipe(fd_nice);
>> pipe(fd_naughty);
>>
>> if (fork() == 0)
>> {
>> return nice_child(fd_nice, fd_naughty);
>> }
>> else
>> {
>> if (fork() == 0)
>> {
>> return naughty_child(fd_naughty, fd_nice);
>> }
>> }
>>
>> close(fd_nice[1]);
>> close(fd_naughty[1]);
>>
>> fd_set fdset;
>> char buffer[64];
>> int fd = (*fd_naughty> *fd_nice) ? *fd_naughty : *fd_nice;
>>
>> FD_ZERO(&fdset);
>> FD_SET(fd_nice[0],&fdset);
>> FD_SET(fd_naughty[0],&fdset);
>>
>> while (1)
>> {
>> int result = select(fd + 1,&fdset, 0, 0, 0);
>> assert(result> 0);
>>
>> if (FD_ISSET(fd_nice[0],&fdset))
>> {
>> int result = read(fd, buffer, sizeof(buffer));
>> buffer[result] = 0;
>>
>> std::cout<< "Nice child sent : "<< buffer<<
>> std::endl;
>> }
>>
>> if (FD_ISSET(fd_naughty[0],&fdset))
>> {
>> int result = read(fd, buffer, sizeof(buffer));
>> buffer[result] = 0;
>>
>> std::cout<< "Naughty child sent : "<< buffer<<
>> std::endl;
>> }
>> }
>>
>> return 0;
>> }
>>
>> I was expecting the output to be like :
>>
>> Nice child sent : I love my wife !
>> Naughty child sent : I love your wife !
>> Nice child sent : I love my wife !
>>
>> But what I actually get is :
>>
>> Nice child sent : I love your wife !
>> Nice child sent : I love your wife !
>> Nice child sent : I love your wife !
>> Nice child sent : I love your wife !
>> Nice child sent : I love your wife !
>> Nice child sent : I love your wife !
>>
>> Can somebody throw some light on what might be wrong ?
>>
>>
>> Thank you&
>> Regards
>>
>> Manish Jain
>> invalid.pointer@gmail.com
>>
>> _______________________________________________
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>
>
>
> It looks like you are always reading from the same file descriptor
> 'fd'. Instead you should read from fd_naughty[0] or fd_nice[0] based
> on your FD_ISSET checks. Also, don't forget to wait() for your child
> processes.
>
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