Friday, October 7, 2022

[SOLVED] Software written in grapa can write to stderr when redirected to a file, but not to a terminal

Issue

I have been having some issues redirecting stderr to stdout with linux shell.

The program I'm trying to test with is written as follows (referred to below as some-binary):

#!/bin/sh
grapa -ccmd '$file().set("/dev/fd/2","Error\n")'

The following does work:

$ ./some-binary < ./my_input 2>test.txt
$ cat test.txt
Error
$

(The output is saved to file)

But this does not for me:

$ ./some-binary < ./my_input 2>&1
$

(There is not output)

I don't understand why this is. Could someone please clarify?

This can be reproduced with the following Dockerfile:

FROM ubuntu:focal-20220801
RUN apt update
RUN apt install wget freeglut3 libxft2 libxext6 libxinerama1 libxcursor-dev -y
RUN wget https://github.com/grapa-dev/grapa/raw/master/bin/grapa-ubuntu64.tar.gz
RUN tar -xvf grapa-ubuntu64.tar.gz -C /usr/local/bin

docker run -it $(docker build -q .) /bin/bash
root@db9abeca8b5f:/# grapa -ccmd '$file().set("/dev/fd/2","Error\n")' >/dev/null 2>test.txt
root@db9abeca8b5f:/# cat test.txt 
Error
root@db9abeca8b5f:/# grapa -ccmd '$file().set("/dev/fd/2","Error\n")' >/dev/null 2>&1      
root@db9abeca8b5f:/#

Solution

The redirection is fine. Grapa just doesn't support canonical file IO.

Here's the documentation for Grapa's $file().set:

Updates the column in a row. By default the $VALUE column is updated. But an alternate column can be specified.

As suggested, it's meant for structured file IO. Here's the source code:

        u64 size = pValue.mLength;
        err = mFile.SetSize(size);
        err = mFile.Write(0, 0, 0, size, pValue.mBytes);
        mFile.Purge(size, 1);
        err = mFile.Close();

And the Write function:

    err = GetSize(fileSize);
    if (err)
        return((GrapaError)-1);
    [...]
    endPos = lseek(mFp, (blockPos*blockSize + offset), SEEK_SET);
    if (endPos != (blockPos*blockSize + offset))
        return((GrapaError)-1);

As you can see, it strongly assumes the file is seekable, and bails if it isn't. This means you can't use this function to write to terminals, pipes, or anything that's not a basic file.

The class says "This class/libraries will be enhanced over time to support navigating data types beyond the file system and the grapa database". You can consider filing a feature request.



Answered By - that other guy
Answer Checked By - Candace Johnson (WPSolving Volunteer)
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