fflush(stdout)

This is a discussion on fflush(stdout) within the C Programming forums, part of the General Programming Boards category; Hi all, Just a quick one. I had been forced to use fflush(stdout) on certain instances. Whats i wanted to ...

  1. #1
    Registered User ssharish2005's Avatar
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    fflush(stdout)

    Hi all,

    Just a quick one. I had been forced to use fflush(stdout) on certain instances. Whats i wanted to know is, why doesn't the data doesn't get written unless i call the fflush(stdout) on certain cases. Is obvious symptoms for this cause?

    Thanks in advance!

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

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    C++ Witch laserlight's Avatar
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    Quote Originally Posted by ssharish2005
    Just a quick one. I had been forced to use fflush(stdout) on certain instances. Whats i wanted to know is, why doesn't the data doesn't get written unless i call the fflush(stdout) on certain cases. Is obvious symptoms for this cause?
    The standard output stream is typically line buffered, so in the event that the buffer is not yet full or a newline is not yet encountered (or if there has not been intervening input), the output would remain in the buffer, hence fflush() is useful to flush the buffer.
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    Registered User ssharish2005's Avatar
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    But what i thought was that printf itself internally would do that. I might wrong here.

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

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    C++ Witch laserlight's Avatar
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    Quote Originally Posted by ssharish2005
    But what i thought was that printf itself internally would do that. I might wrong here.
    Your own experience tells you that you are wrong
    Of course, if the output stream is line buffered and you print a line then the buffer will be flushed.
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    Registered User ssharish2005's Avatar
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    >Your own experience tells you that you are wrong
    haha i might be. But wasn't sure.

    -ssharish
    Life is like riding a bicycle. To keep your balance you must keep moving - Einstein

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    spurious conceit MK27's Avatar
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    Quote Originally Posted by ssharish2005 View Post
    >Your own experience tells you that you are wrong
    haha i might be. But wasn't sure.

    -ssharish
    Yep. That's why if you are trying to track a segfault using printf(); always use "printf();fflush(stdout);" -- otherwise, you can be misled as to where the fault occurs because the output was left in the buffer (implying, wrongly, that execution had not yet reached the printf()).
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    C++ Witch laserlight's Avatar
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    Quote Originally Posted by MK27
    That's why if you are trying to track a segfault using printf(); always use "printf();fflush(stdout);"
    It may be easier to just use fprintf() with the standard error stream, printing a new line at the end, since the standard error stream is guaranteed to not be fully buffered by default.
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