usleep() function

This is a discussion on usleep() function within the C Programming forums, part of the General Programming Boards category; Hi, I was writing/upgrading a network program that sends a synflood ( Series of TCP SYN packets). This is being ...

  1. #1
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    usleep() function

    Hi,

    I was writing/upgrading a network program that sends a synflood ( Series of TCP SYN packets). This is being used for the testing. Now I send a series of SYN packets without any delay from a linux machine. It sends it at approx 14-150000 packets per seconds. This I have implemented using setitimer() by setting alarm for 1 sec and counted the total packets sent for the whole time, till i hit Control+C to stop it, where I used SIGINT and a interrupt handler to avg them out.

    Now my question is when I add a user configurable delay between 2 successive sendto() calls using usleep(),the rate drops to 50 packets per seconds. Well I am not sure if this is right. How can a delay of microseconds, drop the rate so much.? Morover whatever delay I provide, rate remains the same 50 pps.

    Here is snippet of this code where I call usleep().

    Code:
    send_syn(so, pPkt, sip, sport, dip, dport, pktid, seq);
    pktcount++;
    if((lmt != 0) && (pktcount == lmt)) goto done;
    signal(SIGINT,exit_handler);
    if(usl) {usleep(usl);} /* If delay is provided in command, add delay */
    Can someone clarify this for me ?

    Thanks,

  2. #2
    and the hat of wrongness Salem's Avatar
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    All sleep functions are a MINIMUM guarantee, not an absolute request.
    usleep(1); which returns a day later would meet it's requirements, the rest is down to the vagaries of whatever else your OS is up to, and the granularity of the scheduler.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Quote Originally Posted by Salem
    All sleep functions are a MINIMUM guarantee, not an absolute request.
    usleep(1); which returns a day later would meet it's requirements, the rest is down to the vagaries of whatever else your OS is up to, and the granularity of the scheduler.
    My OS is Linux with gcc complier. Do you have/know any code samples that shows how to add delay on Linux platforms.

    Thanks,

  4. #4
    cwr
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    As Salem said, sleep functions are a minimum guarantee. The reason for it being such a dramatic delay compared to your expected 1 microsecond is because a sleep function is a signal to the kernel that it can context switch to another process. So, no matter what argument to usleep, you'll trigger a context switch to another process that is waiting to run.

    For more information on accurate, short delays under Linux, see http://www.linux.com/howtos/IO-Port-Programming-4.shtml

    If the delay length is critical, you might want to consider "real time linux" at http://www.fsmlabs.com/

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    Quote Originally Posted by cwr
    As Salem said, sleep functions are a minimum guarantee. The reason for it being such a dramatic delay compared to your expected 1 microsecond is because a sleep function is a signal to the kernel that it can context switch to another process. So, no matter what argument to usleep, you'll trigger a context switch to another process that is waiting to run.

    For more information on accurate, short delays under Linux, see http://www.linux.com/howtos/IO-Port-Programming-4.shtml

    If the delay length is critical, you might want to consider "real time linux" at http://www.fsmlabs.com/
    Thanks for your reply. I read the link. Is it possible to find out the CPU frequency at runtime using any API ?

    Thanks,

  6. #6
    cwr
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    Under Linux, you could take the cpu MHz line from /proc/cpuinfo, but what does knowing the CPU frequency buy you? Not much.

    Why don't you use the nanosleep function in combination with sched_setscheduler() as the article says?

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