Adventures in Debugging C/XS 2: Debugging Boogaloo


... or "Ask Not To Whom The Pointer Points, It Points To Thee."

TL;DR: A pointer is not a reference. A pointer knows nothing about the data being pointed to. Returning multiple values requires actual work.

Everything went wrong when I wanted a string with a NUL character inside it. C strings are not Perl scalars, they don't know how long they are. So to mark the end of a string, C uses the NUL character, \0. The strcpy function will copy to your destination until the first \0 from your source. When you want to have a string with a \0 inside of it that does not mark the end of the string, you need to know exactly how long the string is. This is not difficult to do, but you also have to return that length from the function that creates your string.

C functions do not have more than one return value.

(char* buffer, int bufferSize) = get_string_with_nuls();
// You thought it could be that easy?

So in order for your function to result in more than one value, you have to pass in references to be used to fill in with actual values.

char* buffer;
int bufferSize = get_string_with_nuls( buffer );
// C programmers will already know what I did wrong here

Thinking like a Perl programmer, I thought I could just pass in the pointer to the function and the function could fill it with data. Two problems:

  1. I passed in the pointer itself, not a reference to the pointer: &buffer
  2. I did not initialize the pointer to anything.

A more correct way would be:

char* buffer = malloc( 128 * sizeof( char ) );
int bufferSize = get_string_with_nuls( &buffer );

But this suffers from another problem: I have to know beforehand how big my string is going to be and allocate that much memory beforehand.

The way I ended up succeeding was:

int bufferSize;
char* buffer = get_string_with_nuls( &bufferSize );

This way, get_string_with_nuls can handle the malloc with exactly the correct size and give it to me. I don't have to guess at a size beforehand.

Of course, a struct could do this better, or since I'm actually in C++, an object. I'll be planning a new API as soon as I confirm this one actually works and has proper tests (written in Perl, of course).

Adventures in Debugging C/XS


Originally posted as: Adventures in Debugging C/XS on

... or Why A Good Perl Developer Is Not Automatically A Good C Developer, the Story of C Programming via Google.

My tests failed, but only sometimes. I was building an XS module to interface with a C wrapper around a C++ library (wrapper unnecessary? probably). make test was failing with exit code 11. Some quick searching revealed that I had an intermittent segfault. Calling a function as_xml would fail with a SEGV in strlen(). This only happened in perl after as_xml when perl was making a SV out of the return value. This also only mainly happened during make test. Doing prove myself would succeed 19 times out of 20, where make test would fail 19 times out of 20. Worse, my C test program would never fail at all.

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