How to flush output to stdout with vvp? about iverilog

[martinwhitaker]

For me, running under Linux. the output appears immediately. What version of Icarus and O/S are you using?

[getvictor]

@martinwhitaker, what do you mean by immediately? What I'm looking for is for the first $display to appear first, then the second, etc. Currently I'm seeing them all appear on the screen at the same time at the end of the run.

I tried it with Icarus 0.9.6 on Ubuntu 13.04, and with a recent Icarus 0.10.0 on RHEL5

[martinwhitaker]

By immediately I mean as soon as the simulation reaches that point, i.e. exactly what you expect/want.

I would only expect the output to be delayed if you have redirected or piped standard output, e.g. something like 'vvp a.out | tee run.log' will give the behaviour you are seeing. Are you running vvp directly from a command prompt in a terminal window?

[caryr]

What you are looking for is the following (from 1364-2005)

17.2.6 Flushing output

For example:
$fflush ( mcd );
$fflush ( fd );
$fflush ( );

writes any buffered output to the file(s) specified by mcd, to the file specified by fd, or if $fflush is invoked
with no arguments, to all open files.

and this snippet from 17.2.1

Three file descriptors are pre-opened; they are STDIN, STDOUT, and STDERR, which have the values 32'h8000_0000, 32'h8000_0001, and 32'h8000_0002, respectively. STDIN is pre-opened for reading, and STDOUT and STDERR are pre-opened for append.

If you don't have other files open for writing that would benefit from buffering just calling $fflush(); should be enough otherwise try $fflush(32'h8000_0001); to just flush STDOUT.

Cary

[getvictor]

Yes, using $fflush() works.
Thank you.

[getvictor]

@martinwhitaker, yes, I ran vvp directly from command prompt and also from inside a bash script. I always see results flushed out at the end of the sim.

[martinwhitaker]

Glad to hear Cary's solution works for you, but it shouldn't be needed! stdout should be line-buffered when connected to a terminal, so each $display should produce output immediately. Unfortunately I can't think of any explanation for why this is not happening for you.

[getvictor]

Ok. I see the difference in behavior now. I'm running vvp like this:
vvp a.out | tee log.txt. Whenever the vvp output gets piped or redirected, the results don't get flushed out. This is inconvenient -- I'm writing a Java-based wrapper (intended for quick Verilog prototyping using different simulators) and I must do redirection. Any chance this behavior can be changed?

[steveicarus]

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The behavior you are seeing is the typical behavoir of stdout
as defined by the C library. It is possible to override this
(by explicitly setting stdout to line buffered) but the reason
the people in the olden days did this is because output that is
not going to the tty is typically being sent to a file or other
non-interactive destination and the C library chooses the most
efficient way to do that.

What is typically done with programs that want to interactively
wrap interactive programs is to use ptty devices to link the
wrapper program to the interactive program.

On 07/17/2013 07:04 AM, getvictor wrote:

Ok. I see the difference in behavior now. I'm running vvp like
this: |vvp a.out | tee log.txt|. Whenever the vvp output gets piped
or redirected, the results don't get flushed out. This is
inconvenient since I'm writing a Java-based wrapper (intended for
quick Verilog prototyping using different simulators). Any chance
this behavior can be changed?

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[getvictor]

@steveicarus, thank you for the explanation. I found a way to enable line buffering while redirecting: unbuffer vvp a.out | tee log.txt