Trouble understanding how to process C string

Question

I\'m trying to use Mac OS X\'s listxattr C function and turn it into something useful in Python. The man page tells me that the function returns a string buffer, which is a \"si

Answer 1

Actually, since I'm going to send it to Python I don't have to process it C-style after all. Just use the Py_BuildValue passing it the format character s#, which knows what do with it. You'll also need the size.

return Py_BuildValue("s#", buffer, size);

You can process it into a list on Python's end using split('\x00'). I found this after trial and error, but I'm glad to have learned something about C.

Answer 2

It looks like listxattr returns the size of the buffer it has filled, so you can use that to help you. Here's an idea:

for(int i=0; i<res-1; i++)
{
    if( buffer[i] == 0 )
        buffer[i] = ',';
}

Now, instead of being separated by null characters, the attributes are separated by commas.

Answer 3

1. char *p = buffer;
2. get the length with strlen(p). If the length is 0, stop.
3. process the first chunk.
4. p = p + length + 1;
5. back to step 2.

Answer 4

So you guessed pretty much right.

The listxattr function returns a bunch of null-terminated strings packed in next to each other. Since strings (and arrays) in C are just blobs of memory, they don't carry around any extra information with them (such as their length). The convention in C is to use a null character ('\0') to represent the end of a string.

Here's one way to traverse the list, in this case changing it to a comma-separated list.

int i = 0;
for (; i < res; i++)
   if (buffer[i] == '\0' && i != res -1) //we're in between strings
       buffer[i] = ',';

Of course, you'll want to make these into Python strings rather than just substituting in commas, but that should give you enough to get started.