Question on select() : why am I getting absurd output ?

[Manish Jain]

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 at 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 at 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.
>