When you know about buffering it's trivial, but when you don't you can get some very unexpected behaviour from UNIX.

For example, the following program will print nothing.

#include 

int main(void)
{
        printf("hello world");

        while (1)
                sleep(1);
}

This is because by default your stdout stream (which printf uses) will be in line buffering mode, where output will not be printed until a newline (\n) is seen.

You can either add the newline (as was probably intended, but it's an easy enough thing to leave off!) or manually call fflush to flush the buffer.

There are three buffer modes available; unbuffered, line and block buffered. You can access them with the setbuf call. Unbuffered is the default for stderr, while your tty by default takes on line buffering. Anything connected to a block device (e.g. a file on a disk, or pipes) take on block buffering, where the best amounts of data for the device are written at once.

But essentially, if your output is missing check for those newlines!

posted at: Fri, 26 Aug 2005 23:51 | in /code/c | permalink | add comment (0 others)