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Full Discussion: help with unix command exec1 in C
Top Forums Programming help with unix command exec1 in C Post 302504484 by Corona688 on Monday 14th of March 2011 05:53:53 PM
Old 03-14-2011
Quote:
Originally Posted by omega666
i dont see where in your version are u recording the amount of bytes written
It happens here:

Code:
bytes=read(fd[0], pipeout, 512);

'bytes' gets set to the number of bytes it was able to read, or 0 on EOF, see 'man 2 read'.
Quote:
this line reads

bytes=read(fd[0], pipeout, 512);
and it reads 512 bytes which i think is more than needed, how do i know the amount written from execl?
512 is just a maximum. It reads as much as it can up to the maximum and tells you where it stopped. You don't need to know the exact amount the child wrote -- you can read its output in as big or as small chunks as you want, though you don't always get what you asked for. Check the return value of read() to find out what you actually got.

You may not get the exact amount that the child wrote, by the way, since the pipe is also a buffer. The child could do ten writes of 10 bytes and you get one read of 100 bytes; up to a few kilobytes of data may pile up in the pipe before it actually lets anything through. It flushes when the writing end closes, which is why you're still able to get less (and why you need to close the writing end if you're not using it.)
Last edited by Corona688; 03-14-2011 at 06:59 PM..
 

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LEARN ABOUT V7

rk

RK(4)							     Kernel Interfaces Manual							     RK(4)

NAME

       rk  -  RK-11/RK03 or RK05 disk

DESCRIPTION

       Rk?   refers  to an entire disk as a single sequentially-addressed file.  Its 256-word blocks are numbered 0 to 4871.  Minor device numbers
       are drive numbers on one controller.

       The rk files discussed above access the disk via the system's normal buffering mechanism and may be read  and  written  without	regard	to
       physical  disk  records.   There  is  also a `raw' interface which provides for direct transmission between the disk and the user's read or
       write buffer.  A single read or write call results in exactly one I/O operation and therefore raw I/O is considerably more  efficient  when
       many  words  are transmitted.  The names of the raw RK files begin with rrk and end with a number which selects the same disk as the corre-
       sponding rk file.

       In raw I/O the buffer must begin on a word boundary, and counts should be a multiple of 512 bytes (a  disk  block).   Likewise  seek  calls
       should specify a multiple of 512 bytes.

FILES

       /dev/rk?, /dev/rrk?

BUGS

       In  raw	I/O  read  and	write(2) truncate file offsets to 512-byte block boundaries, and write scribbles on the tail of incomplete blocks.
       Thus, in programs that are likely to access raw devices, read, write and lseek(2) should always deal in 512-byte multiples.

																	     RK(4)
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