====== A quick one ======

Just a quick something to keep this site alive.

These two issues were nothing that got //me// fuming angry, but my boss was not amused when I was blindsided by these two issues in a production environment.

===== One: bash ''tee'' and return values =====

There is this nice UNIX tool named `tee` that writes its standard input to a file without actually consuming it. It is very useful in scripts where you want to print the proceedings to screen while at the same time logging them somewhere:

<code bash>
someCommand | tee -a logfile.txt # '-a' to append instead of overwriting
</code>

It gets ugly, though, when you add this kind of logging to a script retroactively, //and fail to realize that the next step in the original script was to check if ''someCommand'' was successful//:

<code bash>
someCommand | tee -a logfile.txt
if [ $? != 0 ]; then …
</code>

Congratulations: Because ''$?'' returns the exit code of the //last// command executed, you just checked whether ''tee'' succeeded (e.g., in logging ''someCommand''’s failure).

Luckily, bash offers a way out: The array ''PIPESTATUS'' carries the individual exit codes of all commands in the pipe chain.

<code bash>
someCommand | tee -a logfile.txt
if [ {$PIPESTATUS[0]} != 0 ]; then …
</code>

===== Two: ''va_arg'' and C++ references don’t mix. =====

Consider:

<code cpp>
#include <string>
#include <cassert>

using namespace std;

void foo( std::string & parmN, ... )
{
    va_list ap;
    va_start( ap, parmN );
    int i = va_arg( ap, int );
    va_end( ap );
}

int main()
{
    std::string broken;
    foo( broken, 42 );
    return 0;
}
</code>

//The variable ''i'' will most likely not be set to 42.//

The reason is that most implementations of ''va_arg'' will take the address of ''parmN'' using the address operator ''&''. For a C++ reference, like in this case, this will yield the address //of the referenced variable//. You will not get the address of ''parmN'', but the address of ''broken'', and whatever you pull out of ''va_arg'' will not be what you expected.

----

These two [[http://en.wikipedia.org/wiki/Lobo_%28DC_Comics%29 | bastiches]] caused me some hurt. Be warned.